Converter with integrated active clamp circuit and bias circuit

1. Field of Invention

The present invention relates generally to converters and, more particularly, to converters having integrated active clamp and primary bias circuits.

2. Description of the Prior Art

A DC-DC converter, such as an SMPS (Switch Mode Power Supply) converter, is a device that converts a DC input voltage to a different DC output voltage.

SMPS converters are very versatile. They can be used, for example, to:

1. step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS, 2. step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost converter or Step-Up SMPS, 3. step up or step down an unregulated dc input voltage to produce a regulated dc output voltage using a Buck-Boost Converter.

Converters are also characterized as either flyback converters, forward converters or combinations thereof.

A flyback converter generally has a Buck-Boost SMPS topology in which, during the first half of the switching period, energy is stored in the transformer and during the second half of the period this energy is transferred to the transformer secondary and into the load.

A forward converter generally has a Buck-derived SMPS topology in which energy is transferred to the transformer secondary winding and the load when the switching transistor is on.

Whether a converter is a flyback converter, a forward converter or a combined forward-flyback converter, it is necessary to provide a primary bias circuit for the controller which controls the switching cycle and also to provide a primary clamp circuit.

In the case of forward converters, the purpose of the clamp circuit is to reset the core flux of the transformer. More specifically, in a forward converter only positive voltage is supplied across the transformer core during the conduction of the primary switch. As a result, the flux continues to increase with the application of the input voltage. The flux increases until the core saturates and circuit failure occurs. Accordingly, it is necessary to supply negative voltage to reset the core flux. This is generally effected with a clamp circuit.

In the case of flyback transformers used in flyback converters, although they do not require flux reset circuitry, they do require clamp circuitry to clamp any voltage spike on the primary power switch.

There are several types of clamp circuits employed which generally fall into two categories: active and passive. Active clamp circuits are generally preferred. This is particularly so in the case of flyback converters when a high voltage is used. This is because in this case there is considerable insulation material between the primary and the secondary of the transformer which leads to large leakage inductance. This, in turn, results in large voltage spikes on the primary which require an active clamp circuit for effective clamping.

The present invention is directed to a converter having an integrated active clamp circuit and primary bias circuit.

According to the invention, the converter may include a transformer having a primary winding, a secondary winding and an auxiliary winding, a first switching device for connecting an input voltage to the primary winding of the transformer, an output circuit connected to the secondary winding, a controller, and an integrated active clamp and primary bias. The integrated active clamp and primary bias circuit includes the auxiliary winding and a second switch for providing a bias voltage to the controller to alternately operate the first and second switches between conductive and non-conductive states such that when the first switch is conductive the second switch is nonconducting and power is transferred from the primary winding to the output circuit via the secondary winding and from the auxiliary winding to the controller and, when the first switch is nonconducting, the switch is conducting to provide voltage clamping of the primary winding via the second switch and the auxiliary winding.

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

FIG. 1 illustrates a circuit diagram of an embodiment of a conventional forward converter with a separate active clamp circuit and a separate primary bias circuit;

FIG. 2 illustrates the switching waveforms occurring in the converter illustrated in FIG. 1 ;

FIG. 3 illustrates a circuit diagram of an embodiment according to the present invention showing a forward converter having an integrated active clamp circuit and a primary bias circuit;

FIG. 4 illustrates the switching waveforms occurring in the converter illustrated in FIG. 3 ;

FIG. 5 illustrates an application o the novel combination of an active clamp circuit with a primary bias circuit of the present invention applied to flyback converter.

FIG. 6 illustrates the switching waveforms occurring in the converter illustrated in FIG. 5 ;

FIG. 7 illustrates an application of the novel combination of an active clamp circuit with a primary bias circuit of the present invention applied to forward-flyback converter; and

FIG. 8 illustrates the switching waveforms occurring in the converter illustrated in FIG. 7 .

By way of example and not of limitation, the present invention will be described in connection with its applicability to a forward converter, a flyback converter and a forward-flyback converter. Components of the various embodiments having the same or similar functions have the same reference designations.

Forward Converter

FIG. 1 shows the circuit diagram of a conventional forward converter with an active clamp circuit. As shown, a DC voltage at the input is connected to the primary winding Np of a transformer T 1 via a main power switch S 1 . The conventional active clamp circuit is formed by an auxiliary switch S 2 and a capacitor C 1 in parallel with an auxiliary winding Nb of the transformer T 1 . The conventional forward converter also includes a separate primary bias circuit comprising an auxiliary winding Na of the transformer T 1 connected to a rectifier diode D 1 , a freewheeling diode D 2 , and an LC filter comprising an inductor L 1 and a capacitor C 2 . The secondary winding Ns of the transformer T 1 is connected to an output load through an output filter comprising inductor Lo, capacitor Co and a synchronous rectifier including switches S 3 and S 4 .

In a forward converter as shown in FIG. 1 , the switching waveforms of which are shown in FIG. 2 , when power switch S 1 is conducting, auxiliary switch S 2 is off and energy is transferred via the secondary winding Ns of the transformer T 1 to the output load and is transferred via the auxiliary winding Na to the primary bias circuit. During this period, diode D 1 conducts to transfer energy to the control circuit 10 and store energy in filter inductor L 1 . During the off period of S 1 , auxiliary switch S 2 is turned on to reset transformer T 1 , while the control circuit 10 is biased with the energy stored in inductor L 1 through D 2 . Accordingly, reset of transformer T 1 is effected by the separate active clamp circuit comprising auxiliary switch S 2 , capacitor C 1 and auxiliary winding Nb. As a result, an average bias voltage for the control circuit 10 is formed separately in the conventional forward converter of FIG. 1 .

FIG. 3 is a circuit diagram of a forward converter according to the invention incorporating an integrated active clamp circuit and primary bias circuit. In this converter, the active clamp circuit comprising switch S 2 and capacitor C 1 is now connected in parallel with auxiliary winding Na that also transfers energy for the primary bias circuit.

In operation, the main switch S 1 and the active clamp switch S 2 are turned on and off alternately by the control circuit. During [t 0 , t 1 ] ( FIG. 4 ), main power switch S 1 is turned on and winding Na is positively biased, so diode D 1 conducts to allow energy to be transferred to the control circuit and stored in filter inductor L 1 at the same time. The active clamp switch S 2 is off during this period with a voltage as show in FIG. 4 . At time t 1 , S 1 is turned off. The voltage on the primary winding Np of transformer T 1 reverses as do the voltages on the secondary winding Ns and the auxiliary winding Na. Then diode D 1 is reverse biased and freewheeling diode D 2 conducts to bias the control circuit with the energy stored in inductor L 1 during the conduction of switch S 1 . The auxiliary switch S 2 is also turned on by the control circuit during this nonconduction period of S 1 to reset the transformer actively via winding Na. As seen, the auxiliary winding Na is a common part of both the active clamp circuit and the primary bias circuit. The selection of the number of auxiliary winding turns is determined by the bias voltage the control circuit requires.

Flyback Converter

FIG. 5 illustrates an application of the novel combination of an integrated clamp circuit with a primary bias circuit of the present invention applied to a flyback converter having a flyback transformer T 1 , and FIG. 6 illustrates the switching waveforms occurring in the converter illustrated in FIG. 5 .

In operation, the power switch S 1 and the active clamp switch S 2 are turned on and off alternately by the control circuit 10 . During [t 0 , t 1 ]( FIG. 6 ), power switch S 1 is on and winding Nc is positively biased, i.e., the dot end of winding Nc is positive compared to the non-dot end. The active clamp switch S 2 is off during this period with a voltage as shown in FIG. 6 . At time t 1 , S 1 is turned off. The energy stored in transformer T 1 during the conduction of S 1 is transferred to the control circuit 10 and the voltage spike (not shown) on S 1 due to the leakage inductance between primary and secondary windings is clamped via the capacitor C 1 , auxiliary winding Nc and the switch S 2 . S 2 is still turned on during the turn-off of S 1 as a part of the bias circuit. As seen, the auxiliary winding Nc is a common part of both the active clamp circuit and the primary bias circuit. The selection of the number of winding turns of the auxiliary winding Nc is determined by the bias voltage the control circuit requires.

Forward-Flyback Converter

FIG. 7 illustrates an application of the novel combination of an active clamp circuit with a primary bias circuit of the present invention applied to forward-flyback converter having a flyback transformer T 1 , and FIG. 8 illustrates the switching waveforms occurring in the converter illustrated in FIG. 7 .

In operation, the main switch S 1 and the active clamp switch S 2 are turned on and off alternately by the control circuit. During [t 0 , t 1 ] ( FIG. 8 ), power switch S 1 is turned on and winding Nc is positively biased, i.e., the dot end of winding Nc is positive compared to the non-dot end. As a result diode D 1 conducts to allow energy to be transferred to the control circuit 10 and stored in filter inductor L 1 at the same time. The active clamp switch S 2 is off during this period with a voltage stress as shown in FIG. 8 . At time t 1 , S 1 is turned off. The voltage on the primary winding Np of transformer T 1 reverses, as do the voltages on the secondary winding Ns and the auxiliary winding Nc. Then diode D 1 is reverse biased and freewheeling diode D 2 is conducting to bias the control circuit 10 with the energy stored in inductor L 1 during the conduction of switch S 1 . The auxiliary switch S 2 is also turned on by the control circuit 10 during this turn-off period of S 1 to reset the transformer actively via winding Nc. As seen, the auxiliary winding Nc is a common part of both the active clamp circuit and the primary bias circuit. The selection of the number of winding turns of the auxiliary winding Nc is determined by the bias voltage the control circuit requires.

As described above, in converters incorporating an integrated active clamp circuit with a primary bias circuit, an auxiliary winding (e.g., the winding Nb) is omitted. This enables an increase in the power density of DC-DC converters. This is because the ever-increasing demand in power density requirements for isolated DC-DC converters, has made it common for a PCB (Printed Circuit Board) transformer to be used instead of using a traditional transformer with several windings mounted on a bobbin. However, if a PCB transformer contains too many windings, it will require an expensive multi-layer PCB and a considerable number of terminal connections. This makes it difficult to achieve a high power density. By saving an auxiliary winding, as in the present invention, it is much easier to design a PCB transformer with fewer windings to satisfy the desire for high power density.

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. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.