JP6917274B2 - Synchronous rectifying forward converter - Google Patents

Description

The present invention relates to a synchronous rectification type forward converter, which is one of the DC / DC converters.

In order to improve the efficiency of the DC / DC converter, the synchronous rectifier converter replaces the diode, which is a switch in the asynchronous rectifier converter, with a transistor such as a FET, and controls the on / off timing to operate in the same manner as the asynchronous rectifier converter. Is to be done.

FIG. 4 schematically shows an example of a basic circuit configuration of a synchronous rectification type forward converter and a timing chart for explaining its operation. The synchronous rectification type forward converter here is an isolated type converter in which the input side and the output side are insulated by using a transformer. For example, it is described in Patent Documents 1 and 2.

The transformer T has a primary coil Np and a secondary coil Ns having the same polarity direction (the winding start end is indicated by a black circle). The DC input voltage + Vin applied to the primary coil Np is switched by the main switching element Q1. The rectifying switching element Q2 and the choke coil L are connected in series to the secondary coil Ns. Further, the commutation switching element Q3 is connected in parallel to the secondary coil Ns and the rectifying switching element Q2. A smoothing capacitor C is connected in parallel to the secondary coil Ns, the rectifying switching element Q2, and the choke coil L. Both ends of the smoothing capacitor C are output ends p and n of the DC output voltage. The PWM (pulse width modulation) signal that controls the main switching element Q1 and the rectifying switching element Q2 on and off and the PWM (pulse width modulation) signal that controls the commutation switching element Q on and off are in a reciprocal relationship. And vice versa.

PWM ICs are widely used to generate PWM signals. The PWM IC usually determines a predetermined duty ratio in a pulse waveform having a constant frequency, generates a PWM signal, and outputs the PWM signal. In this example, a control unit (Cont.) Is provided to adjust the on / off timing of the commutation switching element Q3.

Japanese Unexamined Patent Publication No. 11-178334 Japanese Unexamined Patent Publication No. 2013-90432

Problems of a general synchronous rectification type forward converter will be described with reference to FIG.
FIG. 4A shows a PWM signal, FIG. 4B shows a voltage between the gate and source of the main switching element Q1 and the rectifying switching element Q2 (that is, turning on / off Q2), and FIG. 4C shows a connection point between the choke coil L and the commutation switching element Q3. (Hereinafter referred to as “choke coil potential”) _VL , (d) shows the respective waveforms of the gate-source voltage (that is, on / off of Q3) of the commutation switching element Q3. Modes I to III will be described along the time axis.

Mode I: During the ON period of the PWM signal, the main switching element Q1 and the rectifying switching element Q2 are conducted (ON). When the current flows through the primary coil Np, the positive electromotive voltage generated in the secondary coil Ns causes the current to flow through the rectifying switching element Q2 and the choke coil L, and is supplied to the load from the output terminals p and n. Magnetic energy is stored in the choke coil L. The commutation switching element Q3 is cut off (off).

Mode II: When the PWM signal is turned off, the main switching element Q1 and the rectifying switching element Q2 are cut off (off), and a negative counter electromotive voltage is generated _{in the choke coil voltage VL.} At this time, the commutation switching element Q3 is still off. The period of mode II is a so-called dead time provided to prevent the rectifying switching element Q2 and the commutation switching element Q3 from being turned on at the same time. The control unit monitors, for example, the choke coil potential _VL , and turns on the commutation switching element Q3 after a predetermined dead time has elapsed. Although different from the illustrated example, control may be performed to turn on the rectifying switching element Q2 after a predetermined dead time elapses after the commutation switching element Q3 is turned off. Such dead time control is very complicated, and a processor or the like is usually used for arithmetic processing.

Mode III: A current flows through the choke coil L through the commutation switching element Q3 that has been turned on, and is supplied to the load from the output terminals p and n. As a result, the magnetic energy stored in the choke coil L is released. When the current becomes zero, the counter electromotive voltage returns to zero. Then, the next ON of the PWM signal is reached. This is repeated. In the illustrated example, the control unit _{monitors the choke coil L potential VL} (or current) and controls to turn off the commutation switching element Q3 when it becomes zero. However, since the potential _VL usually fluctuates near zero, chattering in which the commutation switching element Q3 repeatedly turns on and off is likely to occur (see FIG. 4D). Further, if the choke coil potential _VL is left uncontrolled, there is a problem that oscillation is likely to occur.

From the above problems, the present invention enables safe and reliable synchronous control of each switching element in a synchronous rectification type forward converter without requiring complicated control, and realizes higher efficiency and stability. The purpose is to do.

In order to achieve the above object, the present invention provides the following configurations.
-Aspects of the present invention include a transformer having a primary coil and a secondary coil, a main switching element connected in series with the primary coil, a rectifying switching element and a choke coil connected in series with the secondary coil, and the above two. A commutation switching element connected in parallel to the next coil and the rectifying switching element, the secondary coil, the rectifying switching element, and a smoothing capacitor connected in parallel to the choke coil are provided, and the main switching element and the rectifying element are provided. In a synchronous rectification type forward converter in which the switching element and the commutation switching element are each turned on and off by a PWM signal.
A first delay circuit that controls on / off of the main switching element and the rectifying switching element according to the input PWM signal after each of the PWM signals is input and a predetermined same delay time elapses from the input time. And the second delay circuit,
It has a predetermined reference potential, the PWM signal, and an arithmetic amplifier circuit that controls the on / off of the commutation switching element by inputting the choke coil potential which is the potential of the connection point between the choke coil and the commutation switching element. death,
The arithmetic amplifier circuit
When the PWM signal is off and the choke coil potential is about to drop below the reference potential, the commutation switching element is turned from off to on to keep the choke coil potential at the same potential as the reference potential. As well as working on
The commutation switching element is turned from on to off when the PWM signal is turned on or the choke coil potential exceeds the reference potential.
-In the above embodiment, it is preferable that the reference potential is a voltage having the same polarity as the counter electromotive voltage generated in the secondary coil and having an absolute value smaller than the counter electromotive voltage.
In the above embodiment, one end of the secondary coil and one end of the smoothing capacitor are arranged so as to be in the forward direction with respect to the counter electromotive voltage generated in the secondary coil when the rectifying switching element is turned off. It is preferable to have additional rectifying elements connected in between.
-In the above embodiment, it is preferable that the PWM signal is input to the first delay circuit via a photocoupler.

In the synchronous rectification type forward converter of the present invention, safe and reliable synchronous control of each switching element is performed without requiring complicated control. In addition, higher efficiency and stability are achieved.

FIG. 1 is a diagram schematically showing a circuit configuration example of an embodiment of the synchronous rectification type forward converter of the present invention. FIG. 2 is a diagram showing an example of a timing chart of the circuit shown in FIG. 1 under heavy load. FIG. 3 is a diagram showing an example of a timing chart of the circuit shown in FIG. 1 when a light load is applied. It is a figure which showed an example of the basic circuit configuration and the timing chart of a synchronous rectification type forward converter.

(1) Circuit Configuration FIG. 1 is a diagram schematically showing a circuit configuration example of an embodiment of the synchronous rectification type forward converter of the present invention. The same components as those in the basic form of the synchronous rectification type forward converter of FIG. 4 described above are designated by the same reference numerals. As used herein, the term "input" to an input end of an operational amplifier includes the case where a potential is input via a diode or a resistor.

The transformer T has a primary coil Np and a secondary coil Ns having the same polarity direction. The winding start end of the coil is indicated by a black circle. The main switching element Q1 is connected in series to the primary coil Np. In this example, the main switching element Q1 is an n-channel FET, the drain is connected to the primary coil Np, and the source is connected to the ground end on the primary side. The main switching element Q1 is on / off controlled by the voltage applied to the gate. As a result, the DC input voltage + Vin applied to the primary coil Np is switched.

The rectifying switching element Q2 and the choke coil L are connected in series to the secondary coil Ns of the transformer T. In this example, the rectifying switching element Q2 is an n-channel FET, and the drain is connected to the secondary coil Ns and the source is connected to the output terminal n which is the ground end on the secondary side. The rectifying switching element Q2 plays the role of a rectifying diode of an asynchronous rectifying forward converter. The rectifying switching element Q2 is on / off controlled by the voltage applied to the gate. One end of the choke coil L is connected to the secondary coil Ns, and the other end is connected to the output end p.

Further, the commutation switching element Q3 is connected in parallel to the secondary coil Ns and the rectifying switching element Q2 connected in series. In this example, the commutation switching element Q3 is an n-channel FET, and the drain is connected to one end of the choke coil L and the source is connected to the output end n which is the ground end on the secondary side. Further, a diode D6 is connected in parallel to the commutation switching element Q3 in the same direction as the body diode. Although the diode D6 is not indispensable, it is preferable to connect the diode D6 because the loss increases when a current flows through the body diode of the FET. As the diode D6, for example, a Schottky barrier diode having a fast response and a small voltage drop is suitable. The commutation switching element Q3 plays the role of a commutation diode of the asynchronous rectifying forward converter. The commutation switching element Q3 is on / off controlled by the voltage applied to the gate.

Further, a smoothing capacitor C is connected in parallel to the secondary coil Ns, the rectifying switching element Q2, and the choke coil L connected in series. Both ends of the smoothing capacitor C are output ends p and n of the DC output voltage.

Further, a diode D7 is connected between the secondary coil Ns and the positive output end p. In the diode D7, the anode is connected to the winding end of the secondary coil Ns, and the cathode is connected to the output end p.

Next, a circuit that controls each of the switching elements Q1, Q2, and Q3 on and off will be described. As described above, in the synchronous rectification type forward converter, the PWM signal that controls the main switching element Q1 and the rectifying switching element Q2 and that that controls the commutation switching element Q3 are basically in a reciprocal relationship. In addition, it is necessary to ensure that both are turned on at the same time.

In the circuit of FIG. 1 according to the present invention, the voltage for on / off control of the switching elements Q1, Q2, and Q3 is generated by using one PWM signal output from the out terminal of the illustrated PWM IC. The PWM IC usually determines a predetermined duty ratio in a pulse waveform having a constant frequency, generates a PWM signal, and outputs the PWM signal. The method of determining the duty ratio in the PWM IC is not described here because it is outside the scope of the present invention.

First, a circuit configuration for on / off control of the main switching element Q1 and the rectifying switching element Q2 will be described. The PWM signal (for example, H: + 15V, L: 0V pulse signal) output from the out terminal of the PWM IC is rectified with the main switching element Q1 via the OP amplifier A1 and the OP amplifier A2 that substantially function as comparators. It is applied to each gate of the switching element Q2. The OP amplifier A1 and the OP amplifier A2 have the same predetermined response speed.

The signals input to the OP amplifier A1 and the OP amplifier A2 are output to the output terminal after a delay time corresponding to a predetermined response speed has elapsed. Therefore, the OP amplifier A1 and the OP amplifier A2 provide a first delay circuit and a second delay circuit for delaying the PWM signal by a predetermined time and applying the PWM signal to the gates of the main switching element Q1 and the rectifying switching element Q2. It is configured.

For the main switching element Q1, a PWM signal is applied to the non-inverting input end of the OP amplifier A1 via the resistor R1. The inverting input end of the OP amplifier A1 is connected to the ground end. The diode D1 is for voltage limiting. The PWM signal applied to the non-inverting input terminal is output to the output terminal of the OP amplifier A1 with a delay of a predetermined delay time due to the response speed of the OP amplifier A1, and is applied to the gate of the main switching element Q1. The same applies to the OP amplifier A2. As a result, the main switching element Q1 and the rectifying switching element Q2 are turned on and off at the same time.

In the illustrated example, the PWM signal is directly input to the OP amplifier A1, and the inverting input ends of the OP amplifier A1 and the OP amplifier A2 are connected to each other, but the primary side is insulated from the secondary side by using a photocoupler or the like. Is preferable.

Next, a circuit configuration for on / off control of the commutation switching element Q3 will be described. The PWM signal output from the out terminal is applied to the inverting input end of the OP amplifier A3 via the resistor R3 and the diode D3. The OP amplifier A3 is a high-speed amplifier having a faster response speed than the above-mentioned OP amplifiers A1 and A2. A constant reference potential Vref is applied to the non-inverting input end of the OP amplifier A3.

The reference potential Vref is a potential having the same polarity as the counter electromotive voltage generated at the winding start end of the secondary coil Ns and having an absolute value smaller than the counter electromotive voltage. Preferably, the ground potential is set to a potential close to zero potential. In the circuit of FIG. 1, for example, a negative potential slightly lower than the zero potential (for example, −10 mV) is set.

The OP amplifier A3 is driven by a positive power supply + Vcc (for example, + 15V) and a negative power supply-Vcc (for example, -15V). The output end of the OP amplifier A3 is connected to the gate of the commutation switching element Q3 via a push-pull emitter follower circuit composed of two transistors. The push-pull emitter follower circuit is provided for current supply.

Further, the connection point between the drain of the commutation switching element Q3 and the choke coil L (which is also the winding start end of the secondary coil Ns) is connected to the inverting input end of the OP amplifier A3 via the resistor R4, whereby the OP amplifier It forms a feedback path when A3 operates as an operational amplifier circuit.

A diode D4 and a diode D5 connected in antiparallel between the inverting input end and the ground end are provided for limiting the input voltage at the inverting input end.

The on (H) or off (L) potential of the PWM signal is applied to the inverting input end of the OP amplifier A3 via the resistor R3 and the diode D3, and the choke coil potential _VL is applied via the resistor R4. Will be done. A positive potential (H) or a negative potential (L) is output to the output terminal depending on the relationship between the input potentials to these two inverting input ends and the reference potential Vref of the non-inverting input end. This relationship is schematically shown in Table 1.

Only in the mode III of Table 1, a positive potential (H) is output to the output end of the OP amplifier A3, is applied between the gate sources of the commutation switching element Q3, and the commutation switching element Q3 is turned on.

The condition of mode III is that the PWM signal is off (L) and the choke coil potential _VL is lower than the reference potential Vref (or tries to drop to a potential lower than the reference potential Vref). However, as will be described later, once the commutation switching element Q3 is turned on in the mode III, the choke coil _VL potential is held so as to be the same as the reference potential Vref by the amplification operation of the OP amplifier A3.

On the other hand, in modes I, II and IV, the commutation switching element Q3 is turned off. The condition of these modes is when the PWM signal is on (H) _{or at least one of the choke coil potential VL} is higher than the reference potential Vref is satisfied.

(2) Circuit Operation Hereinafter, the circuit operation of the synchronous rectification type forward converter of FIG. 1 will be described for each of the cases of heavy load and light load. The heavy load and the light load are relative, but here we distinguish whether the output current is in continuous mode or discontinuous mode.

<Operation under heavy load>
FIG. 2 is a diagram showing an example of a timing chart when a load (not shown) is heavy in the circuit shown in FIG. This is when the load requires a lot of current. The duty ratio of the PWM signal is relatively large, that is, the on period is long. Vgs shown in FIGS. 2 (b) and 2 (d) means the voltage between the gate and source of the FET (the same applies to FIG. 3).

The timing chart of FIG. 2 shows the periods substantially corresponding to the modes I, II, III, and IV shown in Table 1, respectively. Hereinafter, the description will be given according to the order of the modes.

・ Mode I
The start point of the mode I is not the start point of the ON period of the PWM signal, but a time point delayed by a predetermined delay time from the start point (FIG. 2A). The predetermined delay time is determined by the response delay of the OP amplifiers A1 and A2. The main switching element Q1 and the rectifying switching element Q2 are turned on after a predetermined delay time from the PWM signal due to the response delay of the OP amplifiers A1 and A2 (FIG. 2B). When the switching elements Q1 and Q2 are turned on, the start point of the mode I is.

When the main switching element Q1 is turned on, a current flows through the primary coil Np, an electromotive voltage (for example, + 30V) is generated in the secondary coil Ns, and the choke coil potential _VL becomes a positive potential (FIG. 2C). The electromotive voltage of the secondary coil Ns is determined by the turn ratio of the input DC voltage Vin and the transformer T.

_{As a result, the current IL} flows to the output end p through the rectifying switching element Q2 and the choke coil L (FIG. 2 (f)). The current _IL is supplied to the load. Current I _L, while electromotive force is generated in the secondary coil Ns, increases linearly. When the current _IL flows through the choke coil L, magnetic energy is stored in the choke coil L. In addition, magnetic energy is also stored in the transformer T.

In mode I, the commutation switching element Q3 is off (FIG. 2 (d)). Further, since the diode D6 and the body diode are also reverse biased, no current flows in this path. Further, since the diode D7 also has a reverse bias, no current flows (FIG. 2 (g)).

・ Mode II
The starting point of mode II is the time when the PWM signal changes from on to off (FIG. 2A). When the main switching element Q1 and the rectifying switching element Q2 are off, the response delay of the OP amplifiers A1 and A2 causes a delay compared to when the PWM signal is off. When the switching elements Q1 and Q2 are off, the end point of the mode II is set (FIG. 2 (b)).

In mode II, since the switching elements Q1 and Q2 are still in the ON state, the winding start end of the secondary coil Ns and the choke coil potential _VL hold a positive potential (FIG. 2C).

In mode II, the commutation switching element Q3 is still in the off state (FIG. 2 (d)). _{This is because the choke coil potential VL} is still a positive potential even when the PWM signal is turned off.

・ Mode III
The starting point of the mode III is when the main switching element Q1 and the rectifying switching element Q2 are off (FIG. 2B). As a result, the current of the primary coil Np is cut off, and a counter electromotive voltage is generated in the primary coil Np, the secondary coil Ns, and the choke coil L. As a result, the choke coil potential _VL changes from a positive potential to a negative potential (FIG. 2 (c)). Then, when the potential at the inverting input end of the OP amplifier A3 drops below the reference potential Vref (FIG. 2 (e), for example, -10 mV) at the non-inverting input end, the output end of the OP amplifier A3 becomes a positive potential and turns. The flow switching element Q3 is turned on (FIG. 2 (d)).

In the process from the turning off of the main switching element Q1 and the rectifying switching element Q2 to the turning on of the commutation switching element Q3 at the start point of the mode III, the switching elements Q1 and Q2 and the switching element Q3 are not turned on at the same time. .. This is because after the main switching element Q1 and the rectifying switching element Q2 are turned off, the choke coil potential _VL drops to a negative potential, and then the commutation switching element Q3 is turned on.

Accordingly, current I _L, the commutation switching elements Q3 and flow through the choke coil L to the output p (FIG. 2 (f)), the magnetic energy stored in the choke coil L is released. Current I _L, with the release of magnetic energy in the choke coil L, decreases linearly.

Further, when a counter electromotive voltage is generated in the secondary coil Ns, the diode D7 becomes a forward bias and the current _ID flows to the output end p (FIG. 2 (g)). As a result, the magnetic energy stored in the transformer T can be taken out as electric power to the secondary side. The current _ID decreases linearly as the magnetic energy is released.

In a general forward converter, it is necessary to provide a reset circuit for resetting the magnetic energy stored in the transformer T during the on period and a snubber circuit for countermeasures against the counter electromotive voltage generated in the primary coil Ns when the transformer T is off. There is. In this circuit, the magnetic energy stored in the transformer T can be output to the secondary side via the diode D7, so that the reset circuit and snubber circuit on the primary side can be simplified.

Further, when the commutation switching element Q3 is turned on in the mode III, a feedback path is formed such that the output end of the OP amplifier A3 → one end of the choke coil L (drain of Q3) → the resistor R4 → the inverting input end of the OP amplifier A3. NS. As a result, the OP amplifier A3 substantially functions as an arithmetic amplifier circuit of the voltage follower. Therefore, while the commutation switching element Q3 is on, the choke coil potential _VL is held at the same potential as the reference potential Vref at the non-inverting input end of the OP amplifier A3 (FIG. 2C).

The function of this arithmetic amplifier circuit stabilizes the _{choke coil potential VL.} In the case of a conventional synchronous rectification type forward converter without such an arithmetic amplifier circuit, the choke coil potential _VL may fluctuate in the vicinity of the ground end potential (0V) and become unstable or easily oscillate. be.

・ Mode IV
The starting point of mode IV is the time when the PWM signal changes from off to on (FIG. 2A). The main switching element Q1 and the rectifying switching element Q2 are still in the off state in mode IV due to the response delay of the OP amplifiers A1 and A2 (FIG. 2B).

On the other hand, since the PWM signal is turned on at the inverting input end of the OP amplifier A3, it becomes a positive potential (upper limit voltage due to the diode D4, for example, + 0.6V), the potential at the output end of the OP amplifier A3 is inverted, and commutation switching is performed. The element Q3 is turned off (FIG. 2 (d)).

Even commutation switching element Q3 is turned off, if there are remaining magnetic energy in the choke coil L, the current I _L continues to flow through the diode D6 (FIG. 2 (f)). At this time, the choke coil potential _VL becomes the potential (for example, −0.6 V) due to the voltage drop of the diode D6 (FIG. 2 (c)).

In mode IV, the current I _L through diode D6, but commutation switching element Q3 is larger power loss as compared to when on period of the mode IV is short, since also decreased current I _L, less It's not a big problem. Current I _L, the main switching element Q1 and the rectifying switching element Q2 is turned on at the beginning of the following modes I, starts to increase again. The current _IL is in continuous mode and never goes to zero.

Even if the current _ID is still flowing in the mode IV, the current ID becomes zero at the start point of the next mode I because the diode D7 has a reverse bias (FIG. 2 (g)).

In mode IV, even if the PWM signal is turned from off to on, the main switching element Q1 and the rectifying switching element Q2 remain off due to the response delay of the OP amplifiers A1 and A2. Therefore, even when the PWM signal is turned from off to on, the switching elements Q1 and Q2 and the switching element Q3 are not turned on at the same time.

<Operation under light load>
FIG. 3 is a diagram showing an example of a timing chart when the load (not shown) is light in the circuit shown in FIG. This is when the load does not require much current. The duty ratio of the PWM signal is relatively small, that is, the on period is short.

Similar to FIG. 2, the timing chart of FIG. 3 shows the periods substantially corresponding to the modes I, II, III, and IV shown in Table 1, respectively. At light loads, another mode appears between Mode III and Mode IV. This is referred to as mode IIIa. Hereinafter, the part of the operation under a light load that is different from the operation under a heavy load will be described.

・ Mode I
The operation of mode I under a light load is the same as the operation under a heavy load. However, in the light load, the current I _L flowing through the secondary side is discontinuous mode. Therefore, current _{I L} in mode I is increased from zero linearly (FIG. 3 (f)).

・ Mode II
The operation of Mode II under light load is the same as the operation under heavy load.

・ Mode III
The operation of mode III is different from the operation under heavy load when the _{current IL is in the discontinuous mode.} The operation at the start point of the mode III is the same as that at the time of heavy load, and starts when the main switching element Q1 and the rectifying switching element Q2 are turned off (FIG. 3 (b)). As a result, the current of the primary coil Np is cut off, and a counter electromotive voltage is generated in the primary coil Np, the secondary coil Ns, and the choke coil L. Therefore, the choke coil potential _VL changes from a positive potential to a negative potential (FIG. 3 (c)). Then, when the potential at the inverting input end of the OP amplifier A3 drops below the reference potential Vref (for example, -10 mV, FIG. 3 (e)) at the non-inverting input end, the output end of the OP amplifier A3 becomes a positive potential. The commutation switching element Q3 is turned on (FIG. 3 (d)).

In the process from the turning off of the main switching element Q1 and the rectifying switching element Q2 to the turning on of the commutation switching element Q3 at the start point of the mode III, the switching elements Q1 and Q2 and the switching element Q3 are not turned on at the same time. ..

Thus, a current I _L flows through the commutation switching element Q3 and the choke coil L to the output p (Fig. 3 (f)), the magnetic energy stored in the choke coil L is released. When the load is light, the magnetic energy stored in the choke coil L is small. Current I _L of the discontinuous mode, gradually decreases linearly with the release of magnetic energy in the choke coil L, eventually becomes zero (FIG. 3 (f)).

At the time of light load _{, the mode III ends when the current IL} becomes zero, and the mode IIIa shifts to the mode IIIa. When the current _IL becomes zero, the choke coil potential _VL rises to a positive potential (FIG. 3 (c)). This corresponds to the potential of the output end p (for example, + 12V). When this positive choke coil potential _VL is applied to the inverting input end of the OP amplifier A3 via the resistor R4, the potential at the output end of the OP amplifier A3 is inverted from the positive potential to the negative potential. As a result, the commutation switching element Q3 is turned off (FIG. 3 (d)).

In mode III, a counter electromotive voltage is generated in the secondary coil Ns, so that the diode D7 becomes a forward bias as in the case of a heavy load, and the current _ID flows to the output end p (FIG. 3 (g)). The current _ID decreases linearly as the magnetic energy of the transformer T is released, and becomes zero when the magnetic energy disappears.

・ Mode IIIa
In mode IIIa, the PWM signal continues to be off (FIG. 3 (a)), the main switching element Q1 and the rectifying switching element Q2 are off (FIG. 3 (b)), and the choke coil potential _VL is at the output end. It is an electric potential (FIG. 3 (c)), the commutation switching element Q3 is off (FIG. 3 (d)), and the current _IL is zero (FIG. 3 (f)).

・ Mode IV
Mode IV under heavy load is a mode following mode IIIa, which is not under heavy load, and therefore operates differently from mode IV under heavy load.

The starting point of the mode IV is the time when the PWM signal changes from off to on as in the case of heavy load (FIG. 3 (a)). The main switching element Q1 and the rectifying switching element Q2 are still in the off state in mode IV due to the response delay of the OP amplifiers A1 and A2 (FIG. 3B).

Therefore, in mode IV, the same state as in mode IIIa is maintained on both the primary side and the secondary side of the transformer T. The commutation switching element Q3 also remains in the off state.

Mode IV continues until the next mode I start point. As described above, the start point of the mode I is the time when the main switching element Q1 and the rectifying switching element Q2 are turned on after a predetermined delay time.

At the time of light load, the commutation switching element Q3 is already turned off when the _{current IL becomes zero during the off period of the PWM signal.} Therefore, the commutation switching element Q3 does not turn on at the same time as the main switching element Q1 and the rectifying switching element Q2, which turn on later than when the next PWM signal is turned on.

The circuit configuration that realizes the operation of the synchronous rectification type forward converter of the present invention described above is not limited to that shown in FIG. 1, and various circuit configurations are possible.

p Positive output end n Negative output end (secondary side grounded end)
T transformer Np primary coil Ns secondary coil Q1, Q2, Q3 switching element (FET)
D1-D7 Rectifying element (diode)
C capacitor (smoothing capacitor)

Claims (4)

A transformer having a primary coil and a secondary coil, a main switching element connected in series with the primary coil, a rectifying switching element and a choke coil connected in series with the secondary coil, the secondary coil and the rectifying switching element. A commutation switching element connected in parallel to the secondary coil, a rectifying switching element, and a smoothing capacitor connected in parallel to the choke coil, the main switching element, the rectifying switching element, and the commuting switching. In a synchronous rectification type forward converter in which each element is turned on and off by a PWM signal,
A first delay circuit that controls on / off of the main switching element and the rectifying switching element according to the input PWM signal after each of the PWM signals is input and a predetermined same delay time elapses from the input time. And the second delay circuit,
It has a predetermined reference potential, the PWM signal, and an arithmetic amplifier circuit that controls the on / off of the commutation switching element by inputting the choke coil potential which is the potential of the connection point between the choke coil and the commutation switching element. death,
The arithmetic amplifier circuit
When the PWM signal is off and the choke coil potential is about to drop below the reference potential, the commutation switching element is turned from off to on to keep the choke coil potential at the same potential as the reference potential. As well as working on
A synchronous rectification type forward converter, characterized in that the commutation switching element is turned from on to off when the PWM signal is turned on or the choke coil potential exceeds the reference potential. The synchronous rectification type forward converter according to claim 1, wherein the reference potential is a voltage having the same polarity as the counter electromotive voltage generated in the secondary coil and having an absolute value smaller than the counter electromotive voltage. It is connected between one end of the secondary coil and one end of the smoothing capacitor so as to be in the forward direction with respect to the counter electromotive voltage generated in the secondary coil when the rectifying switching element is turned off. The synchronous rectifying forward converter according to claim 1 or 2, further comprising a rectifying element. The synchronous rectification type forward converter according to any one of claims 1 to 3, wherein the PWM signal is input to the first delay circuit via a photocoupler.

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