Forward converter with synchronous rectification

This is a continuation under 35 U.S.C. §111(a) of PCT/JP2006/325153 filed Dec. 18, 2006, and claims priority of JP2006-032883 filed Feb. 9, 2006, incorporated by reference.

1. Technical Field

The present invention relates to forward converters with synchronous rectification for use in, for example, switching power supplies.

2. Background Art

A forward converter with synchronous rectification in related art is disclosed in Japanese Patent Application No. 3339452. The circuit configuration of the forward converter with synchronous rectification disclosed in Japanese Patent Application No. 3339452 is shown in FIG. 1 .

In the circuit shown in FIG. 1 , when a primary switching element 2 at the primary side 4 a of a transformer 4 is turned on, a rectifier-side synchronous rectifier element 5 at the secondary side is turned on by a voltage generated at a second winding 4 b of the transformer 4 while a commutator-side synchronous rectifier element 6 is turned off. If the turning-off of the commutator-side synchronous rectifier element 6 is delayed, a short-circuited path is formed through the two switching elements 5 and 6 . Accordingly, the forward converter with synchronous rectification is configured so as to connect a switching element 7 in series to a third winding 4 c of the transformer 4 and the forward converter with synchronous rectification is also configured such that the switching element 7 is turned on with a control signal supplied through a pulse transformer 11 at a time when the primary switching element 2 at the primary side is turned on.

With the above configuration, the charge of a parasitic capacitance of the commutator-side synchronous rectifier element 6 is discharged through the switching element 7 immediately before the primary switching element 2 at the primary side is turned on to immediately turn off the commutator-side synchronous rectifier element 6 , thus preventing short circuit.

In the configuration described above, the following problems become apparent particularly if the power of the forward converter with synchronous rectification (mainly, the current flowing through the forward converter with synchronous rectification) is high.

First Problem

The first problem is that the waveform of the control voltage for the commutator-side synchronous rectifier element 6 (the waveform of a voltage between the gate and source of the commutator-side synchronous rectifier element 6 ) is varied and can become a negative voltage.

Specifically, when the primary switching element 2 is turned off, a voltage is generated at the third winding 4 c to cause a current to flow toward the gate of the commutator-side synchronous rectifier element 6 , and the current flows through the parasitic diode of the switching element 7 , which is turned off. Then, the parasitic capacitance between the gate and source of the commutator-side synchronous rectifier element 6 is charged and the gate voltage of the commutator-side synchronous rectifier element 6 increases to turn on the commutator-side synchronous rectifier element 6 . As a result, the commutation current flows between the drain and source of the commutator-side synchronous rectifier element 6 , instead of through the parasitic diode of the commutator-side synchronous rectifier element 6 .

When the primary switching element 2 is turned on, a signal used for turning on the primary switching element 2 , output from a control circuit 12 , is also applied to the switching element 7 through the pulse transformer 11 to temporarily turn on the switching element 7 slightly before the primary switching element 2 is turned on. The parasitic capacitance between the gate and source of the commutator-side synchronous rectifier element 6 is discharged to turn off the commutator-side synchronous rectifier element 6 . However, if the switching element 7 is turned on only for a short time, the parasitic capacitance cannot be sufficiently discharged and the commutator-side synchronous rectifier element 6 may not be turned off completely. Accordingly, the switching element 7 is designed so as to be turned on for a slightly long time in consideration of various variations. However, in this case, the primary switching element 2 is turned on while the switching element 7 is turned on to generate a reverse voltage at the third winding 4 c and to positively discharge the charge of the parasitic capacitance of the commutator-side synchronous rectifier element 6 through the switching element 7 , which is turned on. As a result, the charge of the parasitic capacitance is not only discharged but also reversely charged to generate a negative gate voltage at the commutator-side synchronous rectifier element 6 .

In order to turn off the commutator-side synchronous rectifier element 6 , it is sufficient to discharge the charge of the parasitic capacitance and it is not necessary to reversely charge the parasitic capacitance. Accordingly, a current flowing along a path through the parasitic capacitance, the third winding 4 c , the switching element 7 , and the parasitic capacitance when the parasitic capacitance is reversely charged and a current generated when the reverse charge is discharged during the subsequent cycle are useless, and causes an excessive loss due to the resistance of the current path.

Particularly in a high power direct-current (DC) to DC converter (DC-DC converter), there are cases where multiple synchronous rectifier elements are connected in parallel to each other. In such a case, a number of current paths corresponding to the number of the parallel connections exist to increase the loss by an amount corresponding to the existing current paths and to cause a considerable loss in the entire circuit.

(2) Second Problem

When the parasitic capacitance of the commutator-side synchronous rectifier element 6 is reversely charged, the current flowing through the third winding 4 c causes the first winding 4 a to generate a current toward the primary switching element 2 . Although it seems that no problem is caused because the direction of this current is the same as that of the current flowing when the primary switching element 2 is turned on, the primary switching element 2 is not practically turned on completely (it is going to be turned on) at this stage and the primary switching element 2 has a relatively high resistance. The current is forced to be applied to the primary switching element 2 in this state, thus causing an increase in the loss.

(3) Third Problem

In the configuration shown in FIG. 1 , the voltage generated at the third winding 4 c is used to control the commutator-side synchronous rectifier element 6 . If the load is reduced to shorten the turn-on period of the primary switching element 2 , the voltage generated at the third winding 4 c is decreased and, therefore, it is not possible to turn on the commutator-side synchronous rectifier element 6 . Although an increase in the number of turns of the third winding 4 c supposedly resolves this phenomenon, the resistance of the third winding 4 c is increased by an amount corresponding to the increase in the number of turns of the third winding 4 c , thus increasing the loss.

According to various aspects of the present invention, the following embodiments of a forward converter with synchronous rectification are capable of reducing the losses caused by the foregoing problems.

According to several aspects of the invention, a forward converter with synchronous rectification according to the disclosed embodiments of the present invention may be configured as follows:

A forward converter with synchronous rectification includes a transformer (T 1 ) including a first coil and a second coil, a primary switching element (Q 1 ) connected in series to the first coil (N 11 ) of the transformer (T 1 ), a choke coil (L 2 ) connected in series to the second coil (N 12 ) of the transformer (T 1 ), a smoothing capacitor (C 1 ) connected in parallel between output terminals, a rectification switching element (Q 2 ) that is connected in series to the second coil (N 12 ) of the transformer (T 1 ) and that is turned on or off in synchronization with turning on or off of the primary switching element (Q 1 ), a commutation switching element (Q 3 ) that is turned off or on in synchronization with the turning on or off of the primary switching element (Q 1 ) and that serves as a path through which an excitation energy of the choke coil (L 2 ) is released when the commutation switching element (Q 3 ) is turned on, and a switching control circuit ( 23 ) which controls the primary switching element.

The forward converter with synchronous rectification may further include a commutation switch control voltage generating circuit ( 41 ) that generates a control voltage for the commutation switching element (Q 3 ); a commutation switch turning-on control switching element (Q 5 ) that is controlled with a voltage generated at an auxiliary coil (N 14 ) of the transformer (T 1 ) and that controls application of the control voltage to a control terminal of the commutation switching element (Q 3 ); a commutation switch turning-off control switching element (Q 4 ) that is connected to the control terminal of the commutation switching element (Q 3 ) and, when the commutation switch turning-off control switching element (Q 4 ) is turned on, controls a voltage at the control terminal of the commutation switching element (Q 3 ) to turn off the commutation switching element (Q 3 ); and a control switching element drive circuit ( 24 ) that transmits an on-timing signal for the primary switching element (Q 1 ) from the first side of the transformer to the second side thereof in an insulated state and that turns on the commutation switch turning-off control switching element (Q 4 ) in response to the on-timing signal for the primary switching element, transmitted to the second side.

A driving voltage adjustment capacitor (C 5 ) may be connected in series between the auxiliary coil (N 14 ) of the transformer and the commutation switch turning-on control switching element (Q 5 ).

A differentiation circuit including a capacitor (C 5 ) and a resistor (R 5 ) may be connected to the auxiliary coil (N 14 ) of the transformer. An output from the differentiation circuit may be connected to a control terminal of the commutation switch turning-on control switching element (Q 5 ).

A charging circuit including the capacitor (C 5 ) and a diode (D 5 ) may be connected to the auxiliary coil (N 14 ) of the transformer. A diode (D 6 ) for preventing application of a reverse voltage to the control terminal of the commutation switch turning-on control switching element (Q 5 ) may be connected in series to the capacitor (C 5 ).

A rectification switching element turning-on control switching element (Q 7 ) may be connected between an output of the commutation switch control voltage generating circuit ( 41 ) and a control terminal of the commutation switching element (Q 2 ). A switching element Q 6 for applying a turn-on signal to a control terminal of the rectification switching element turning-on control switching element (Q 7 ) when the primary switching element (Q 1 ) is turned on may be connected to the control terminal of the rectification switching element turning-on control switching element (Q 7 ). A diode (D 14 ) for discharging the charge of a parasitic capacitance generated at the commutation switch turning-on control switching element (Q 5 ) may be connected between the control terminal of the commutation switch turning-on control switching element (Q 5 ) and the control terminal of the rectification switching element turning-on control switching element (Q 7 ).

According to these embodiments of the present invention, at least the following advantages are presented:

The commutation switch control voltage generating circuit ( 41 ) is separately provided as a power supply for driving the commutation switching element (Q 3 ), the commutation switch turning-on control switching element (Q 5 ) is provided between the power supply and the commutation switching element (Q 3 ), and the commutation switch turning-on control switching element (Q 5 ) is turned on with a voltage generated at the auxiliary coil (N 14 ) in order to turn on the commutation switching element (Q 3 ) with a voltage supplied from the commutation switch control voltage generating circuit ( 41 ). Accordingly, since the fourth coil does not exist on the path through which the charge of the commutation switching element (Q 3 ) is discharged and the reverse charging is not performed, the problems of the circuit disclosed in Japanese Patent Application No. 3339452 are not caused.

Consequently, the gate of the commutation switching element (Q 3 ) does not have a negative value, thus preventing any loss due to the negative voltage.

In addition, the commutation switch turning-on control switching element (Q 5 ) can be made considerably smaller than the commutation switching element (Q 3 ) and the capacitance between the gate and source of the commutation switch turning-on control switching element (Q 5 ) can be decreased. Accordingly, a reverse current flowing through the auxiliary coil (N 14 ) is decreased to reduce any loss at the primary switching element (Q 1 ) due to the current flowing toward the primary side.

Since a separate power supply is used as the driving power supply of the commutation switching element (Q 3 ), it is sufficient to minimally provide the auxiliary coil (N 14 ). Accordingly, a problem in that the commutation switching element (Q 3 ) cannot be driven with a lighter load is not caused.

Connecting the driving voltage adjustment capacitor (C 5 ) in series between the auxiliary coil (N 14 ) of the transformer and the commutation switch turning-on control switching element (Q 5 ) allows the control voltage of the commutation switch turning-on control switching element (Q 5 ) to be correctly adjusted, thus preventing application of an excessive voltage to the gate of the commutation switch turning-on control switching element (Q 5 ).

Connecting the differentiation circuit including the capacitor (C 5 ) and the resistor (R 5 ) to the auxiliary coil (N 14 ) of the transformer and connecting the output of the differentiation circuit to the control terminal of the commutation switch turning-on control switching element (Q 5 ) cause the output voltage from the auxiliary coil (N 14 ) to be differentiated. Accordingly, it is possible to advance the turning on of the commutation switch turning-on control switching element (Q 5 ) and the commutation switching element (Q 3 ) and to shorten the period during which the commutation current flows through the parasitic diode of the commutation switching element (Q 3 ), thus reducing any loss due to the rectification circuit.

Connecting the charging circuit including the capacitor (C 5 ) and the diode (D 5 ) to the auxiliary coil (N 14 ) of the transformer, and connecting the diode (D 6 ) for preventing application of a reverse voltage to the control terminal of the commutation switch turning-on control switching element (Q 5 ) in series to the capacitor (C 5 ), eliminate the charging voltage (the voltage of the cathode of the D 6 ) to the commutation switch turning-on control switching element (Q 5 ) when the commutation switch turning-off control switching element (Q 4 ) is turned on. Accordingly, it is possible to shorten a time when the commutation switch turning-off control switching element (Q 4 ) is turned on simultaneously with the turning on of the commutation switch turning-on control switching element (Q 5 ), thus reducing any loss caused by the driving of the commutation switching element (Q 3 ).

The rectification switching element turning-on control switching element (Q 7 ) is connected between the output of the commutation switch control voltage generating circuit ( 41 ) and the control terminal of the rectification switching element (Q 2 ); the switching element (Q 6 ) for applying an turn-on signal to the control terminal of the rectification switching element turning-on control switching element (Q 7 ) when the primary switching element (Q 1 ) is turned on is connected to the control terminal of the rectification switching element turning-on control switching element (Q 7 ); and the diode (D 14 ) for discharging the charge at the control terminal of the commutation switch turning-on control switching element (Q 5 ) is connected between the control terminal of the commutation switch turning-on control switching element (Q 5 ) and the control terminal of the rectification switching element turning-on control switching element (Q 7 ). Consequently, since the source of the switching element (Q 6 ) is grounded when the switching element (Q 6 ) is turned on at a turn-on timing of the primary switching element (Q 1 ), the capacitance between the gate and source of the commutation switch turning-on control switching element (Q 5 ) is discharged through the diode (D 14 ) to turn off the Q 5 . At the same time, the turning on of the switching element (Q 6 ) turns on the rectification switching element turning-on control switching element (Q 7 ) and also turns on the rectification switching element (Q 2 ) and the commutation switch turning-off control switching element (Q 4 ). The charge of the control terminal (the parasitic capacitance) of the Q 5 has been discharged through the diode (D 14 ) before the commutation switch turning-off control switching element (Q 4 ) is turned on to turn off the commutation switch turning-on control switching element (Q 5 ), so that the problem of the simultaneous turning-on of the Q 4 and the Q 5 is resolved.

Other features and advantages of the present invention will become apparent from the following description of embodiments of invention which refers to the accompanying drawings.

FIG. 1 is a schematic circuit diagram showing the configuration of a converter disclosed in Japanese Patent Application No. 3339452.

FIG. 2 is a schematic circuit diagram of a forward converter with synchronous rectification according to a first embodiment of the present invention.

FIG. 3 is a schematic circuit diagram of a forward converter with synchronous rectification according to a second embodiment of the present invention.

FIG. 4 includes diagrams showing waveforms of the main parts of the converter shown in FIG. 3 .

FIG. 5 is a schematic circuit diagram of a forward converter with synchronous rectification according to a third embodiment of the present invention.

FIG. 6 includes diagrams showing waveforms of the main parts of the converter shown in FIG. 5 .

FIG. 7 is a schematic circuit diagram of a forward converter with synchronous rectification according to a fourth embodiment of the present invention.

FIG. 8 includes diagrams showing waveforms of the main parts of the converter shown in FIG. 6 .

FIG. 9 is a schematic circuit diagram of a forward converter with synchronous rectification according to a fifth embodiment of the present invention.

FIG. 10 is a schematic circuit diagram of a forward converter with synchronous rectification according to a sixth embodiment of the present invention.

FIG. 11 includes diagrams showing waveforms of the main parts of the converter shown in FIG. 10 .

FIG. 12 is a schematic circuit diagram of a forward converter with synchronous rectification according to a seventh embodiment of the present invention.

Reference Numerals

21 input terminals

22 third rectification smoothing circuit

23 switching control circuit

24 control switching element drive circuit

32 output terminals

41 commutation switch control voltage generating circuit

C 1 smoothing capacitor

C 2 capacitor

C 3 capacitor

D 1 , D 2 diodes

L 1 choke coil

L 2 choke coil

N 11 first coil

N 12 second coil

N 13 third coil

N 14 fourth coil

Q 1 primary switching element

Q 2 rectification switching element

Q 3 commutation switching element

Q 4 commutation switch turning-off control switching element

Q 5 commutation switch turning-on control switching element

Q 6 , Q 7 control switching elements

Q 8 rectification switch control switching element

T 1 main transformer

N 11 first coil

N 12 second coil

N 13 third coil

N 14 fourth coil

L 1 , L 2 choke coils

C 1 smoothing capacitor

DV driver circuit

First Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a first embodiment of the present invention will now be described with reference to FIG. 2 .

FIG. 2 is a schematic circuit diagram of the forward converter with synchronous rectification, part of which is represented by blocks or symbols. As shown in FIG. 2 , a main transformer T 1 includes a first coil N 11 , a second coil N 12 , a third coil N 13 , and a fourth coil N 14 (an auxiliary coil according to the claims). A primary switching element Q 1 is connected in series to the first coil N 11 , and a capacitor C 2 is connected between the input terminals 21 ( 21 a and 21 b ). A choke coil L 2 and a rectification switching element Q 2 are connected in series to the second coil N 12 of the main transformer T 1 , and a smoothing capacitor C 1 is connected between the output terminals 32 ( 32 a and 32 b ). The second coil N 12 , the choke coil L 2 , and the smoothing capacitor C 1 form a loop. A commutation switching element Q 3 is provided at a position on a commutation path through which an excitation energy of the choke coil L 2 is released.

A third rectification smoothing circuit 22 including diodes D 1 and D 2 , a choke coil L 1 , and a capacitor C 3 is connected to the third coil N 13 of the main transformer T 1 . A switching control circuit 23 receives an output from the third rectification smoothing circuit 22 as an indirect power and output-voltage detection signal and outputs a switching control signal to the primary switching element Q 1 .

The circuit is configured so as to apply an electromotive voltage of the second coil N 12 of the main transformer T 1 to the control terminal of the rectification switching element Q 2 .

First, when the primary switching element Q 1 is turned off, the capacitance between the gate and source of a commutation switch turning-on control switching element Q 5 is charged to a voltage exceeding a threshold voltage of the Q 5 by a voltage generated at the fourth coil N 14 , to turn on the Q 5 . The capacitance between the gate and source of the commutation switching element Q 3 is charged by a current flowing from a commutation switch control voltage generating circuit 41 through the Q 5 to turn on the Q 3 . At this time, a commutation switch turning-off control switching element Q 4 is turned off. Accordingly, the commutation switching element Q 3 is kept in the turn-on state.

Next, when the primary switching element Q 1 is turned on under the control of the switching control circuit 23 , the rectification switching element Q 2 is turned on by a voltage generated at the second coil N 12 of the main transformer T 1 while the commutation switching element Q 3 is turned off.

A control switching element drive circuit 24 receives the switching control signal for the primary switching element Q 1 (an on-timing signal for the Q 1 ), supplied from the switching control circuit 23 , to drive the commutation switch turning-off control switching element Q 4 in synchronization with the reception of the switching control signal. The turning-on of the commutation switch turning-off control switching element Q 4 discharges the charge of the capacitance between the gate and source of the commutation switching element Q 3 to force the Q 3 to be turned off.

The provision of the commutation switch control voltage generating circuit 41 and the commutation switch turning-on control switching element Q 5 separates the gate of the commutation switching element Q 3 from the fourth coil N 14 . Accordingly, the gate voltage of the Q 3 does not become a negative value, so that it is possible to reduce any loss caused by the driving of the Q 3 .

In addition, since the voltage is supplied from the commutation switch control voltage generating circuit 41 , which is separated from the fourth coil N 14 of the main transformer T 1 , it is possible to keep the control voltage of the commutation switching element Q 3 constant to achieve stable control.

Second Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a second embodiment of the present invention will now be described with reference to FIGS. 3 and 4 .

FIG. 3 is a schematic circuit diagram of the forward converter with synchronous rectification according to the second embodiment of the present invention.

As shown in FIG. 3 , a switching control circuit 23 is provided with a switching control integrated circuit (IC) 230 that operates by using an output from a third rectification smoothing circuit 22 as a power and that receives a divided voltage resulting from division of the output by resistors R 2 and R 3 . The switching control IC 230 supplies a switching control signal to the gate of a primary switching element Q 1 through a first coil N 21 of a pulse transformer T 2 . The switching control circuit 23 performs pulse width modulation (PWM) control to the primary switching element Q 1 such that the divided voltage is equal to a reference voltage on the basis of the received divided voltage and the reference voltage.

A diode D 3 for resetting the excitation of the pulse transformer T 2 is connected to the first coil N 21 of the pulse transformer T 2 . The connection of the diode D 3 in this direction causes a pulse of an on-timing signal to be output through a second coil N 22 only when a signal used for turning on the primary switching element Q 1 is output from the switching control circuit 23 . The pulse transformer T 2 and the diode D 3 form a control switching element drive circuit 24 .

One end of a second coil N 12 is connected to the gate of a rectification switching element Q 2 .

A capacitor C 5 is connected between one end of a fourth coil N 14 of a main transformer T 1 and the gate of a commutation switch turning-on control switching element Q 5 .

A capacitor C 4 is connected to a second coil of a choke coil L 2 at the secondary side through a diode D 4 . A series circuit including the commutation switch turning-on control switching element Q 5 and a resistor R 4 is connected to both ends of the capacitor C 4 . The circuit is configured such that the voltage between both ends of the resistor R 4 is applied between the gate and source of a commutation switching element Q 3 .

A commutation switch turning-off control switching element Q 4 is connected between the gate and source of the commutation switching element Q 3 .

A commutation switch control voltage generating circuit 41 rectifies the voltage of the second coil of the choke coil L 2 with the diode D 4 , and the capacitor C 4 is charged with this voltage to generate a voltage used for applying a control voltage to the commutation switching element Q 3 . The commutation switch control voltage generating circuit 41 is not limited to the configuration described above and may have any configuration.

The capacitor C 5 is used to divide a voltage generated at the fourth coil N 14 between the capacitor C 5 and the capacitance between the gate and source of the commutation switch turning-on control switching element Q 5 and is not essential. The remaining configuration of the circuit in FIG. 3 is basically the same as that of the circuit according to the first embodiment of the present invention.

FIG. 4 includes diagrams showing waveforms of the main parts of the forward converter with synchronous rectification shown in FIG. 3 .

A graph (a) in FIG. 4 shows the waveform of a voltage between both ends of the fourth coil N 14 of the main transformer T 1 . A graph (b) in FIG. 4 shows the waveform of a voltage between the gate and source of the commutation switching element Q 3 . A graph (c) in FIG. 4 shows the waveform of a voltage between the gate and source of the rectification switching element Q 2 .

The states of periods A to E in FIG. 4 are as follows:

Period A: the Q 1 is turned on and the transformer is turned on.

Period B: the choke coil L 2 is inverted, the Q 1 is turned off, and the transformer is turned on.

Period C: the parasitic diode of the Q 3 is conducted.

Period D: the Q 1 is turned off and the Q 3 is turned on.

Period E: the Q 4 is turned on.

As described above, the turn-off of the primary switching element Q 1 under the control of the switching control circuit 23 inverts the electromotive voltage of the second coil N 12 to invert the voltage at the control terminal of the rectification switching element Q 2 , thereby turning off the Q 2 . The control switching element drive circuit 24 turns on the commutation switch turning-off control switching element Q 4 in synchronization with the turning-on of the primary switching element Q 1 . Accordingly, a commutation current flows through a path of L 2 →Q 3 →C 1 →L 2 .

The turning on and off of the primary switching element Q 1 repeats the rectification and the commutation described above.

Direct driving of the commutation switching element Q 3 with the fourth coil N 14 of the main transformer T 1 will cause the gate voltage of the commutation switching element Q 3 to be a negative value. However, since the Q 3 is indirectly driven by the commutation switch turning-on control switching element Q 5 according to the second embodiment of the present invention, as shown in FIG. 4 , the gate of the Q 3 does not have a negative voltage, thus reducing the driving power of the Q 3 .

If the commutation switching element Q 3 is directly driven with the N 14 , the driving terminal of the Q 3 will be charged to a negative voltage to apply a current to the first coil N 11 due to a current flowing through the N 14 , thereby increasing the switching loss when the primary switching element Q 1 is turned on. However, the switching loss of the Q 1 can also be reduced in the configuration according to the second embodiment of the present invention.

In addition, since the driving voltage of the commutation switching element Q 3 does not exceed the charging voltage of the capacitor C 4 , it is possible to prevent an excessive voltage from being applied to the gate of the Q 3 . As a result, an element having a lower gate breakdown voltage can be adopted as the commutation switching element Q 3 .

Third Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a third embodiment of the present invention will now be described with reference to FIGS. 5 and 6 .

FIG. 5 is a schematic circuit diagram of the forward converter with synchronous rectification according to the third embodiment of the present invention. The circuit in FIG. 5 differs from the circuit shown in FIG. 3 in that a differentiation circuit including a capacitor C 5 and a resistor R 5 is connected to a fourth coil N 14 of a main transformer T 1 and that an output from the differentiation circuit is connected to the gate of a commutation switch turning-on control switching element Q 5 . The remaining configuration of the circuit in FIG. 5 is similar to the one shown in FIG. 3 .

Driving the commutation switch turning-on control switching element Q 5 with a signal resulting from the differentiation of a voltage of the fourth coil N 14 can advance the on-timings of the Q 5 and a commutation switching element Q 3 and can shorten a period during which a current flows through the parasitic diode of the commutation switching element Q 3 (particularly, a period during which a strong commutation current begins to flow). As a result, it is possible to reduce any loss caused by the rectification circuit.

FIG. 6 includes diagrams showing waveforms of the main parts of the forward converter with synchronous rectification shown in FIG. 5 .

A graph (a) in FIG. 6 shows the waveform of a voltage between both ends of the fourth coil N 14 of the main transformer T 1 . A graph (b) in FIG. 6 shows the waveform of a voltage between the gate of the Q 5 and the source of the Q 3 . A graph (c) in FIG. 6 shows the waveform of a voltage between the gate and source of the Q 3 . A graph (d) in FIG. 6 shows the waveform of a voltage between the gate and source of a Q 2 . The states of periods A to E in FIG. 6 is the same as those of the periods A to E shown in FIG. 4 according to the second embodiment of the present invention.

The waveform of the voltage between the gate of the Q 5 and the source of the Q 3 , shown in (b) in FIG. 6 , is given by adding the differentiated waveform of the voltage between both ends of the fourth coil N 14 to the waveform of the voltage between the gate and source of the Q 3 . According to the first embodiment, as shown in FIG. 4 , the period C is relatively long during which a current flows through the parasitic diode of the Q 3 until the voltage between the gate and source of the commutation switching element Q 3 reaches a threshold value of the gate voltage of the Q 3 . In contrast, according to the third embodiment, the on-period of the parasitic diode of the Q 3 can be shortened because the voltage between the gate and source of the Q 3 sharply rises as in the waveform example in FIG. 6 . Accordingly, it is possible to further reduce any loss.

Fourth Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a fourth embodiment of the present invention will now be described with reference to FIGS. 7 and 8 .

FIG. 7 is a schematic circuit diagram of the forward converter with synchronous rectification according to the fourth embodiment of the present invention. The circuit in FIG. 7 differs from the circuit shown in FIG. 5 in that a charging circuit including a capacitor C 5 and a diode D 5 is connected to a fourth coil N 14 of a main transformer T 1 and that a diode D 6 for preventing a reverse-voltage from being applied to the gate of a commutation switch turning-on control switching element Q 5 is connected in series to the capacitor C 5 . The remaining configuration of the circuit in FIG. 7 is similar to the one shown in FIG. 5 .

FIG. 8(A) shows current paths when a primary switching element Q 1 is turned on and off. Referring to FIG. 8(A) , outline arrows represent the directions of a current flowing when the primary switching element Q 1 is turned on. The current is used to charge the capacitor C 5 . Hatched arrows represent a current path when the primary switching element Q 1 is turned off. The current is used to apply a voltage given by differentiation of a charging voltage of the C 5 and an output voltage of the fourth coil N 14 to the gate of the commutation switch turning-on control switching element Q 5 .

As described above, the capacitor C 5 is charged with the current through the diode D 5 when the primary switching element Q 1 is turned on, and the charged voltage is added to the voltage generated at the fourth coil N 14 of the main transformer T 1 when the Q 1 is turned off. Furthermore, the voltage given by the differentiation of the added voltage is applied to the gate of the commutation switch turning-on control switching element Q 5 . Accordingly, it is possible to advance the on-timing of a commutation switching element Q 3 .

A resistor R 5 is used to discharge the charge of the capacitance between the gate and source of the commutation switch turning-on control switching element Q 5 . When a commutation switch turning-off control switching element Q 4 is turned on, the gate of the commutation switch turning-on control switching element Q 5 is grounded through the resistor R 5 . After the Q 4 is turned on, the charge of the capacitance between the gate and source of the Q 5 is immediately discharged to turn off the Q 5 . Accordingly, the Q 4 is not turned on simultaneously with the turning-on of the Q 5 .

A current loop through a capacitor C 4 , the Q 5 , the Q 4 , and the C 4 is supposed in the circuit shown in FIG. 7 . If the Q 4 is turned on simultaneously with the turning-on of the Q 5 , a short-circuit current for discharging the capacitance of the capacitor C 4 flows to cause problems of any loss and heating of the Q 4 and Q 5 due to the short-circuit current. However, since the Q 5 is not turned on simultaneously with the turning-on of the Q 5 , as described above, such problems are not caused in circuit according to the fourth embodiment of the present invention.

FIG. 8(B) includes diagrams showing waveforms of the main parts of the forward converter with synchronous rectification shown in FIG. 7 .

A graph (a) in FIG. 8(B) shows the waveform of a voltage between both ends of the fourth coil N 14 of the main transformer T 1 . A graph (b) in FIG. 8(B) shows the waveform of a voltage between the anode of the diode D 6 and the source of the Q 3 . A graph (c) in FIG. 8(B) shows the waveform of a voltage between the gate of the Q 5 and the source of the Q 3 . A graph (d) in FIG. 8(B) shows the waveform of a voltage between the gate and source of the Q 3 . A graph (e) in FIG. 8(B) shows the waveform of a voltage between the gate and source of a Q 2 .

The waveform of the voltage between the anode of the diode D 6 and the source of the Q 3 , shown in (b) in FIG. 8(B) , is given by differentiation of the voltage between both ends of the N 14 . The waveform of the voltage between the gate of the Q 5 and the source of the Q 3 , shown in (c) in FIG. 8(B) , is given by peak charging of the differentiated waveform. The states of periods A to E in FIG. 8(B) is the same as those of the periods A to E shown in FIG. 4 according to the second embodiment of the present invention.

Fifth Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a fifth embodiment of the present invention will now be described with reference to FIG. 9 .

A field effect transistor (FET) is used as the commutation switch turning-on control switching element Q 5 in the example shown in FIG. 7 . In contrast, an NPN transistor is used as a switching element Q 5 in the example shown in FIG. 9 . In association with the use of the NPN transistor, two resistors R 5 and R 6 are connected in series between the cathode of the diode D 6 and the emitter of the commutation switching element Q 5 , and the base of the commutation switching element Q 5 is connected between the resistor R 5 and the resistor R 6 . The remaining configuration of the circuit in FIG. 9 is similar to the one shown in FIG. 7 .

The entire operation of the forward converter with synchronous rectification according to the fifth embodiment of the present invention is similar to that of the forward converter with synchronous rectification according to the fourth embodiment of the present invention. An effect similar to that of the forward converter with synchronous rectification according to the fourth embodiment is achieved in the forward converter with synchronous rectification according to the fifth embodiment of the present invention.

Sixth Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a sixth embodiment of the present invention will now be described with reference to FIGS. 10 and 11 .

FIG. 10 is a schematic circuit diagram of the forward converter with synchronous rectification according to the sixth embodiment of the present invention. In the forward converter with synchronous rectification shown in FIG. 10 , a single pulse transformer T 2 is used to transmit an on-trigger pulse and an off-trigger pulse for a primary switching element Q 1 to the secondary side. A diode bridge including diodes D 9 and D 10 is provided in a second coil N 22 of the pulse transformer T 2 . Diodes D 7 and D 8 at the primary side reset the excitation when the pulse transformer T 2 is turned on and off. According to the sixth embodiment of the present invention, turning on of Q 6 and Q 7 in response to an on-timing signal of the Q 1 , output from the pulse transformer T 2 , applies an output voltage of a commutation switch control voltage generating circuit 41 to the gate of a Q 4 to turn on the Q 4 , unlike the first to fifth embodiments of the present invention.

A connection point between one end of a second coil N 22 of the pulse transformer T 2 and the cathode of the diode D 9 is connected to the gate of a rectification switch control switching element Q 8 through a diode D 11 for preventing counter flow. A resistor R 8 is connected between the gate of the Q 8 and the ground. A resistor R 7 is connected between the other end of the second coil N 22 of the pulse transformer T 2 and the ground. The two resistors R 7 and R 8 are used to adjust a voltage generated at the second coil N 22 of the pulse transformer T 2 when the off-trigger pulse for the primary switching element Q 1 is generated.

A connection point between one end of the second coil N 22 of the pulse transformer T 2 and the cathode of the diode D 10 is connected to the gate of the control switching element Q 6 through a diode D 13 for preventing counter flow. A resistor R 10 is connected between the gate of the Q 6 and the ground. A resistor R 6 is connected between the other end of the second coil N 22 of the pulse transformer T 2 and the ground. The two resistors R 6 and R 10 are used to adjust a voltage generated at the second coil N 22 of the pulse transformer T 2 when the on-trigger pulse for the primary switching element Q 1 is generated.

A diode D 12 is connected between the drain of the control switching element Q 6 and the gate of the rectification switch control switching element Q 8 . Since the presence of the diode D 12 causes the Q 8 to be turned off when the Q 6 is turned on (when the Q 7 is turned on), it is possible to prevent the Q 7 from being turned on simultaneously with the turning on of the Q 8 to prevent an increase in loss at the Q 8 .

The remaining configuration of the circuit in FIG. 10 is similar to the one shown in FIG. 7 .

FIG. 11 includes diagrams showing waveforms of the main parts of the forward converter with synchronous rectification shown in FIG. 10 . A graph (a) in FIG. 11 shows the waveform of a gate voltage of the primary switching element Q 1 . A graph (b) in FIG. 11 shows the waveform of a voltage between both ends of a first coil N 11 of a main transformer T 1 . A graph (c) in FIG. 11 shows an off-timing signal in the voltage between both ends of the second coil N 22 of the pulse transformer T 2 . A graph (d) in FIG. 11 shows an on-timing signal (an on-timing signal of the Q 6 ) in the voltage between both ends of the second coil N 22 of the pulse transformer T 2 . A graph (e) in FIG. 11 shows the waveform of a gate voltage of a Q 3 . A graph (f) in FIG. 11 shows the waveform of a gate voltage of a Q 2 .

As shown by a time Tb in FIG. 11 , the commutation switching element Q 3 is turned off in advance of the turning-on of the primary switching element Q 1 and a time when the voltage of the main transformer T 1 is inverted. As shown by a time Td in FIG. 11 , the rectification switching element Q 2 is activated before the Q 1 rises. In addition, since the primary switching element Q 1 is driven through a driver circuit DV as shown in FIG. 10 , the Q 1 is turned off in advance of the turn-off of the rectification switching element Q 2 , as shown by a time Tc, thus reducing the switching loss at the Q 1 . Since the rectification switching element Q 2 is turned on in advance of the turning-on of the primary switching element Q 1 , it is possible to reduce any loss caused by a current flowing through the parasitic diode of the Q 4 due to a delay of the turning-on of the Q 2 when the voltage between both ends of the main transformer T 1 is generated.

Furthermore, since the commutation switch turning-off control switching element Q 4 is turned on with a turn-on voltage of the rectification switching element Q 2 , a simultaneous turning-on of the Q 2 and the commutation switching element Q 3 does not occur simultaneously with the generation of the voltage between both ends of the main transformer T 1 , thus preventing the main transformer T 1 from being short-circuited. If the gate voltage of the rectification switching element Q 2 is applied through the coil (the second coil N 12 ) of the main transformer T 1 , the above effect of preventing the short-circuit is not achieved because the generation of the voltage at the main transformer T 1 occurs simultaneously with the simultaneous turning-on of the Q 2 and the Q 3 even if the gate of the commutation switch turning-off control switching element Q 4 is connected to the gate of the Q 2 .

Since the gate of a commutation switch turning-on control switching element Q 5 is connected to the drain of the control switching element Q 6 through a diode D 14 , the Q 6 is turned on with a signal transmitted through the diode D 13 when a signal used for turning on the primary switching element Q 1 is transmitted to the secondary side through the pulse transformer T 2 . At this time, the source of the control switching element Q 6 is grounded to discharge the capacitance between the gate and source of the commutation switch turning-on control switching element Q 5 through the diode D 14 , thereby turning off the Q 5 . At the same time, the turning on of the control switching element Q 6 turns on the control switching element Q 7 and also turns on the rectification switching element Q 2 and the commutation switch turning-off control switching element Q 4 . Accordingly, the commutation switch turning-on control switching element Q 5 has been turned off before the commutation switch turning-off control switching element Q 4 is turned on, as shown by a time Ta, so that the problem of the simultaneous turning-on of the Q 4 and the Q 5 is not caused.

Seventh Embodiment

An example of the configuration of a forward converter with synchronous rectification according to a seventh embodiment of the present invention will now be described with reference to FIG. 12 .

In the examples according to the second to sixth embodiments of the present invention, the commutation switch control voltage generating circuit 41 is configured so as to rectify and smooth the voltage at the second coil of the choke coil L 2 at the secondary side to yield the control voltage for the commutation switching element Q 3 . In contrast, according to the seventh embodiment of the present invention, a commutation switch control voltage generating circuit 41 having a different configuration is provided in the forward converter with synchronous rectification.

Referring to FIG. 12 , a Q 3 charging circuit CH corresponds to the commutation switch turning-on control switching element Q 5 connected to the gate of the Q 3 according to the second to sixth embodiments of the present invention.

The commutation switch control voltage generating circuit 41 includes a diode D 4 and a capacitor C 4 . A voltage (commutation switching control voltage) to be supplied to the Q 3 charging circuit CH can be generated by applying a voltage to the commutation switch control voltage generating circuit 41 from any of points a, b, c, and d in FIG. 12 .

A voltage charged in the capacitor C 4 in the commutation switch control voltage generating circuit 41 may be increased by a voltage regulator.

A diode rectifier circuit may be provided at the output of a coil (for example, a second coil N 12 ) of a main transformer T 1 or another coil to configure a power supply circuit.

A voltage from an output terminal 32 may be applied to configure another power supply circuit.

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 is not limited by the specific disclosure herein.