Switching power supply unit

1. Field of the Invention

The present invention relates to a ringing choke converter type switching power supply unit.

2. Description of the Related Art

Up to now, a ringing choke converter (hereinafter, referred to as RCC) type switching power supply unit has been known for use as a switching power supply unit. This RCC type switching power supply unit is to provide a stabilized DC power supply using a commercially available AC power supply, and, as the construction is relatively simple and the power supply unit is small-sized, light-weight, and highly efficient, it is widely used in various devices such as electronic computers, communication devices, office automation equipment, etc. Furthermore, this type of power supply unit is also utilized as an auxiliary unit and a pre-regulator in small capacity series regulators.

FIG. 8 is a circuit diagram of a conventional RCC type switching power supply unit. As shown in the drawing, the switching power supply unit 10 is composed of an input circuit 2 , a DC-DC converter circuit 3 , a voltage detector circuit 4 , and a control circuit 5 .

The input circuit 2 is composed of a fuse F, a filter circuit LF, and a rectifying diode bridge DB, and an alternating-current power supply AC is connected to the input terminals. Furthermore, one input terminal of the input circuit 2 is connected to one end of the input-side of the filter circuit LF through the fuse F and the other input terminal of the input circuit 2 is directly connected to the other end of the input side of the filter circuit LF. Furthermore, the terminals on the output side of the filter circuit LF are connected to respective input terminals of the diode bridge DB. The output terminals a and b of the diode bridge DB are connected to the input terminals a and b of the DC—DC converter circuit 3 .

The DC—DC converter circuit 3 is composed of capacitors C 1 and C 4 , a transformer T, a main switching element such as MOS field-effect transistor (FET) Q 1 as resistors R 1 and R 10 , and a diode D 1 . Furthermore, the transformer T contains a primary winding N 1 , a secondary winding N 2 which is opposite in polarity to the primary winding N 1 , and a feedback winding Nb which is the same in polarity as the primary winding N 1 . The smoothing capacitor C 1 is provided between the input terminals a and b of the DC—DC converter circuit 3 , and also a series circuit of the primary winding N 1 and the FET Q 1 is provided so as to be parallel to the capacitor C 1 . Thus, the starting point of the primary winding N 1 of the transformer T is connected to the input terminal a of the DC—DC converter circuit 3 , the drain of the FET Q 1 is connected to the end of the primary winding N 1 , and the source of the FET Q 1 is connected to the input terminal b of the DC—DC converter circuit 3 . Furthermore, one end of the starter resister R 1 is connected to the starting point of the primary winding N 1 , and the other end of the resistor R 1 is connected to the gate of the FET Q 1 . The resistor R 10 is connected between the gate and the source of the FET Q 1 . Moreover, the anode of the rectifying diode D 1 is connected to the end of the secondary winding N 2 of the transformer T, and the smoothing capacitor C 4 is connected between the cathode of the diode D 1 and the starting point of the secondary winding N 2 . The diode D 1 and capacitor C 4 constitute a rectifying and smoothing circuit.

The voltage detector circuit 4 is provided on the output side of the DC—DC converter circuit 3 , and is composed of resistors R 5 , R 6 , and R 7 , a light-emitting diode PD as a light-emitting device of a photo coupler PC, and a shunt regulator Sr. The output terminals of the voltage detector circuit 4 are provided so as to be parallel to the capacitor C 4 of the DC—DC converter circuit 3 . Furthermore, each of a series circuit of the resistor R 5 , the light-emitting diode PD, and the shunt regulator Sr, and a series circuit of the resistors R 6 and R 7 is provided so as to be parallel to the capacitor C 4 . Moreover, the cathode of the light-emitting diode PD and the cathode of the shunt regulator Sr are connected. Furthermore, the reference terminal of the shunt regulator Sr is connected to the connection point between the resistors R 6 and R 7 .

The control circuit 5 is composed of resistors R 2 , R 3 , R 4 , and R 13 , capacitors C 2 and C 3 , a diode D 2 , a phototransistor PT as a light receiving element of a photo coupler PC, and an NPN transistor Q 2 . The capacitor C 3 and the resistor R 13 are connected in series between the gate of the FET Q 1 of the DC—DC converter circuit 3 and the starting point of the feedback winding Nb. Furthermore, the collector and the emitter of the transistor Q 2 are connected to the gate and the source of the FET Q 1 , respectively. Moreover, a series circuit of the resistor R 2 and the resistor R 3 is connected between the starting point and the end of the feedback winding Nb. In addition, a series circuit of the resistor R 4 , the diode D 2 , and the phototransistor PT in the photo coupler PC is connected so as to be parallel to the resistor R 2 . Moreover, the cathode of the diode D 2 is connected to the collector of the phototransistor PT in the photo coupler PC. Furthermore, the capacitor C 2 is connected between the base and the emitter of the transistor Q 2 , and the above resistor R 3 is connected so as to be parallel to the capacitor C 2 .

Next, the operation of the switching power supply unit 10 constructed as in the above is described. First, at the start where the power supply is turned on, a voltage is applied to the gate of the FET Q 1 through the starter resistor R 1 and the FET Q 1 is turned on. In this way, the voltage of the power supply is applied to the primary winding N 1 of the transformer T and a voltage having the same polarity as that of the primary winding N 1 is generated in the feedback winding Nb. This voltage signal as a positive feedback signal is provided to the gate of the FET Q 1 through the resistor R 13 and the capacitor C 3 , and the FET Q 1 is quickly turned on. At this time, an excitation energy is stored in the primary winding N 1 .

A charging current flows into the capacitor C 2 through the resistor R 2 because of the electromotive force in the feedback winding Nb. When the charging voltage of the capacitor C 2 exceeds the forward voltage drop between the base and the emitter of the controlling transistor Q 2 , the transistor Q 2 is turned on. In this way, the voltage between the gate and the source of the FET Q 1 becomes substantially zero thereby to turn off the FET Q 1 .

Thus, the excitation energy stored in the primary winding N 1 of the transformer T during the ON-state period of the FET Q 1 is discharged as electric energy through the secondary winding N 2 , rectified by the diode D 1 , smoothed by the capacitor C 4 , and supplied to the load (not shown in the drawing) which is connected to the output terminals of the voltage detector circuit 4 .

When the electric charge in the charged capacitor C 2 is discharged through a discharge circuit of the resistor R 3 , etc., and the charged voltage becomes equal to or less than the forward voltage drop between the base and the emitter of the transistor Q 2 , the transistor Q 2 is turned off. Furthermore, when the excitation energy stored in the primary winding N 1 of the transformer T is discharged through the secondary winding N 2 and the current flowing through the diode D 1 becomes zero, a kickback voltage is generated in the feedback winding Nb in which the starting point of the winding is positive polarity and the FET Q 1 is turned on by the kickback voltage once again. When the FET Q 1 is turned on, a voltage is provided to the primary winding N 1 of the transformer T again, and an excitation energy is stored in the primary winding N 1 .

Thus, the above-described oscillation is repeated in the switching power supply unit 10 .

Next, the feedback control by the voltage detector circuit 4 is described. Under normal conditions, the output voltage on the load side is detected by a voltage divider of the resistors R 6 and R 7 , and the detected voltage and the reference voltage of the shunt regulator Sr are compared. The changes of the output voltage are amplified by the shunt regulator Sr, a current passing through the light-emitting diode PD of the photo coupler PC changes, and the amount of light emission of the light-emitting diode PD also changes. Furthermore, in accordance with the amount of light received by the phototransistor PT of the photo coupler PC, the impedance of the phototransistor PT changes and accordingly the charging time constant of the capacitor C 2 changes. The more the output voltage decreases, the larger the above charging time constant becomes, and accordingly the more the output voltage decreases, the longer the period from the turn on to the forcible turn off, caused by the transistor Q 2 , of the FET Q 1 , that is, the ON-state period of the FET Q 1 , increases, forcing the output voltage to increase. Furthermore, when the output voltage increases, the condition is reversed and the output voltage is forcibly reduced. In this way, a constant-voltage control is achieved so that the output voltage may be constant.

Moreover, the conditions of a light load correspond to the cases where the above-mentioned output voltage increases, and the conditions of a heavy load correspond to the cases where the above output voltage decreases.

In the conventional RCC type switching power supply unit shown in FIG. 8, it is known that the oscillation frequency of the FET Q 1 changes substantially inversely with the input power or output power. This is shown by (b) in FIG. 5, that is, shown as the relationship of the oscillation frequency (switching frequency) to the output (load power).

Generally, as the load becomes lighter, the switching loss is reduced for each switching operation, but, as shown by (b) in FIG. 5, the smaller the output power is, that is, the lighter the load is, the higher the oscillation frequency becomes, and then the higher the oscillation frequency becomes, the greater the number of switching operations and accordingly, switching losses increase. As a result, even if the load becomes lighter, the degree of reduction of the switching losses is very little. Therefore, the lighter the load is, the more the efficiency of the power supply unit is reduced.

In order to reduce the switching loss under such light-load conditions, the circuit constants may be designed so that the oscillation frequency becomes lower at the rated load, but when the power supply unit is required to cope with a wide range of very light loads to heavy loads, it is unavoidable that the oscillation frequency becomes relatively higher at a light load. That is, generally the oscillation frequency at the rated load is mainly determined by effects in the parts such as the magnetic flux density of the transformer and of ripples, noise, etc., and if the oscillation frequency is made too low, there is a problem in that the saturation of the transformer, etc., are caused.

Furthermore, when the device using the conventional RCC type switching power supply unit is on standby, that is, at the above-mentioned light load, as the oscillation frequency becomes higher, heat generation is caused in the main switching element because of the switching losses.

For example, in the switching power supply unit used in a printer, when the power switch is in the OFF-state, it is possible to operate the power source in an optimized operation mode during standby by using a switching signal. On the other hand, once the power switch of the printer is turned on for printing, etc., the switching power supply unit returns to the optimized operation mode for normal work. Furthermore, when the power switch of the printer is in the ON-state, if printing is not performed, there is a waiting time. Because the printer is light-loaded during this waiting time, heat is generated in the main switching element of the switching power supply unit used in the printer as described above.

Regarding this problem, in Japanese Unexamined Patent Application Publication No. 11-235036, the inventors of the application concerned have disclosed a technology concerning a switching power supply unit in which the loss on standby is improved by inputting a switching signal on standby and reducing the oscillation frequency.

Furthermore, in Japanese Patent Application No. 11-253550, the inventors of the application concerned have proposed a technology concerning a switching power supply unit in which the loss on standby is improved by continuously lowering the oscillation frequencies at the rated range of use on standby.

However, in the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-235036, when the switching signal is for normal operation, the power supply unit functions as an RCC. Therefore, when the power supply unit is on standby as in the above-described printer, no means for reducing the loss is provided and there is a problem in that the increase of the input power and heat generation in the main switching element are caused. Furthermore, there is also an operation of an intermittent oscillation and in this case there is a problem in that the output ripple voltage increases.

Furthermore, in the switching power supply unit proposed in Japanese Patent Application No. 11-253550, when the power supply unit is lightly-loaded, the oscillation frequency is automatically reduced. In this case, when the oscillation frequency is reduced too much, the response characteristic of the load worsens, and accordingly the frequency is limited to high frequencies when compared to the technology described in Japanese Unexamined Patent Application Publication No. 11-235036. Because of this, there is a problem in that the effect of lowering the loss is inferior to a switching signal input type.

The present invention is to solve the above-mentioned problems, and it is an object of the present invention to provide an RCC type switching power supply unit in which by lowering or keeping the oscillation frequency substantially at a constant value at a light load and on standby and thus avoiding the increase of the oscillation frequency, the suppression of the standby power, the suppression of heat generation of the main switch, the standby efficiency of the RCC, and the improvement of the output ripple voltage caused by the intermittent oscillation are objectives, and in which by switching the operation mode by an external signal or switching the operation mode after a certain period of time after the load current has become on standby, the oscillation frequency is further lowered and the loss on standby is greatly reduced.

The ringing choke converter type switching power supply unit of the present invention comprises a transformer having a primary winding, a secondary winding, and a feedback winding; a main switching element receiving a feedback signal from the feedback winding and turning on and off the current of the primary winding; a rectifying and smoothing circuit comprising a rectifying element and a smoothing element, being connected to the secondary winding; and a control circuit connected between the feedback winding and the control terminal of the main switching element, a delay circuit for prohibiting the turning on of the main switching element for a certain period of time after the current flowing through the rectifying element has become zero is provided, and a switching circuit for switching the delay time of the delay circuit in two stages or more than two stages is provided.

According to this construction, the turning on of the main switching element is prohibited for a certain period of time by the delay circuit and the OFF-state period is extended. Furthermore, the delay time can be switched in two stages or more than two stages by the switching circuit. Therefore, the switching frequency of the main switching element is lowered when compared to the cases where such a delay circuit is not used. Furthermore, it becomes possible to set the switching frequency of the main switching element at a desired value by the switching circuit.

Furthermore, in the switching power supply unit of the present invention, the delay circuit contains a switching device and a time constant circuit which are provided between the feedback winding of the transformer and the control terminal of the main switching element is provided, and a switching circuit for switching the time constant of the time constant circuit is provided. In this way, the ON-state period of the switching device changes in accordance with the time constant of the time constant circuit, and it becomes possible to switch the time constant of the time constant circuit by the switching circuit.

Moreover, in the switching power supply unit of the present invention, an output power detector circuit for rectifying and smoothing the voltage of the feedback winding of the transformer by a rectifying element and a smoothing element is provided, and a delay time extension circuit connected between the output power detector circuit and the control terminal of the delay circuit through a resistor element is provided.

In addition, in the switching power supply unit of the present invention, the switching circuit can be switched by an external signal. In this way, it becomes possible to externally switch the switching frequency of the main switching element easily.

Furthermore, in the switching power supply unit of the present invention, a current detector circuit for detecting the current flowing through the secondary winding is provided, and the switching circuit for switching the delay time of the delay circuit after a certain period of time when the current detector circuit detects the load current or load power being on standby is provided. Thus, it becomes possible to switch the switching frequency of the main switching element automatically.

FIG. 1 is a circuit diagram showing a switching power supply unit according to a first embodiment of the present invention;

FIG. 2 is a circuit diagram showing a switching power supply unit according to a second embodiment of the present invention;

FIG. 3 is a circuit diagram showing a switching power supply unit according to a third embodiment of the present invention;

FIG. 4 is a graph showing the change of the absolute value of the charging voltage of a capacitor constituting the output power detector circuit on the primary winding side of the switching power supply unit shown in FIG. 3;

FIG. 5 is a graph showing the change of the switching frequency of a switching power supply unit of the present invention and a conventional one;

FIG. 6 is a circuit diagram showing a switching power supply unit according to a fourth embodiment of the present invention;

FIG. 7 is a circuit diagram showing a switching power supply unit according to a fifth embodiment of the present invention; and

FIG. 8 is a circuit diagram of a conventional RCC type switching power supply unit.

First Embodiment

The construction of an RCC type switching power supply unit according to a first embodiment of the present invention is described in accordance with FIG. 1 . FIG. 1 is a circuit diagram showing the switching power supply unit according to a first embodiment of the present invention. Moreover, in the switching power supply unit 1 shown in the drawing, a delay circuit 6 and a delay-time switching circuit 7 are added to the switching power supply unit 10 shown in FIG. 8, and the other circuits are the same as those shown in FIG. 8 . Therefore, the same parts are given the same reference numerals, and their detailed description is omitted.

In the illustrative embodiment of FIG. 1, the delay circuit 6 in the switching power supply unit 1 is provided between the DC—DC converter circuit 3 and the control circuit 5 , and is composed of resistors R 8 , R 13 , and R 14 , a capacitor C 5 , a diode D 3 , and a PNP transistor Q 3 as a switching device. Moreover, the resistor R 13 has been transferred to the delay circuit 6 from the control circuit 5 in FIG. 8 . In the delay circuit 6 , a series circuit of the resistor R 13 and the transistor Q 3 and a series circuit of the resistor R 14 and the diode D 3 are provided in parallel between the gate as a control terminal of the FET Q 1 in the DC—DC converter circuit 3 and the capacitor C 3 in the control circuit 5 . Moreover, in the above delay circuit 6 , one end of the resistor R 13 and the collector of the transistor Q 3 are connected, and one end of the resistor R 14 and the anode of the diode D 3 are connected. Furthermore, the capacitor C 5 and the resistor R 8 are connected in series, and these are connected between the collector and the emitter of the transistor Q 2 in the control circuit 5 . The capacitor C 5 and the resistor R 8 constitute a time constant circuit. The base of the transistor Q 3 is connected to the connection point of the capacitor C 5 and the resistor R 8 , and the capacitor C 5 is provided between the base and the emitter of the transistor Q 3 .

The delay time switching circuit 7 is provided in the delay circuit 6 and is composed of a switch SW 1 and a resistor R 15 . A series circuit of the switch SW 1 and the resistor R 15 is connected so as to be parallel to the resistor R 8 in the delay circuit 6 . Usually the switch SW 1 is on.

Next, the operation of the thus constructed switching power supply unit 1 is described. Moreover, the main part of the total operation is described here.

First, when a voltage is applied to the gate of the FET Q 1 and the FET Q 1 is turned on, a power-supply voltage is applied to the primary winding N 1 of the transformer T and a voltage having the same polarity as that in the primary winding N 1 is generated in the feedback winding Nb. This voltage signal as a positive feedback signal is provided to the gate of the FET Q 1 through the capacitor C 3 , the transistor Q 3 which is turned on, and the resistor R 13 , and the FET Q 1 is quickly turned on. At this time, an excitation energy is stored in the primary winding N 1 .

A charging current also flows into the capacitor C 2 through the resistor R 2 by the electromotive force of the feedback winding Nb. When the charging voltage of the capacitor C 2 exceeds the forward voltage drop between the base and the emitter of the transistor Q 2 , the transistor Q 2 is turned on. Because of this, the voltage between the gate and the source of the FET Q 1 becomes substantially zero and, as a result, the FET Q 1 is turned off.

Thus, the excitation energy stored in the primary winding N 1 of the transformer T while the FET Q 1 is turned on is discharged as electrical energy through the secondary winding N 2 , rectified by the diode D 1 , smoothed by the capacitor C 4 , and supplied to a load, which is not shown in the drawing, connected to the output terminals of the voltage detector circuit 4 .

When the electric charge in the charged capacitor C 2 is discharged through the discharge circuit comprising the resistor R 3 , etc., and the charged voltage of the capacitor C 2 becomes lower than the forward voltage drop between the base and the emitter of the transistor Q 2 , the transistor Q 2 is turned off. Furthermore, when the whole excitation energy stored in the primary winding N 1 of the transformer T is discharged through the secondary winding N 2 and the current flowing through the diode D 1 becomes zero, the capacitor C 5 is charged by a kickback voltage Vnb generated in the feedback winding Nb.

When the charged voltage Vc 2 of the capacitor C 5 reaches the voltage Von at which the transistor Q 3 is turned on, the transistor Q 3 is turned on, a voltage is applied to the gate of the FET Q 1 , and the FET Q 1 is turned on.

As described above, the turning on of the FET Q 1 can be delayed until the charged voltage Vc 2 of the transistor Q 2 in the delay circuit 6 reaches the voltage Von. Moreover, the period up to the turning on of the transistor Q 3 is determined by the time constant of the time constant circuit comprising the capacitor C 5 and the parallel connection of the resistor R 8 and the resistor R 15 .

When the resistor R 15 is disconnected from the resistor R 8 by the turning off of the switch SW 1 in the delay-time switching circuit 7 , the time constant is determined by the capacitor C 5 and the resistor R 8 . Therefore, the delay time for the turning on of the FET Q 1 is extended. In this way, the turning on of the FET Q 1 is delayed and the OFF-state period of the FET Q 1 is extended.

In this way, a substantially constant oscillation frequency can be maintained at a light load because of the delay circuit 6 , and, by avoiding an increase in the switching frequency, the efficiency of the RCC during standby can be improved or the output ripple voltage can be improved. Furthermore, by the turning on and off of the switch SW 1 in the delay-time switching circuit 7 , the delay time until the turning on of the FET Q 1 can be adjusted and the efficiency at a light load can be greatly improved.

Second Embodiment

Next, the construction of an RCC type switching power supply unit according to a second embodiment of the present invention is described in accordance with FIG. 2 . FIG. 2 is a circuit diagram showing the switching power supply unit according to a second embodiment of the present invention. In FIG. 2, the delay circuit 6 of the switching power supply unit 1 a is constructed in substantially the same way as the delay circuit 6 of the switching power supply unit 1 shown in FIG. 1 . Furthermore, a delay-time switching circuit 7 a in the switching power supply unit 1 a is composed of the switch SW 1 and the resistor R 15 . In a series circuit of the switch SW 1 and the resistor R 15 , one end of the resistor R 15 is connected to the gate of the FET Q 1 in the DC—DC converter circuit 3 , and one end of the switch SW 1 is connected to the base of the transistor Q 3 in the delay circuit 6 .

The switching power supply unit 1 a constructed in this way operates in the same way as the switching power supply unit 1 . Accordingly, a substantially constant oscillation frequency can be maintained at a light load, and, by avoiding increase in the switching frequency, the efficiency of the RCC on standby can be improved or the output ripple voltage can be improved, and at the same time the efficiency at a light load can be greatly improved by the turning on and off of the switch SW 1 .

Furthermore, because, in the switching power supply unit 1 a, the delay-time switching circuit 7 a is provided at a location shown in FIG. 2, when the resistor R 15 has a low resistance, the switching power supply unit 1 a has the following effect in comparison with the switching power supply unit 1 . That is, when the switching power supply unit 1 is started while the switch SW 1 of the delay-time switching circuit 7 is switched on, there are cases where the FET Q 1 cannot be turned on and accordingly the switching power supply unit 1 cannot be started. This is because, as the voltage between the gate and the source of the FET Q 1 is determined by a ratio between the composite resistance of the parallel-connected resistors R 8 , R 15 , and R 10 and the resistance of the resistor R 1 , the voltage between the gate and the source of the FET Q 1 does not reach the threshold level.

On the other hand, because, in the switching power supply unit 1 a, the delay-time switching circuit 7 a is provided at a location shown in FIG. 2, the switching power supply unit 1 a can be started, even if the switching power supply unit 1 a is started while the switch SW 1 is on. This is because, as a series circuit of the resistor R 15 and the resistor R 10 is connected in parallel to the resistor R 8 with the switch SW 1 switched on, the voltage between the gate and the source of the FET Q 1 is not adversely affected by the resistor R 15 , even if the resistor R 15 has a low resistance. Accordingly, the voltage between the gate and the source of the FET Q 1 reaches the threshold level to turn on the FET Q 1 even with switch SW 1 turned on.

Third Embodiment

Next, the construction of an RCC type switching power supply unit according to a third embodiment of the present invention is described in accordance with FIG. 3 . FIG. 3 is a circuit diagram showing the switching power supply unit according to a third embodiment of the present invention. In the switching power supply unit 1 b shown in FIG. 3, an output power detector circuit 8 and a resistor element (impedance) connecting the delay-time switching circuit 7 and the output power detector circuit 8 are added to the switching power supply unit 1 shown in FIG. 1, and the other circuits are the same as those shown in FIG. 1 . Accordingly, the same parts are given the same reference numerals, and their detailed description is omitted.

In FIG. 3, the output power detector circuit 8 of the switching power supply unit 1 b is provided between the feedback winding Nb of the DC—DC converter circuit 3 and the control circuit 5 , and the series circuit is composed of a diode D 4 as a rectifying element and a capacitor C 6 as a smoothing element. The diode D 4 and the capacitor C 6 are connected in series, and is provided between both terminals of the feedback winding Nb. That is, the cathode of the diode D 4 is connected to the starting point of the feedback winding Nb and one end of the capacitor C 6 is connected to the end of the feedback winding Nb.

Furthermore, in the delay circuit 6 , one end of a resistor R 16 as a resistance element is connected to the connection point of the capacitor C 5 and the resistor R 8 , and the other end of the resistor R 16 is connected to the connection point of the diode D 4 and the capacitor C 6 in the output power detector circuit 8 .

Next, the operation of the thus constructed switching power supply unit 1 b is described. Moreover, the main part of the switching power supply unit 1 b is described here.

First, a voltage is applied to the gate of the FET Q 1 and the FET Q 1 is turned on. Because of this, the voltage of the power supply is applied to the primary winding N 1 of the transformer T and a voltage of the same polarity as that in the primary winding N 1 is generated in the feedback winding Nb. This voltage signal as a positive feedback signal is provided to the gate of the FET Q 1 through the capacitor C 3 , the transistor Q 3 , and the resistor R 13 , and the FET Q 1 is quickly turned on. At this time, an excitation energy is stored in the primary winding N 1 .

A charging current also flows into the capacitor C 2 through the resistor R 2 because of the electromotive force in the feedback winding Nb. When the charging voltage of the capacitor C 2 exceeds the forward voltage drop between the base and the emitter of the controlling transistor Q 2 , the transistor Q 2 is turned on. Thus, the voltage between the gate and the source of the FET Q 1 becomes substantially zero and the FET Q 1 is turned off.

When the OFF-state period of the FET Q 1 starts, the capacitor C 6 in the output power detector circuit 8 is charged by a voltage generated in the feedback winding Nb of the transformer T. Furthermore, the excitation energy stored in the primary winding N 1 of the transformer T during the ON-state period of the FET Q 1 is discharged as electric energy through the secondary winding N 2 , rectified by the diode D 1 , smoothed by the capacitor C 4 , and supplied to a load, not shown in the drawing, connected to the output terminal of the voltage detector circuit 4 .

Then, when the electric charge in the charged capacitor C 2 is discharged through the discharge circuit comprising the resistor R 3 , etc., and the charged voltage becomes lower than the forward voltage drop between the base and the emitter of the transistor Q 2 , the transistor Q 2 is turned off. Furthermore, the whole excitation energy stored in the primary winding N 1 of the transformer T is discharged through the secondary winding N 2 and the current flowing through the diode D 1 becomes zero, the charged voltage Vc 1 of the capacitor C 6 is discharged, a voltage is applied to the emitter of the transistor Q 3 by the charged voltage Vc 1 and the voltage Vnb generated in the feedback winding Nb, and the capacitor C 5 in the delay circuit 6 is charged. When the switch SW 1 is on, the voltage of the capacitor C 5 increases in accordance with the time constant of the capacitor C 5 , the parallel connection of the resistor R 8 and resistor R 15 , and the resistor R 16 in series with capacitor C 6 .

FIG. 4 is a graph showing the change of the absolute value of a charging voltage Vc 1 of the capacitor C 6 constituting the output power detector circuit 8 on the primary winding side of the switching power supply unit 1 b shown in FIG. 3 . Here, the absolute value of the charging voltage Vc 1 increases in proportion to the load power of the switching power supply unit 1 b because of the influence of the leakage inductance of the transformer T, etc., as shown in FIG. 4 .

Therefore, at a light load, the absolute value of the charging voltage Vc 1 of the capacitor C 6 is relatively small, and the time required until the charging voltage Vc 2 of the capacitance C 5 constituting the time constant circuit reaches the voltage Von at which the transistor Q 3 is turned on becomes longer. Because of this, the turning on of the FET Q 1 is delayed and the OFF-state period of the FET Q 1 is extended, and, as a result, the switching frequency of the FET Q 1 is lowered. On the other hand, at a heavy loading, the absolute value of the charging voltage Vc 1 of the capacitor C 6 becomes relatively large, and the time required until the charging voltage Vc 2 of the capacitor C 5 reaches the voltage Von at which the transistor Q 3 is turned on becomes shorter. Thus, the turning on of the FET Q 1 is quickened and the OFF-state period of the FET Q 1 is shortened.

Because the capacitor C 6 as a voltage source is disposed in the path of the capacitor C 5 and the resistor R 16 , the larger the absolute value of the voltage of the capacitor C 6 is, the more quickly the voltage of the capacitor C 5 increases.

When the charging voltage Vc 2 of the capacitor C 5 reaches the voltage Von, the transistor Q 3 is turned on, a voltage is applied to the gate of the FET Q 1 , and the FET Q 1 is turned on.

In this way, the turning on of the FET Q 1 can be delayed until the charging voltage Vc 2 of the transistor Q 2 in the delay circuit 6 reaches the voltage Von.

When the resistor R 15 is disconnected from the resistor R 8 by the turning off of the switch SW 1 in the delay-time switching circuit 7 , the time constant is determined by the capacitor C 5 and the resistor R 8 . Accordingly, the delay time until the FET Q 1 is turned on is extended. Because of this, the turning on of the FET Q 1 is delayed and the OFF-state period of the FET Q 1 is extended.

Here, the change of the switching frequency which is a consequence of the increase of the load power of the switching power supply unit 1 b is shown in FIG. 5 . FIG. 5 is a graph showing the change of the switching frequency in the switching power supply unit of the present invention and a conventional one. In FIG. 5, (a) and (a)′ show the change of the switching frequency of the switching power supply unit 1 b and (b) shows the change of the switching frequency of the conventional switching power supply unit 10 . The frequency (a) of the switching power supply unit of the present invention is particularly low at a light load compared with the frequency (b) of the conventional switching power supply unit 10 , and when the load increases, the frequency (a) changes in the same way as the frequency (b) of the conventional one beyond a certain point of the load. Furthermore, when the switch SW 1 is switched off, the frequency is shown by the line (a)′.

Fourth Embodiment

Next, the construction of an RCC type switching power supply unit according to a fourth embodiment of the present invention is described in accordance with FIG. 6 . FIG. 6 is a circuit diagram showing the switching power supply unit according to a fourth embodiment of the present invention. In FIG. 6, the delay circuit 6 of the switching power supply unit 1 c is constructed in substantially the same way as the delay circuit 6 of the switching power supply unit 1 shown in FIG. 1 . Furthermore, a delay-time switching circuit 7 b of the switching power supply unit 1 c is composed of the resistor R 15 and a phototransistor PT 2 as part of a photo coupler PC 2 . A series circuit of the resistor R 15 and the phototransistor PT 2 is connected so as to be parallel to the resistor R 8 . That is, one end of the resistor R 15 is connected to the connection point of the capacitor C 5 and the resistor R 8 in the delay circuit 6 , and the emitter of the phototransistor PT 2 is connected to the source of the FET Q 1 in the DC—DC converter circuit 3 .

Furthermore, an external remote signal REM is provided to the anode of a light-emitting diode in the photo coupler PC 2 , and the cathode of the light-emitting diode PD 2 is grounded through a resistor R 17 .

The switching power supply unit 1 c constructed in this way operates in the same way as the switching power supply unit 1 . That is, when the external remote signal REM is a high signal, the light-emitting diode PD 2 lights up and the phototransistor PT 2 as a light receiving element is turned on. Thus, the resistor R 15 is connected so as to be parallel to the resistor R 8 , and the time constant of the time constant circuit composed of the capacitor C 5 and the resistor R 8 changes. Accordingly, when this switching power supply unit 1 c is used in the above-mentioned printer, the waiting mode can be changed to the normal operation mode by switching the remote signal to a high signal.

Fifth Embodiment

Next, the construction of an RCC type switching power supply unit according to a fifth embodiment of the present invention is described in accordance with FIG. 7 . FIG. 7 is a circuit diagram showing the switching power supply unit according to a fifth embodiment of the present invention. In the switching power supply unit 1 d shown in FIG. 7, a resistor R 16 is provided between the output terminal and one end of the resistor R 6 of the voltage detector circuit 4 of the switching power supply unit 1 shown in FIG. 1, and a current detector circuit 9 for detecting the current of the resistor R 16 is provided. Furthermore, a phototransistor PT 2 as a light receiving element of a photo coupler PC 2 provided in the current detector circuit 9 is provided so as to take the place of the switch SW 1 in the delay-time switching circuit 7 . The current detector circuit 9 is composed of the resistors R 16 to R 23 , the capacitor C 11 , the comparators COMP 1 and COMP 2 , and the reference voltage Vref. The resistor R 16 is connected between the output terminal of the voltage detector circuit 4 and one end of the resistor R 6 . Between the output terminals of the voltage detector circuit 4 , the resistor R 17 and the resistor R 18 are connected in series. The positive input terminal of the comparator COMP 1 is connected to the connection point of the resistor R 17 and the resistor R 18 . A parallel circuit composed of a series circuit of the resistor R 19 and the resistor R 20 , a series circuit of the resistor R 21 and the capacitor C 11 , and a series circuit of the resistor R 22 and the photodiode PD 2 of the photo coupler PC 2 is connected between the connection point of the resistor R 16 and the resistor R 6 and the starting point of the secondary winding N 2 of the transformer T. Moreover, one end of the resistor R 22 and the anode of the photodiode PD 2 are connected. Furthermore, the negative input terminal of the comparator COMP 1 is connected to the connection point of the resistor R 19 and the resistor R 20 . Furthenmore, the output terminal of the comparator COMP 1 , the negative input terminal of the comparator COMP 2 , and one end of the resistor R 23 are connected to the connection point of the resistor R 21 and the capacitor C 11 . Furthermore, the output terminal of the comparator COMP 2 and the other end of the resistor R 23 are connected to the connection point of the resistor R 22 and the photodiode PD 2 . The reference voltage Vref is connected to the positive input terminal of the comparator COMP 2 .

Furthermore, the delay-time switching circuit 7 c in the switching power supply unit 1 d is constructed in the same way as the delay-time switching circuit 7 b in the switching power supply unit 1 c shown in FIG. 5 and is composed of the resistor R 15 and the phototransistor PT 2 of the photo coupler PC 2 . A series circuit of the resistor R 15 and the phototransistor PT 2 is connected so as to be parallel to the resistor R 8 . That is, one end of the resistor R 15 is connected to the connection point of the capacitor C 5 and the resistor R 8 in the delay circuit 6 , and the emitter of the phototransistor PT 2 is connected to the source of the FET Q 1 in the DC—DC converter circuit 3 .

Next, the operation of the switching power supply unit 1 d constructed as in the above is described. Moreover, here, the main part of the switching power supply unit 1 d is described.

When the load of the switching power supply unit 1 d changes from the rated load (heavy load) to the standby load (light load) and the output current decreases, the output of the comparator COMP 1 in the current detector circuit 9 goes to an open state.

At this time, in the current detector circuit 9 , when the voltage of the capacitor C 11 in the RC-time-constant circuit rises at a speed to be determined by the time constant and exceeds the reference voltage Vref, the output of the comparator COMP 2 becomes a low level signal. Then, the flow of the current in the photodiode PD 2 stops, the light-emitting diode PD 2 goes off, and the phototransistor PT 2 as a light receiving element is turned off. Thus, the time constant becomes unaffected by the resistor R 15 , and the time constant of the time constant circuit composed of the capacitor C 5 and the resistor R 8 changes. Therefore, when this switching power supply unit 1 d is used in the above-mentioned printer, the normal operation mode changes into the waiting mode because of the output signal of the comparator COMP 2 which changes into a low level signal.

When a current detector circuit is provided in the switching power supply unit as described above, an effect can be obtained where, even if the rated load changes into a standby load, the operation mode does not change during a period of time determined by the time constant.

Next, when a waiting load changes into the rated load, the output of the comparator COMP 1 in the current detector circuit 9 goes low. When the voltage of the capacitor C 11 in the RC time constant circuit decreases and becomes less than or equal to the reference voltage Vref, the output of the comparator COMP 2 goes to a high level signal and the waiting mode changes into the normal rated operation mode.

When a current detector circuit is provided in the switching power supply unit, an instantaneous mode change from a waiting load to the rated load takes place and the most appropriate operation can be realized under any load condition.

As described in the above, in the fifth embodiment of the present invention, the modes can be automatically changed by detecting the current of the output load using the current detector circuit. Furthermore, as a timer circuit is provided, any sudden load change can be coped with.

For example, in a printer, the load may quickly change because of the printing, form feed, printer control, etc. When a conventional high-efficiency power supply unit on standby in which modes are automatically changed is used under such a condition, the switching between a waiting mode and the normal operation mode frequently takes place in accordance with the load on the printer side, and accordingly it is required that a switching power supply unit for the printer has good responsiveness. Therefore, as the responsiveness had priority, the switching frequency could not be dynamically lowered.

On the contrary, when a switching power supply unit of the present invention is used in the printer, because the normal operation mode does not change into a waiting mode for a certain period of time even if the printer comes under a waiting load condition, and accordingly the printer operates under the normal operation mode when the load frequently and quickly changes, and, as a result, the switching power supply unit of the invention is reliable. Furthermore, a waiting mode is instantly switched into the normal operation mode, and the load at the time of the switching is often relatively light as in the treatment before printing, etc., and there is no problem in responsiveness.

Moreover, in each of the embodiments of the present invention, the switch and the phototransistor of the photo coupler in the delay-time switching circuit are shown as two-state devices which are operated in either of two conditions, but these are not limited in the present invention. A delay-time switching circuit in which devices to be operated in any of more than two conditions are used may be constructed by combination of a switch, a phototransistor in a photo coupler, etc. In that case, it becomes possible to make a finer adjustment in accordance with the switching frequency.

According to the present invention, the main switching element is prohibited from being turned on for a certain period of time by the delay circuit and an OFF-state period of the oscillation is extended, and also the delay time can be changed in two or more stages by using the switching circuit, and accordingly, when compared with the cases where such a delay circuit is not used, the switching frequency of the main switching element can be lowered, and the switching frequency of the main switching element can be set at a desired frequency by using the switching circuit. Therefore, the increase of the oscillation frequency at a light load and on standby is suppressed, the standby power is suppressed, the heat generation in the main switch is suppressed, or the output ripple voltage caused by the intermittent oscillation can be suppressed.

Furthermore, because the delay circuit including the switching device and the time constant circuit, provided between the feedback winding of the transformer and the control terminal of the main switching element, and the switching circuit for switching the time constant of the time constant circuit are provided, although an ON-state period of time of the switching device changes in accordance with the time constant of the time constant circuit, the time constant of the time constant circuit can be changed by the switching circuit and the efficiency can be greatly improved at light load.

Moreover, because the output power detector circuit in which the voltage of the feedback winding of the transformer is rectified by the rectifying element and is smoothed by the smoothing element and the delay-time extension circuit as the resistor element through which the output power detector circuit is connected to the control terminal of the delay circuit are provided, the turning on of the main switching element is further delayed and the switching frequency of the main switching element can be lowered, and, as a result, the oscillation frequency can be lowered at a light load and the efficiency of the RCC on standby and the output ripple voltage can be improved.

In addition, because the switching circuit can be switched by an external signal, the switching frequency of the main switching element can be easily externally changed.

Furthermore, because the current detector circuit for detecting the current flowing through the secondary winding is provided and further the switching circuit for switching the delay time of the delay circuit after a certain period of time when the current detector circuit detects the load current or the load power on standby is provided, the switching frequency of the main switching element can be automatically changed.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention should be limited not by the specific disclosure herein, but only by the appended claims.