FLYBACK CONVERTER

A flyback converter includes: a transformer including a primary winding and a secondary winding; a first switch coupled to the primary winding; a first control module configured to control the first switch; a second switch coupled to the secondary winding; and a second control module configured to control the second switch, such that the second switch operates in an ON state during a first time period and during a second time period which follows the first time period, and such that a current flowing through the secondary winding has a direction during the second time period opposite to that during the first time period.

FIELD

The disclosure relates to power conversion, and more particularly to a flyback converter.

BACKGROUND

A conventional flyback converter disadvantageously has a relatively high switching loss, and thus has a relatively low conversion efficiency.

SUMMARY

Therefore, an object of the disclosure is to provide a flyback converter that can alleviate the drawback of the prior art.

According to one aspect of the disclosure, the flyback converter includes a transformer, a first switch, a first control module, a second switch and a second control module. The transformer includes a primary winding and a secondary winding. Each of the windings has a first terminal and a second terminal. The first terminals respectively of the primary and secondary windings have the same voltage polarity. The first switch is coupled to the first terminal of the primary winding. The first control module is coupled to the first switch, and is configured to control the first switch. The second switch is coupled to the secondary winding. The second control module is coupled to the second switch, and is configured to control the second switch, such that the second switch operates in an ON state during a first time period and during a second time period which is right after the first time period, and such that a current flowing through the secondary winding has a direction during the second time period opposite to that during the first time period.

According to another aspect of the disclosure, the flyback converter includes a transformer, a first switch, a first control module, a second switch and a second control module. The transformer includes a primary winding and a secondary winding. Each of the windings has a first terminal and a second terminal. The first terminals respectively of the primary and secondary windings have the same voltage polarity. The first switch is coupled to the first terminal of the primary winding. The first control module is coupled to the first switch, and is configured to control the first switch. The second switch is coupled to the secondary winding. The second control module is coupled to the second switch, and is configured to control the second switch, such that the second switch operates in an ON state during a first time period and during a second time period which follows the first time period, and such that a current flowing through the secondary winding has a direction during the second time period opposite to that during the first time period.

DETAILED DESCRIPTION

Before describing this disclosure in detail, it should be noted herein that throughout this disclosure, when two elements are described as being “coupled in series,” “connected in series” or the like, it is merely intended to portray a serial connection between the two elements without necessarily implying that the currents flowing through the two elements are identical to each other and without limiting whether or not an additional element is coupled to a common node between the two elements. Essentially, “a series connection of elements,” “a series coupling of elements” or the like as used throughout this disclosure should be interpreted as being such when looking at those elements alone.

Referring toFIGS. 1 to 3, an embodiment of a flyback converter according to the disclosure is used to convert an input voltage (Vi) into an output voltage (Vo), and includes a transformer1, a first switch2, a first control module3, a second switch4, an output capacitor5and a second control module6.

The transformer1includes a primary winding11, a secondary winding12and an auxiliary winding13. Each of the primary, secondary and auxiliary windings11,12,13has a first terminal (e.g., a dot-marked terminal shown inFIG. 1) and a second terminal (e.g., a non-dotted terminal shown inFIG. 1). The first terminals respectively of the primary, secondary and auxiliary windings11,12,13have the same voltage polarity. The primary winding11has a number of turns N times that of the secondary winding12(i.e., a turn ratio of the primary winding11to the secondary winding12is N), and is used to receive the input voltage (Vi) at the second terminal thereof.

The first switch2has a first terminal that is coupled to the first terminal of the primary winding11, a second terminal that is grounded, and a control terminal. In this embodiment, the first switch2is an N-type metal oxide semiconductor field effect transistor (nMOSFET) having a drain terminal, a source terminal and a gate terminal that respectively serve as the first, second and control terminals of the first switch2.

The first control module3is coupled to the first terminal of the auxiliary winding13and the control terminal of the first switch2, generates a first control signal (Vgs1) in response to a voltage (Vaux) at the first terminal of the auxiliary winding13and a predetermined time threshold (Tth), and outputs the first control signal (Vgs1) to the control terminal of the first switch2so as to control operation of the first switch2between an ON state and an OFF state. The first control signal (Vgs1) switches between a first state (e.g., being at a logic high level, and corresponding to the ON state of the first switch2) and a second state (e.g., being at a logic low level, and corresponding to the OFF state of the first switch2). Under the control of the first control module3, the first switch2operates in the OFF state for at least the predetermined time threshold (Tth), and transitions from the OFF state to the ON state when a voltage (Vds1) across the first switch2is determined, in response to the voltage (Vaux) at the first terminal of the auxiliary winding13, to reach its valley.

It should be noted that the flyback converter of this embodiment further includes components (not shown) for providing signals to assist the first control module3in deciding when to make the first switch2transition from the ON state to the OFF state. Configuration and operation of such components and how the first control module3makes the decision are well known to those skilled in the art, and details thereof are omitted herein for the sake of brevity.

The second switch4and the output capacitor5are coupled in series across the secondary winding12. The second switch4has a first terminal that is coupled to the second terminal of the secondary winding12, a second terminal and a control terminal. The output capacitor5is coupled between the first terminal of the secondary winding12and the second terminal of the second switch4, and a voltage thereacross serves as the output voltage (Vo). In this embodiment, the second switch4is an nMOSFET having a drain terminal, a source terminal and a gate terminal that respectively serve as the first, second and control terminals of the second switch4.

The second control module6is coupled to the first, second and control terminals of the second switch4, and generates, in response to a voltage (Vds2) across the second switch4, a voltage detection signal indicating the input voltage (Vi). The second control module6further generates a second control signal (Vgs2) in response to the voltage (Vds2) across the second switch4, the predetermined time threshold (Tth) and the voltage detection signal, and outputs the second control signal (Vgs2) to the control terminal of the second switch4so as to control operation of the second switch4between an ON state and an OFF state. The second control signal (Vgs2) switches between a first state (e.g., being at a logic high level, and corresponding to the ON state of the second switch4) and a second state (e.g., being at a logic low level, and corresponding to the OFF state of the second switch4). Under the control of the second control module6, the second switch4operates in the ON state during a first time period that has a duration of t1, and a second time period that has a duration of t2and that follows the first time period, and operates in the OFF state otherwise. In the first time period, a current (Is) flowing through the secondary winding12is determined, in response to the voltage (Vds2) across the second switch4, to have a non-zero magnitude and a direction from the second terminal of the secondary winding12to the first terminal of the secondary winding12. As shown inFIG. 2, when the duration of the first time period is determined to be greater than the predetermined time threshold (Tth), i.e., t1>Tth, the second time period starts from an end of the first time period (i.e., the second time period is right after the first time period). As shown inFIG. 3, when the duration of the first time period is determined to be less than the predetermined time threshold (Tth), i.e., t1<Tth, the second time period starts from a time point which lags a start of the first time period by at least the predetermined time threshold (Tth), and at which the voltage (Vds2) across the second switch4is determined to reach its valley. The duration of the second time period is a function of the input voltage (Vi), i.e., t2=f(Vi).

As a result, during the second time period, the current (Is) flowing through the secondary winding12has the non-zero magnitude and the direction opposite to that during the first time period; during a third time period that has a duration of t3and that starts from an end of the second time period, a current (Ip) flowing through the primary winding11has a non-zero magnitude and a direction from the first terminal of the primary winding11to the second terminal of the primary winding11, and the voltage (Vds1) across the first switch2gradually decreases from an initial value (Vinit) of (Vi+N×Vo) to a valley value (Vval) that is smaller than the initial value (Vinit) and that is greater than or equal to zero (i.e., 0≦Vval<Vinit).

In this embodiment, in order to make the voltage (Vds1) across the first switch2decrease to a predetermined target valley value at an end of the third time period, the duration of the second time period is determined according to the following equation:

and the duration of the third time period required for the voltage (Vds1) across the first switch2to reach the predetermined target valley value meets the following equation:

where Lm denotes a magnetizing inductance of the primary winding11, C denotes a parasitic capacitance seen across the first switch2, and Vval_t denotes the predetermined target valley value. When the predetermined target valley value is determined to be zero and the duration of the second time period is determined according to Equation 1, the first switch2transitions from the OFF state to the ON state with zero voltage switching.

The second time period generally includes a range of 0.1 μs to 3 μs in duration, and the third time period generally includes a range of 0.1 μs to 0.7 μs induration. In an example where Vi=380V, Vo=20V, N=6, Lm=600 μH, C=60 pF, Vinit=500V and Vval_t=0V, the duration of the second time period is 0.57 μs (t2=0.57 μs), and the duration of the third time period is 0.354 μs (t3=0.359 μs).

In view of the above, the flyback converter of this embodiment has the following advantages:

1. With the first switch2operating in the ON state during the properly determined second time period in addition to the first time period, the voltage across (Vds1) the first switch2can decrease to a sufficiently low valley value (Vval), which results in a relatively low switching loss of the first switch2and thus a relatively high conversion efficiency of the flyback converter of this embodiment.

2. With the predetermined time threshold (Tth), each of the first and second switches2,4operates at a switching frequency that is limited below a certain frequency, which results in a relatively low switching loss of each of the first and second switches2,4and thus a relatively high conversion efficiency of the flyback converter of this embodiment.

3. With the duration of the second time period being a function of the input voltage (Vi), the valley value (Vval) can be unchanged over a relatively wide range of input voltages (Vi).

It should be noted that in other embodiments, the following modifications may be made to this embodiment:

1. The second terminal of the second switch4may be coupled to the first terminal of the secondary winding12, and the output capacitor5may be coupled between the first terminal of the second switch4and the second terminal of the secondary winding12.

2. The predetermined time threshold (Tth) may be omitted. In this case, the first switch2may transition from the OFF state to the ON state when the voltage across the first switch2(Vds1) is determined, in response to the voltage at the first terminal of the auxiliary winding13(Vaux), to reach its valley, and the second time period may be always right after the first time period.

3. The voltage detection signal may be omitted. In this case, the duration of the second time period may be predetermined according to Equation 1 in a design phase of the flyback converter.

While the disclosure has been described in connect ion with what is considered the exemplary embodiment(s), it is understood that the disclosure is not limited to the disclosed embodiment but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.