WO2019001218A1

WO2019001218A1 - Forward switch power supply

Info

Publication number: WO2019001218A1
Application number: PCT/CN2018/089578
Authority: WO
Inventors: 王保均
Original assignee: 广州金升阳科技有限公司
Priority date: 2017-06-30
Filing date: 2018-06-01
Publication date: 2019-01-03
Also published as: CN107395018A

Abstract

A forward switch power supply, which, on the basis of an LCL forward converter, keeps a common-polarity end of a first primary winding (NP1) in a transformer (B) connected to the positive end of a power supply, and a common-polarity end of a second primary winding (NP2) connected to the negative end of the power supply; the first primary winding and the second primary winding are double-wire parallel windings, and one end of a first capacitor (C1) is connected to an opposing-polarity end of the first primary winding, while another end is connected to an opposing-polarity end of the second primary winding; the opposing-polarity end of the first primary winding is connected to the power supply by means of a first field-effect transistor (Q1), and the opposing-polarity end of the second primary winding is connected to the power supply by means of a clamping network (400) in which a second field-effect transistor (Q2) is connected in series with a third capacitor (C3). After an appropriate control strategy is selected, a duty cycle greater than 0.5, zero-voltage switching (ZVS), and lossless absorption may be achieved, while having high power density and high conversion efficiency.

Description

Forward switching power supply

Technical field

The invention relates to the field of switching power supplies, and in particular to a single-ended forward switching power supply.

Background technique

At present, switching power supplies are widely used in the industry, often referred to as converters. The basic forward converter in the forward switching power supply is an ideal isolated version of the Buck converter. The common topology is a single-ended forward converter. Symmetrically driven half-bridge converters, full-bridge converters, push-pull converters, symmetric push-pull forward converters, and the like. Need to mention is a symmetric push-pull forward converter, as shown in Figure 1-1, the picture is taken from Dr. Zhang Xingzhu's book, ISBN 978-7-5083-9015-4 "Switching Power Supply Converter Topology And Figure 5-14 on page 91 of the design, which is referred to herein as: Reference 1.

Symmetrical push-pull forward converters are referred to in the patent literature as straight-in converters, which were first seen in the 1999 Institute of Electrical and Electronics Engineers (IEEE) Proceedings (0-7803-5160-6/99), page 279 "A Novel High-input-voltage, High Efficiency and Fast Transient Voltage Regulator Module" by Xunwei Zhou, Bo Yang, Luca Amoroso, Fred C. Lee and Pit-leong Wong;

And in the IEEE Proceedings of 2002 (0-7803-7404-5/02), page 843, "Single Magnetic Push-Pull Forward Converter featuring Built-in Input Filter and Coupled-Inductor Current Doubler for 48V VRM", author Peng Xu, Mao Ye (Ye Mao) and Fred C. Lee, the paper also mentioned "PUSH-PULL FORWARD CONVERTER";

And on page 6 of the 2004 Master's thesis published by Dai Weili of Nanjing University of Aeronautics and Astronautics, "Push-Pull Forward Circuit", its output is called "Push-Pull Forward Circuit". Full-wave rectification with freewheeling inductor L.

Various forward converters have their preferred uses due to their different circuit topologies:

Single-ended forward converter: good loop response, suitable for powering dynamic loads such as motors. Japan's COSEL industrial power supply is still implemented with PFC + three-winding demagnetization single-ended forward converter, for this reason, but the power is generally around 150W;

Half-bridge converter: suitable for applications with high working voltage, such as power supply for desktop computers; high power;

Full-bridge converter: suitable for high-voltage and high-power applications, common in power sections above 1Kw;

Push-pull converter: mostly used for low voltage, power less than 300W;

Symmetric push-pull forward converter: low voltage and high power, but did not see the practical product launch market;

As mentioned above, the single-ended forward converter is suitable for powering dynamic loads due to good loop response. Therefore, the circuit still has a large amount of use, especially in the case of low voltage operation, the circuit topology of the three winding demagnetization is as follows. Figure 1-2 shows Nc, Figure 1-2 from Figure 3-8 (a) on page 33 of Reference 1, the output of which is a common single-ended forward topology output rectifier circuit, diode VD1 is the switch tube ( Or the power tube) the rectifier tube that is synchronously turned on when the V saturation is turned on, the diode VD2 is the freewheeling tube when the switching tube V is turned off, and the current in the inductor L continues to supply the output filter capacitor C and the load R through the VD2. The third winding demagnetization forward converter is also referred to as a "three-winding absorption forward converter".

In the Chinese application number 201710141802.8 named "A Forward Switching Power Supply", the technical scheme of Figure 1-3 is shown, which solves some problems in Figure 1-2. The third winding realizes the excitation and realizes at the same time. For non-destructive absorption, the inventors have defined the topology used by the forward switching power supply for convenience, and will include a flyback topology using the inventive concept, and the basic topology excluding the demagnetization mode is defined as: LCL The converter is derived from a source comprising two primary side magnetizing inductances and a capacitor in series with them. Such as LCL forward converter, also refers to LCL forward switching power supply.

The technical solution in Figure 1-3 also introduces some new problems, such as the duty ratio can not be greater than 0.5, resulting in low power density, can not achieve the zero voltage switch of the switch Q1 in Figure 1-3 (Zero Voltage Switch, abbreviated as ZVS).

Summary of the invention

In view of this, the present invention solves the shortcomings of the existing LCL forward switching power supply, and provides a forward switching power supply, the duty ratio can be greater than 0.5, the power density is high, and the conversion efficiency is not reduced, and the switching tube can be realized. The zero voltage switch further enhances the conversion efficiency.

The object of the present invention is achieved by a forward switching power supply including a transformer, a first N-channel field effect transistor, a first capacitor, a second capacitor, a clamp network, a first diode, and a second The pole tube, the first inductor, the transformer comprises a first primary winding, a second primary winding and a secondary winding, the clamping network comprises at least an anode and a cathode, and the secondary winding has the same name end connected to the second diode anode, the second The diode cathode is simultaneously connected to the cathode of the first diode and one end of the first inductor, and the other end of the first inductor is connected to one end of the second capacitor, and the output is positive, and the opposite side of the secondary winding is simultaneously connected with the first two The anode of the pole tube and the other end of the second capacitor are connected, and the output is negative; the positive end of the input DC power source is simultaneously connected with the cathode of the same name of the first primary winding and the cathode of the clamp network, and the first primary winding has a different name and the same The drain of an N-channel FET is connected; the anode of the clamp network is connected to the opposite end of the second primary winding, and the source of the first N-channel FET is connected to the same end of the second primary winding, and the connection point is simultaneously connected Input the negative side of the DC power supply The gate of the first N-channel FET is connected to the driving control signal; the first primary winding and the second primary winding are wound in a double line, and one end of the first capacitor is connected to the different end of the first primary winding, first The other end of the capacitor is connected to the second end of the second primary winding, wherein the clamp network includes at least a third capacitor and a second N-channel FET, and the third capacitor and the second N-channel FET are connected in series, in series The way is one of the following two ways:

(1) One end of the third capacitor is the cathode of the clamp network, the other end of the third capacitor is connected to the drain of the second N-channel FET, and the source of the second N-channel FET is the anode of the clamp network, a gate connection clamp control signal of the two N-channel FET;

(2) one end of the third capacitor is the anode of the clamp network, the other end of the third capacitor is connected to the source of the second N-channel FET, and the drain of the second N-channel FET is the cathode of the clamp network, The gate of the two N-channel FET is connected to the clamp control signal.

As an alternative to the first solution: the first N-channel FET can be replaced by a P-channel FET, and the body diode inside the P-channel FET is in the same polarity as the body diode inside the first N-channel FET.

The present invention also provides an equivalent solution of the foregoing solution 1. The second embodiment of the present invention can also be implemented. A forward switching power supply includes a transformer, a first N-channel field effect transistor, a first capacitor, and a second capacitor. a clamp network, a first diode, a second diode, a first inductor, the transformer includes a first primary winding, a second primary winding, and a secondary winding, and the clamp network includes at least an anode and a cathode, and a secondary winding The same name end is connected to the second diode anode, and the second diode cathode is simultaneously connected to the cathode of the first diode and one end of the first inductor, and the other end of the first inductor is connected to one end of the second capacitor, and an output is formed. Positive, the opposite side of the secondary winding is simultaneously connected to the anode of the first diode and the other end of the second capacitor, and forms an output negative; the positive terminal of the input DC power supply is simultaneously connected with the drain of the first N-channel FET, The two primary windings are connected at different ends, and the source of the first N-channel field effect transistor is connected to the same name end of the first primary winding; the second primary winding has the same name end connected to the cathode of the clamp network, and the first primary winding is different. Name and clamp network The anode is connected, and the connection point is simultaneously connected to the negative end of the input DC power supply; the gate of the first N-channel FET is connected to drive the control signal; the first primary winding and the second primary winding are double-wired and wound, the first capacitor One end is connected to the same end of the first primary winding, and the other end of the first capacitor is connected to the same end of the second primary winding, wherein the clamp network includes at least a third capacitor and a second N-channel FET, and a third The capacitor is connected in series with the second N-channel FET, and the series connection is one of the following two ways:

As an alternative to the second solution: the first N-channel FET can be replaced by a P-channel FET, and the body diode inside the P-channel FET has the same polarity as the body diode inside the first N-channel FET.

As an improvement of the above two schemes, the first primary winding and the second primary winding have the same wire diameter.

Preferably, the physical path of the excitation current of the first primary winding and the second primary winding is reversed in the PCB layout.

The working principle will be explained in detail in conjunction with the embodiments. The invention has the beneficial effects that the duty ratio can be greater than 0.5, the power density is high, and the conversion efficiency is not reduced, and the zero voltage switch of the switch tube can be realized, thereby further improving the conversion efficiency.

DRAWINGS

Figure 1-1 is a schematic diagram of a topology of a PPFC converter in a conventional forward switching power supply;

Figure 1-2 is a schematic diagram of the topology of a conventional three-winding demagnetized single-ended forward converter;

Figure 1-3 is a schematic diagram of a technical solution shown in the prior application No. 201710141802.8;

2-1 is a schematic diagram of a first embodiment of the present invention, and the clamp network adopts (1) mode;

2-2 is a second schematic diagram of the first embodiment of the present invention, and the clamp network adopts (2) mode;

2-3 is a schematic diagram of charging the capacitor C1 at the time of power-on in the first embodiment;

2-4 are schematic diagrams showing the generation of two

excitation currents

41, 42 when Q1 is saturated in the first embodiment;

2-5 are schematic diagrams showing the Q1 cutoff in the first embodiment, generating a freewheeling current 43b and a demagnetizing current 44;

3-1 is a schematic diagram of a second embodiment of the present invention, and the clamp network adopts (1) mode;

3-2 is a second schematic diagram of the second embodiment of the present invention, and the clamp network adopts the (2) method.

Detailed ways

First embodiment

2-1 and 2-2 are schematic diagrams showing a forward switching power supply according to a first embodiment of the present invention, including a transformer B, a first N-channel FET Q1, a first capacitor C1, and a second capacitor C2. a clamp network 400, a first diode D3, a second diode D2, a first inductor L1, and a transformer B including a first primary winding N _P1 , a second primary winding N _{P2 ,} and a secondary winding N _S The clamp network 400 includes at least an anode and a cathode, the secondary winding N _S is connected to the anode of the second diode D2 at the same end, and the second diode D2 is simultaneously connected to the cathode of the first diode D3 and the first inductor L1. One end of the first inductor L1 is connected to one end of the second capacitor C2, and forms an output positive, which is the + end of Vout in the figure, and the opposite side of the secondary winding N _S is simultaneously connected with the anode of the first diode D3. The other end of the second capacitor C2 is connected and forms an output negative, which is the end of Vout in the figure; the positive terminal of the input DC power source U _DC is simultaneously connected to the cathode of the clamped network 400 at the same end of the first primary winding N _P1 The first primary winding N _{P1 is} connected to the drain of the N-channel FET Q1; the anode of the clamp network 400 and the second primary winding N _{P 2 The} different name ends are connected, the source s of the N-channel FET Q1 is connected to the same end of the second primary winding N _P2 , and the connection point is simultaneously connected to the negative terminal of the input DC power supply U _DC - the gate of the N-channel FET Q1 g is connected to the driving control signal; the first primary winding N _P1 and the second primary winding N _P2 are double-wired, and one end of the first capacitor C1 is connected to the first primary winding N _P1 , and the first capacitor C1 The other end is connected to the second primary winding N _P2 , and the clamp network 400 includes at least a third capacitor C3 and a second N-channel FET Q2. The third capacitor C3 and the second N-channel FET Q2 are connected in series. The serial connection is one of the following two ways:

(1) One end of the third capacitor C3 is the cathode of the clamp network 400, the other end of the third capacitor C3 is connected to the drain d of the second N-channel FET Q2, and the source s of the second N-channel FET Q2 For the anode of the clamp network 400, the gate g of the second N-channel FET Q2 is connected to the clamp control signal, as shown in FIG. 2-1;

(2) One end of the third capacitor C3 is the anode of the clamp network 400, the other end of the third capacitor C3 is connected to the source s of the second N-channel FET Q2, and the drain d of the second N-channel FET Q2 For the cathode of the clamp network 400, the gate g of the second N-channel FET Q2 is connected to the clamp control signal, as shown in Figure 2-2.

It can be seen that the anode of the clamp network 400, the cathode, and the body diode of the second N-channel FET Q2 therein are corresponding. In Figure 2-1, the anode of the body diode of Q2 is the anode of 400. The cathode of Q2's body diode passes through C3 and is the cathode of 400. In Figure 2-2, the cathode of Q2's body diode is 400 cathode, and the anode of Q2's body diode passes through C3 and is 400 anode. When Q2 is replaced by For a P-channel FET, ensure that the body diode inside the P-channel FET is in the same direction as the body diode in Figure 2-1 or Figure 2-2.

End of the same name: one end of the winding in the figure marked with a black dot;

Heterogeneous end: one end of the winding in the figure where there is no black mark;

Driving control signal: including various pulse waves such as PWM pulse width modulation signal and PFM pulse frequency modulation;

Clamp control signal: includes various square waves such as PWM pulse width modulation signal and PFM pulse frequency modulation, but appears differently from the drive control signal;

Transformer B: the first primary winding N _P1 and the second primary winding N _{P2 are} in the figure, the cores are connected by a broken line, indicating that they are wound around a transformer and share the same core, not a separate transformer, just for The graphics are clear and the connection relationship is simple, and the drawing method in the figure is used.

In Figure 2-1 and Figure 2-2, the source of the N-channel FET Q1 is connected to the same end of the second primary winding N _P2 , and the connection point is simultaneously connected to the negative terminal of the input DC power supply U _DC - that is, the FET The source of Q1 is connected to the negative terminal of the input DC power supply U _DC - which does not exist directly in practical applications. This is because in the field of switching power supply, the analysis of the working principle of the basic topology will omit unnecessary factors. In practical applications, the source of the FET is connected to a current sense resistor or a current transformer to detect the average current or peak current to implement various control strategies. The current sense resistor or current transformer is connected to the source. Equivalent to being directly connected to the source, which is well known in the art, this application follows the industry default rules. If a current transformer is used, the current transformer can appear anywhere in the excitation circuit, such as the drain of a FET, such as the same or different end of the first primary winding, and the current transformer has a conventional primary side. It is also a Hall sensor that is a "wire" and a magnetic core transformer whose secondary side is a multi-turn coil.

Working principle: Referring to Figure 2-1 and Figure 2-2, when the clamp network 400 is replaced by a diode with the same direction as the body diode, it is the prior art circuit of Figures 1-3, but the present invention is clamped. After the network 400, the working principle of the circuit is completely different from the prior art;

Figure 2-1 and Figure 2-2 show the working diagram of the circuit at power-on as shown in Figure 2-3, the second N-channel FET Q2 (for the convenience of analysis, according to the standard of the textbook, hereinafter referred to as the effect tube Q2 or Q2, other The device does not work. Q1 does not work because it does not receive the drive control signal. It is equivalent to an open circuit. Then the power supply U _DC is charged to C1 through the first primary winding N _P1 , and the current is simultaneously passed through the second primary winding N _{P2 .} To the negative end of the power supply U _DC , the charging current of the first primary winding N _P1 is: flowing from the same name end to the different name end; the charging current of the second primary winding N _P2 is: flowing from the different name end to the same name end; N _P1 And N _P2 is a two-wire winding, the two currents are equal in magnitude, and the generated magnetic flux is opposite, completely canceled, that is, at the time of power-on, the power source U _DC charges C1 through the two windings of the transformer B, and the two windings are mutually sensible. The function cancels and does not work. C1 is equivalent to the DC internal resistance of N _P1 and N _P2 in parallel with the power supply U _DC . C1 still functions as power supply filtering and decoupling; as time passes, the terminal voltage of C1 is equal to The voltage of U _DC is positive left and right negative.

When Q1 receives the control signal normally, taking one cycle as an example, when the gate of Q1 is high, Q1 is saturated and its internal resistance is equal to the on-state internal resistance R _ds(ON) . For the convenience of analysis, this is the case. As a straight-through, it is a wire. As shown in Figure 2-4, Q2 is in the off state and does not participate in the work. In the figure, 400 is drawn as an open state; at this time, two excitation currents are generated, 41 in Figure 2-4. And 42;

It can be seen that the excitation currents of 41 and 42 are in parallel. Since the inductances of N _P1 and N _P2 are the same, the excitation voltages are the same, and they are equal to U _DC , 41 and 42 are completely equal. During the excitation process, the secondary winding N _{S is} pressed. The induced voltage is the same. The induced voltage is: a positive voltage is induced at the same name, and a negative voltage is induced at the opposite end. The magnitude is equal to U _DC multiplied by the turns ratio n, that is, N _S induces a positive and negative voltage. The D2 is forwarded and the capacitor C2 is charged through the inductor L1 through the positive conduction D2. The charging current is as shown in 43a, and Vout establishes a voltage or continuously outputs energy. During the Q1 conduction excitation process, the secondary side has an energy output, which is characteristic of the forward converter.

During the excitation process, the excitation currents of 41 and 42 excluding the secondary side mapping current increase linearly upward; the current direction flows from the same name end to the different name end in the inductance;

In order to ensure that the electromagnetic compatibility meets the requirements for use, it is tricky when wiring. Observe 41 and 42 in Figure 2-4, 41 is the clockwise current direction, and 42 is the counterclockwise direction. If it is on the board, it is also guaranteed. The two currents are clockwise and the other is counterclockwise, that is, the physical path of the excitation current of the first primary winding and the second primary winding is opposite when the PCB is wired, and the magnetic flux generated during the excitation is farther. The local observation can be offset, so that the EMI performance of the forward switching power supply of the present invention will be very good.

When the gate of Q1 changes from high level to low level, Q1 also turns off from saturation conduction, because the current in the inductor can not be abrupt, even though Q1 is off at this time, the secondary side mapping current disappears, but 41 and 42 The excitation current still flows from the same name to the opposite end. Although this current is small, since the current loop of the primary side has been cut, the energy in the core flows from the same name to the opposite end on the secondary side, see Figure 2-5. The secondary winding N _S attempts to flow from the same name end to the different name end. This current can turn on D3, but it cannot be generated due to D2 reverse bias, and the current of 43a in Figure 2-4 flows through L1, and the inductance The current cannot be abruptly changed, and the current seeking path of 43a continues to flow, forming a freewheeling current as shown by 43b, starting from the right end of the inductor L1, to the positive end of C2, to the negative end of C2, to the anode of D3, and then to the D3. The cathode returns to the left end of the inductor L1.

The basic forward converter in a forward switching power supply is an ideal isolated version of the Buck converter, which is also commonly referred to as a forward transformer;

The circuit for demagnetizing the circuit of the present invention is composed of a clamp network 400 composed of Q2 and C3 and a second primary winding N _P2 , and the working principle is:

The first primary winding N _P1 and the second primary winding N _P2 are wound in two lines, and the leakage inductance between the two windings is zero. The energy of the exciting current is not transmitted to the secondary side at the instant of Q1 turn-off and after. The electric energy of the exciting current in the second primary winding N _{P2 is} in the same direction as the direction of the excitation, flowing from the same end to the opposite end, that is, in FIG. 2-5, flowing from bottom to top, opening the body diode of Q2 The current flows from the source s of Q2 to the drain d, and this electric energy charges C3 to form a demagnetizing current of the exciting current indicated by 44;

The electric energy of the exciting current in the first primary winding N _P1 is coupled to the second primary winding N _P2 without leakage inductance, and is demagnetized by the body diode of Q2, also forming the demagnetization of the exciting current indicated by 44. Current

In Figure 2-5, the portion of Q2 that is not active is painted in light, and the active body diode is shown in dark.

Then, during the freewheeling of the first diode D3, a variety of operating modes occur, that is, after the C3 absorbs the energy generated by the excitation, there are many working modes of the circuit, and the working principle is as follows:

(1) If Q2 is not conducting, after the body diode charges C3, then Q2 is turned on, then the voltage on C3 is connected in series with U _DC , and the voltage is induced in the first primary winding N _P1 by the forward working mode. And output energy to the power source U _DC through the body diode of Q2, the capacity of the C3 is small, the energy loss is large, and the partial excitation current energy is partially recovered and utilized;

(2) If Q2 and the body diode are synchronously turned on or lag-on, during the D2 cut-off period, the primary side inductor and C3 resonate, and this resonance is used to realize the zero voltage switch of the main power tube Q1 (Zero Voltage Switch is abbreviated as ZVS). Also known as soft-switching technology, the primary side excitation current energy is recycled. This mode is extremely complicated and has dozens of working modes.

(3) If Q2 is not conducting, the body diode charges C3, and then the next cycle is followed by charging. In a certain cycle after multiple cycles, Q2 and the body diode are turned on or lag-on, during the D2 cutoff period, the primary side Inductive, at this time, the primary inductance and C3 resonance, using this resonance to achieve the ZVS mode of the main power tube Q1, to achieve the recovery of the primary side excitation current energy, is also very complicated, there are dozens of working modes.

Because the active clamp is used, the demagnetization can still be well deviated when the duty ratio is greater than 0.5. In the circuit of the prior art as shown in Figures 1-3, the demagnetization of D1 is restricted by the law of volt-second balance. The demagnetization time is equal to the excitation time, which determines that the duty cycle can only be less than 0.5, and the present invention does not have this restriction.

Since the excitation currents of 41 and 42 are the same, the first primary winding and the second primary winding have the same wire diameter, so that the winding is convenient, the wire diameters described herein are the same, and they are all of the same size Litz wire, color. It can be different, that is, multi-strand stranding. For convenience of identification, the same specification wire including the Litz wire can have different colors. As the operating frequency increases, the high frequency current tends to flow on the surface of the enameled wire. In this case, the Litz wire can solve this problem. Of course, two different colors of enameled wire are used to make the Litz wire first, and the first primary winding and the second primary winding are separated by color, or the wire diameter and the number of strands of the two windings are different. , both achieve the purpose of the invention.

It can be seen that, compared with the existing forward converter, the present invention has the following beneficial effects: the duty ratio can be greater than 0.5, so that the power density is higher, and the conversion efficiency is not reduced, and the zero voltage switch of the switch tube can be realized, thereby Further improve the conversion efficiency.

Second embodiment

The present invention also provides the equivalent scheme of the above first embodiment. Corresponding to the second scheme, referring to FIG. 3-1 and FIG. 3-2, a forward switching power supply includes a transformer B, a first N-channel FET Q1, a capacitor C1, a second capacitor C2, a clamp network 400, a second diode D2, a first diode D3, a first inductor L1, and a transformer B includes a first primary winding N _P1 and a second primary winding N _P2 and the secondary winding N _S , the clamp network 400 includes at least an anode and a cathode, the secondary winding N _{S has the} same name end connected to the second diode D2, and the second diode D2 is simultaneously connected to the first diode The cathode of D3 is connected to one end of the first inductor L1, and the other end of the first inductor L1 is connected to one end of the second capacitor C2, and the output is positive, which is the + end of Vout in the figure, and the opposite side of the secondary winding N _S is simultaneously The anode of the first diode D3 and the other end of the second capacitor C2 are connected, and the output is negative, which is the end of Vout in the figure; the positive terminal of the input DC power source U _DC + the drain of the N-channel FET Q1 simultaneously d, a second primary winding _P2 N-phase terminal is connected, N channel field effect transistor Q1 is connected to the source s and the first primary winding _Pl dotted terminal of N Cathode of the second primary winding N _P2 dot end of the clamp network 400 is connected to an anode connected to a first primary winding N _P1 dotted end of the clamp network 400, while the connection point of the negative terminal of the DC power source connected to the input of U _DC - The gate g of the N-channel FET Q1 is connected to the driving control signal; the first primary winding N _P1 and the second primary winding N _P2 are wound in two lines, and one end of the first capacitor C1 and the first primary winding N _P1 is connected to the same name end, and the other end of the first capacitor C1 is connected to the same end of the second primary winding N _P2 . The clamp network 400 includes at least a third capacitor C3 and a second N-channel FET Q2, a third capacitor C3 and a The two N-channel FETs Q2 are connected in series, and the series connection is one of the following two ways:

(1) One end of the third capacitor C3 is the cathode of the clamp network 400, the other end of the third capacitor C3 is connected to the drain d of the second N-channel FET Q2, and the source s of the second N-channel FET Q2 For the anode of the clamp network 400, the gate g of the second N-channel FET Q2 is connected to the clamp control signal, as shown in FIG. 3-1;

(2) One end of the third capacitor C3 is the anode of the clamp network 400, the other end of the third capacitor C3 is connected to the source s of the second N-channel FET Q2, and the drain d of the second N-channel FET Q2 For the cathode of the clamp network 400, the gate g of the second N-channel FET Q2 is connected to the clamp control signal, as shown in Figure 3-2.

In fact, the second embodiment is an equivalent variant of the first embodiment: on the basis of FIG. 2-1 of the first embodiment, the series devices of the two excitation circuits are interchanged, that is, the positions of N _P1 and Q1 are interchanged. At the same time, the clamp network 400 and N _{P2 are} interchanged, and C1 is still connected between the connection points of the two series devices, and the circuit of FIG. 3-1 of the second embodiment is obtained, since the source voltage of Q1 is varied. Therefore, this circuit is floating drive, but obtains the direct drive of the second N-channel FET Q2 for clamping. On the basis of Figure 3-1, C3 and Q2 in the clamp network 400 are mutually Change the position to get the circuit of Figure 3-2.

A brief description of its working principle:

Referring to Figure 3-1 and Figure 3-2, when the circuit is powered on, Q2 does not work. Q1 does not work because it does not receive the control signal. It is equivalent to an open circuit. Then the power supply U _DC charges C1 through N _P2 . Returning to the negative terminal of the power supply U _DC through N _P1 , also at the time of power-on, the power supply U _DC charges C1 through the two windings of the transformer B, and the magnetic fluxes of the two windings cancel out due to the mutual inductance, and the C1 is equivalent. In parallel with the power supply U _DC through the DC internal resistance of N _P2 and N _P1 , C1 still functions as power supply filtering and decoupling;

Over time, the terminal voltage of C1 is equal to the voltage of U _DC , right and left negative;

When Q1 is saturated and turned on, its internal resistance is equal to the on-state internal resistance R _ds(ON) , which is regarded as a wire as before, and two exciting currents are generated at this time.

The first way is: the positive end of the power supply U _DC enters through the drain of Q1, the source of Q1 is out, and then enters through the same name of the first primary winding N _P1 , and the different name of N _P1 is output, returning to the power supply U _DC Negative end

The second way is: the right positive end of the capacitor C1 passes through the same name of the second primary winding N _P2 , the different name of N _P2 is output, the drain of Q1 enters, the source of Q1 is out, and the left negative end of capacitor C1 is returned. ;

The first and second excitation currents are in parallel. Since the inductances of N _P1 and N _P2 are the same and the excitation voltages are the same, they are equal to U _DC . The two paths are completely equal. During the excitation process, the secondary windings N _{S are} pressed. The analogy produces the induced voltage. The same name induces a positive voltage. The opposite end induces a negative voltage. The magnitude is equal to U _DC multiplied by the turns ratio n. That is, N _S induces a positive and negative voltage. This voltage causes D2 to guide. Pass, and through the forward conduction D2, through the inductor L1 to charge the capacitor C2, the charging current is as shown in 43a, Vout establishes the voltage or continuously outputs energy.

During the excitation process, the first and second excitation currents increase linearly upward; the current direction flows from the same name end to the different name end in the excitation inductance of the transformer;

In the second embodiment, the circuit for demagnetizing the circuit of the present invention is composed of a clamp network 400 composed of Q2 and C3 and a second primary winding N _P2 , and the working principle is:

At the instant of Q1 turn-off and after, the energy of the exciting current is not transmitted to the secondary side, and the electric energy of the exciting current in the second primary winding N _{P2 is} in the same direction as the direction of the excitation, flowing from the same end to the opposite end, Flowing downwards and upwards, turning on the body diode of Q2, current flows from the source s of Q2 to the drain d, and this electric energy is charged to C3 to form a demagnetizing current of the exciting current indicated by 44; likewise, the first primary winding N The electric energy of the exciting current in _P1 is coupled to the second primary winding N _P2 without leakage inductance, demagnetization is achieved by the clamping network 400, and a demagnetizing current loop of the exciting current is also formed; in the first diode D3 During the opening of the freewheeling, a variety of operating modes occur, as described in the first embodiment, that is, after C3 absorbs the energy generated by the excitation, there are many operating modes of the circuit, which are not described herein.

The second embodiment is a modification of the first embodiment, and the working principle is equivalent, and the object of the invention is also achieved. Similarly, Q2 can be replaced with a P-channel FET. To ensure that the body diode inside the P-channel FET is in the same direction as the body diode in Figure 3-1 or Figure 3-2, it can work normally.

The above is only a preferred embodiment of the present invention, and it should be noted that the above preferred embodiments are not to be construed as limiting the invention. It will be apparent to those skilled in the art that several modifications and refinements can be made without departing from the spirit and scope of the present invention, such as adding a control loop to achieve regulation of the output, which is apparent from the prior art. Yixian sees, such as the switch tube Q1 with other symbols, the secondary output is added to multiple outputs, and the filter uses π-type filtering; for the purpose of improving efficiency, a parallel between the drain and the source of the FET A diode with the same low voltage drop and fast recovery in the direction of the body diode. This improvement is well known in the art and should be considered equivalent to a body diode; these improvements and retouchings should also be considered as protection scope of the present invention, and no further description is given here. The scope of protection of the present invention should be determined by the scope defined by the claims.

Claims

A forward switching power supply includes a transformer, a first N-channel FET, a first capacitor, a second capacitor, a clamp network, a first diode, a second diode, a first inductor, and a transformer The first primary winding, the second primary winding and the secondary winding, the clamping network comprises at least an anode and a cathode, the secondary winding has the same name end connected to the second diode anode, and the second diode cathode is simultaneously connected with the first two The cathode of the pole tube is connected to one end of the first inductor, and the other end of the first inductor is connected to one end of the second capacitor, and the output is positive, and the opposite end of the secondary winding is simultaneously connected with the anode of the first diode and the second capacitor. One end is connected and the output is negative; the positive terminal of the input DC power supply is simultaneously connected with the same name end of the first primary winding and the cathode of the clamp network, and the first primary winding different name end and the drain of the first N-channel FET Connected; the anode of the clamp network is connected to the opposite end of the second primary winding, the source of the first N-channel FET is connected to the same end of the second primary winding, and the connection point is simultaneously connected to the negative terminal of the input DC power supply; Gate of N-channel FET The driving control signal is connected; the first primary winding and the second primary winding are wound in a double line, and one end of the first capacitor is connected to the different end of the first primary winding, and the other end of the first capacitor and the second primary winding The heteromorphic end is connected, and the clamp network includes at least a third capacitor and a second N-channel FET, and the third capacitor and the second N-channel FET are connected in series, and the series connection is one of the following two ways:

(1) One end of the third capacitor is the cathode of the clamp network, the other end of the third capacitor is connected to the drain of the second N-channel FET, and the source of the second N-channel FET is the anode of the clamp network, a gate connection clamp control signal of the two N-channel FET;

(2) one end of the third capacitor is the anode of the clamp network, the other end of the third capacitor is connected to the source of the second N-channel FET, and the drain of the second N-channel FET is the cathode of the clamp network, The gate of the two N-channel FET is connected to the clamp control signal.
The forward switching power supply according to claim 1, wherein the first N-channel FET is replaced with a P-channel FET, the body diode inside the P-channel FET and the first N-channel FET inside The body diodes have the same polarity.
A forward switching power supply includes a transformer, a first N-channel FET, a first capacitor, a second capacitor, a clamp network, a first diode, a second diode, a first inductor, and a transformer The first primary winding, the second primary winding and the secondary winding, the clamping network comprises at least an anode and a cathode, the secondary winding has the same name end connected to the second diode anode, and the second diode cathode is simultaneously connected with the first two The cathode of the pole tube is connected to one end of the first inductor, and the other end of the first inductor is connected to one end of the second capacitor, and the output is positive, and the opposite end of the secondary winding is simultaneously connected with the anode of the first diode and the second capacitor. One end is connected and forms an output negative; the positive terminal of the input DC power supply is simultaneously connected to the drain of the first N-channel FET and the different end of the second primary winding, and the source of the first N-channel FET is first The primary winding is connected to the same name end; the second primary winding has the same name end connected to the cathode of the clamp network, and the first primary winding different name end is connected to the anode of the clamp network, and the connection point is simultaneously connected to the negative end of the input DC power supply; Gate of an N-channel FET Connecting the driving control signal; the first primary winding and the second primary winding are two-wire winding, one end of the first capacitor is connected to the same end of the first primary winding, and the other end of the first capacitor has the same name as the second primary winding The terminals are connected, and the clamp network includes at least a third capacitor and a second N-channel FET, and the third capacitor and the second N-channel FET are connected in series, and the series connection is one of the following two ways.
The forward switching power supply according to claim 3, wherein the first N-channel FET is replaced with a P-channel FET, the body diode inside the P-channel FET and the first N-channel FET inside The body diodes have the same polarity.
The forward switching power supply according to any one of claims 1 to 4, characterized in that the first primary winding and the second primary winding have the same wire diameter.
The forward switching power supply according to any one of claims 1 to 4, characterized in that the physical path of the exciting current of the first primary winding and the second primary winding is opposite in direction when the PCB is wired.