WO2019001216A1

WO2019001216A1 - Active clamping forward switch power supply circuit

Info

Publication number: WO2019001216A1
Application number: PCT/CN2018/089576
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: CN107196515A

Abstract

An active clamping forward switch power supply. On the basis of an LCL forward converter, the dotted terminal of a first primary winding (N P1) in a transformer (B) is connected to a power supply, and the dotted terminal of a second primary winding (N P2) is grounded; the first and second primary windings are in double-wire parallel winding; one end of a capacitor (C1) is connected to the non-dotted terminal of the first primary winding, while the other end is connected to the non-dotted terminal of the second primary winding; the non-dotted terminal of the first primary winding is connected to the power supply by means of a first field effect transistor (Q1), and the non-dotted terminal of the second primary winding is connected to the power supply by means of a second field effect transistor (Q2) and a clamping network (400) connected in series to a third capacitor (C3). The duty ratio of the switch power supply may be greater than 0.5, and demagnetized energy is recycled to achieve the zero-voltage turning on of a main power switch tube, thereby further reducing loss and especially increasing efficiency in a light load state.

Description

Active clamp forward switching power supply circuit

Technical field

The present invention relates to the field of switching power supplies, and more particularly to single-ended forward switching power supplies that use active clamping.

Background technique

At present, the switching power supply is widely used, and the industry is often called a converter. The basic forward converter in the forward switching power supply is an ideal isolated version of the Buck converter. The common topology has a single-ended forward converter and a symmetric drive. Half-bridge converter, full-bridge converter, push-pull converter, symmetric push-pull forward converter, etc. 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 the convenience of loss, the inventors have defined the topology used by the forward switching power supply, including the flyback topology, and the basic topology excluding the demagnetization mode is defined as: LCL converter, derived from the two The primary side is a magnetizing inductance 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 circuit using an active clamp, the duty ratio can be greater than 0.5, the power density is high, and the conversion efficiency is simultaneously Without reducing, the zero voltage switch of the switch tube can be realized, further improving the conversion efficiency.

The object of the present invention is achieved by an active clamp forward switching power supply circuit including a transformer, a first N-channel FET, a first capacitor, a second capacitor, a clamp network, and a first two a pole tube, a second diode, a 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 and a second pole The anode of the tube is connected, the cathode of the second diode 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 secondary winding is different. The terminal is simultaneously connected with 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 source is simultaneously connected with the cathode of the same name of the first primary winding and the cathode of the clamp network, the first primary side The twisted end of the winding is connected to the drain of the first N-channel FET; 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 second primary winding with the same name End, the connection point is connected at the same time a negative terminal of the current source; a 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 different from the first primary winding The other end of the first capacitor is connected to the second end of the second primary winding, and the clamp network includes at least a third capacitor and a second N-channel FET, a third capacitor and a second N-channel. The MOSFETs 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 active clamp forward switching power supply circuit works by: the first N-channel FET is turned on every cycle, and the second N-channel FET is turned on when the active clamp forward switching power supply is nearly full. Each cycle is open;

The first N-channel FET is turned on every cycle, and the second N-channel FET is turned on every m cycles when the active clamp forward switching power supply is lightly loaded, m is a positive integer, and the active clamp The lighter the load of the forward switching power supply, the larger the value of m.

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 improvement of the above two schemes, when the second N-channel FET Q2 is lightly loaded by the switching power supply, m can jump back and forth between an integer m and a value (m+1) which is larger than the integer by 1 to further Good consideration of design.

As an improvement of the above two schemes, a low-voltage drop and fast recovery in the same direction as the body diode of the second N-channel FET are connected in parallel between the drain and the source of the second N-channel FET. Diode.

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, and at the same time, the energy recovery of the demagnetization circuit is realized, and further, the zero voltage turn-on of the main power switch tube is realized, further reducing the loss and improving the conversion efficiency, especially at light load. When the conversion efficiency is improved.

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 a second embodiment of the present invention, and the clamp network adopts the (2) mode.

Detailed ways

First embodiment

2-1 and 2-2 are schematic diagrams showing an active clamp forward switching power supply according to a first embodiment of the present invention, including a transformer B, a first N-channel FET Q1, and a first capacitor C1. a second capacitor C2, a clamp network 400, a first diode D3, a second diode D2, a first inductor L1, and a transformer B includes a first primary winding N _P1 , a second primary winding N _{P2 ,} and a pair The side winding N _S , the clamp network 400 includes at least an anode and a cathode, the secondary winding N _S is connected to the second diode D2 at the same end, and the second diode D2 is simultaneously connected to the cathode of the first diode D3. One end of the first inductor L1 is connected, and the other 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 secondary winding N _S is simultaneously named with the first diode. The anode of the tube 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 end of the input DC power source U _DC + the same name end of the first primary winding N _P1 , the clamp network The cathode of 400 is connected, and 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 original The side winding N _P2 is connected at different ends, 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 source U _DC - N-channel FET Q1 The gate g is connected to the driving control signal; the first primary winding N _P1 and the second primary winding N _P2 are wound in a double line, and one end of the first capacitor C1 is connected to the different end of the first primary winding N _P1 , The other end of a capacitor C1 is connected to the second-side winding N _P2 , and the clamp network 400 includes at least a third capacitor C3 and a second N-channel FET Q2, a third capacitor C3 and a second N-channel field effect. Tube Q2 is 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. 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: See Figure 2-1 and Figure 2-2. When the clamp network 400 uses one and Q2 (for the convenience of analysis, according to the standard of textbooks, hereinafter referred to as effect tube Q2 or Q2, other devices are the same) body diode direction When the same diode is substituted, it is the prior art circuit of FIG. 1-3. However, after the clamp network 400 is added to the present invention, the working principle of the circuit is completely different from that of the prior art;

Figure 2-1 and Figure 2-2 show the operation diagram of the circuit at power-on as shown in Figure 2-3. The second N-channel FET Q2 does not work. Q1 does not work because it does not receive the drive control signal. It is equivalent to an open circuit. The power supply U _DC is charged to C1 through the first primary winding N _P1 , and the current is simultaneously returned to the negative terminal of the power supply U _DC through the second primary winding N _P2 , and the charging current of the first primary winding N _P1 is: from the same end The flow direction of the second primary winding N _P2 is: flowing from the different name end to the same name end; N _P1 and N _P2 are two lines and winding, the two currents are equal in magnitude, and the generated magnetic flux is opposite, completely Offset, that is, at power-on, the power supply U _DC charges C1 through the two windings of the transformer B. These two windings cancel out due to the mutual inductance, and the C1 is equivalent to the DC internal resistance and power supply through N _P1 and N _P2 . U _{DC is} connected in parallel, and C1 still functions as power supply filtering and decoupling; as time goes by, the terminal voltage of C1 is equal to the voltage of U _DC , and the left is right and the right is 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. When the two currents are clockwise and the other is counterclockwise, the magnetic flux generated during the excitation can be offset at a distance, 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, and there are many working modes of the circuit. Here, the working principle is stated:

The working method of the active clamp forward switching power supply of the present invention is defined as: the first N-channel FET Q1 is turned on every period, and the second N-channel FET Q2 is near the full load of the forward switching power supply. Each cycle is turned on, that is, Q2 and the body diode are synchronously turned on or delayed. During the D2 conduction period, the primary winding is in an excited state; when Q1 is turned from on to off, D2 is turned from on to off. The first diode D3 is turned from off to on, which is a freewheeling of L1, that is, during the off period of D2, the primary side is inductive. At this time, the primary side inductance and C3 resonate, and at the time of design, the value of C3 is calculated. During the resonance process, the terminal voltage of C3 will be close to zero or equal to zero at a certain time, and the moment of resonance to zero at the C3 terminal can occur at the moment of D3 cutoff or later. Since the primary windings N _P2 and N _P1 are double wound and wound, the leakage inductance between the two is zero, that is, the terminal voltages of the primary windings N _P2 and N _P1 remain equal at any time, then the terminal voltage of C3 is close. At or equal to zero, Q2 is in a saturated conduction state, and its terminal voltage is also zero, then the terminal voltage of the clamp network 400 is close to or equal to zero, that is, the terminal voltage of the primary winding N _P2 is equal to the voltage of the DC power source U _DC , and For the upper positive and negative, then, the primary winding N _P1 is lower positive and negative, the terminal voltage of Q2 is twice the U _DC voltage at this moment; and when the terminal voltage of C3 is resonated to twice the U _DC voltage, When the upper positive and negative are negative, that is, the source s voltage of Q2 is -U _DC , since the terminal voltage of C1 is always left positive and negative negative, and equal to U _DC , at this moment, the left terminal voltage of C1 is zero volt, that is, Q1 The terminal voltage is also zero volts. If Q1 is saturated and turned on at this moment, then zero voltage turn-on of Q1 is realized (Zero Voltage Switch is abbreviated as ZVS, where only zero voltage turn-on is implemented, which is quasi-ZVS), also known as soft. Switching technology, to achieve the recycling of the primary side excitation energy, the energy on the output capacitance of Q1, also because of the harmonic They are transferred to achieve recycling primary excitation energy.

In order to achieve a longer resonance time, the capacity of C3 is relatively large. Because C3 is large and the terminal voltage can be increased, the voltage of the terminal voltage and the voltage of the DC power supply U _DC are in series. Using the volt-second balance law, the duty ratio can be greater than 0.5, and all can work normally.

When the active clamp forward switching power supply of the present invention is lightly loaded, if Q2 is turned on every cycle, the Q2 driving loss cannot be ignored, and the Q2 driving loss P _Q21 is: P _Q21 = 0.5 × C _iss × ( U _Q2 ) ² × f, C _iss is the input capacitance of Q2, U _Q2 is the gate drive voltage of Q2, f is the operating frequency, it can be seen that Q1 is turned on every cycle, and Q1 can also be reduced by optimizing the frequency reduction. Loss, but this method still meets the definition of opening every cycle, but the cycle is lengthened; Q2 is turned on every few cycles when the forward switching power supply is lightly loaded, which can reduce the light load when the power supply is light. The loss principle is as follows: when the switching power supply is lightly loaded, the excitation current of the primary side controlled by the loop is small, the energy stored in the primary side inductance is small, and the capacity of C3 is relatively large, so that when Q1 is turned off When Q2 is not turned on, the primary side inductance energy is small, and the primary side inductance energy is: 0.5 × primary inductance + excitation current square; primary inductance energy is charged to C3 through Q2 body diode, if clamped The energy absorbed by the capacitor C3 reaches the threshold, then the next cycle Q2 is On, which may be set in advance by the load, i.e. by the size of the duty ratio set in advance. If C3 does not reach the threshold, the circuit repeats the last cycle of action. Since the energy stored in C3 in the previous cycle is not released, the C3 energy increases again on the basis of the previous cycle, and the voltage across C3 rises again to a stable value. After several cycles, the voltage across C3 will increase in a step-like manner until the threshold is reached, that is, Q2 will be turned on in the next cycle to achieve resonance, and zero voltage turn-on of Q1 is realized. Thus, the drive loss is obtained. Further reduction.

In order to further improve the efficiency, a diode with the same low-voltage drop and fast recovery in the same direction as the Q2 body diode is connected in parallel between the drain and the source of the field effect transistor Q2. This improvement is a well-known technique and should be regarded as equivalent to a body diode. The invention is no longer protected by the embodiments;

Obviously, m is a positive integer, and the lighter the active clamp forward switching power supply load, the smaller the excitation current, then the larger the value of m, the more the C3 can absorb the excitation current energy, so the drive loss is obtained. Further reduction, then this mode of operation is also allowed: when the second N-channel FET Q2 is lightly loaded by the switching power supply, m can be between an integer m and a value (m+1) greater than the integer by one. Change back and forth to better balance the design.

In light load work, perhaps people in the technical field will have a question, such as: the original circuit works in Q1 for 5 cycles, Q2 only participates in the turn-on, then, in the 5 cycles of Q1 work, one of the cycles The duty cycle suddenly increases, the excitation current rises sharply, and the energy is too large, which may cause the voltage across C3 to exceed the threshold value and cause circuit damage. In fact, this situation does not exist. In the paragraph 0050 of the patent application No. 201410186940.4, the principle of the flyback power supply is also discussed. It is also applicable to the forward power supply: the duty ratio of the main power conversion circuit is In the adjacent cycle, it can't be abrupt. For example, the current popular notebook power adapter uses 65KHz. The well-known theory believes that the loop response of its switching power supply can generally be one tenth, that is, 6.5KHz. Part of the principle can refer to Dr. Zhang Xingzhu's paper "Dynamic Small Signal Analysis and Design of Switching Power Supply". The loop response is high and is affected by the optocoupler delay in the switching power supply. It is difficult to improve, that is, the main power in the switching power supply. The duty cycle of the conversion circuit will change significantly in the first cycle and the tenth cycle in ten working cycles. In other words, the duty cycle of the main power conversion circuit cannot be in the adjacent cycle. mutation.

Because the active clamp is used, the demagnetization can be well detuned when the duty ratio is greater than 0.5. In the circuit of Figure 1-3, the demagnetization of D1 is limited by the law of volt-second balance, and the demagnetization time is equal to The excitation time determines that the duty cycle can only be less than 0.5, and the present invention does not have this constraint.

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 LCL converter, the invention has many differences, mainly: the duty ratio can be greater than 0.5, and at the same time, the energy recovery of the demagnetization circuit is realized, and further, the zero voltage of the main power switch tube is realized. Turning on further reduces the loss and improves the conversion efficiency, especially at light loads, the conversion efficiency is improved.

Second embodiment

The present invention further provides an equivalent solution of the above first embodiment. Corresponding to the second scheme, referring to FIG. 3-1 and FIG. 3-2, an active clamp forward switching power supply includes a transformer B, a first N-channel field effect. The tube Q1, the first capacitor C1, the second capacitor C2, a clamp network 400, the second diode D2, the first diode D3, the first inductor L1, and the transformer B includes a first primary winding N _P1 , The primary winding N _P2 and the secondary winding N _S , the clamping 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 A cathode of the diode 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 an output positive is formed, which is the + end of Vout in the figure, and the secondary winding N _{S is} different. The terminal is simultaneously connected to the anode of the first diode D3 and the other end of the second capacitor C2, 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 + simultaneously with the N-channel field effect transistor the drain D of Q1, a second primary winding connected to the phase terminal N _P2, N-channel MOSFET Q1 and the source s of the first primary winding N _P1 with Terminal coupled; N _P2 of the second primary winding is connected to the cathode terminal of the same name as a clamp network 400, a first primary winding N _P1-phase terminal connected to the anode of a clamping network 400, while the connection point of the DC power source connected to the input U The negative terminal of the _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, one end of the first capacitor C1 and the first A primary winding N _P1 is connected to the same end, and the other end of the first capacitor C1 is connected to the same end of the second primary winding N _P2 . The clamping network 400 includes at least a third capacitor C3 and a second N-channel FET Q2. The three capacitor C3 and the second N-channel FET 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.

The active clamp forward switching power supply works by: the first N-channel FET Q1 is turned on every cycle, and the second N-channel FET Q2 is close to full load when the active clamp forward switching power supply is near full load. Every cycle is open;

The first N-channel FET Q1 is turned on every period, and the second N-channel FET Q2 is turned on every m cycles when the active clamp forward switching power supply is lightly loaded, m is a positive integer, and The lighter the load of the source clamp forward switching power supply, the larger the value of m.

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 in the excitation circuit are interchanged, that is, N _P1 and Q1 are interchanged, and 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 fluctuating, This circuit is floating drive, but it obtains the direct drive of the second N-channel FET Q2 for clamping. The cost of driving is relatively low. Based on Figure 3-1, the clamp network 400 is used. The circuit of Figure 3-2 can be obtained by swapping the positions of C3 and Q2.

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 operation principle is the same as that of the first embodiment.

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. As a technical solution using an N-channel FET, it can also be realized by a P-channel field effect transistor. The P-channel FET has a low cost at a low operating voltage, and at this time, the basis of the first embodiment described above. In the above, the polarity of the same name of the power source, the diode, and the transformer is reversed, and the output rectification portion is not reversed. Then, the third embodiment is obtained, which is not described here, and is regarded as a non-creative labor by those skilled in the art using known techniques. It is equivalent to the first and second aspects of the present invention.

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; these improvements are known techniques and should be considered equivalent to body diodes; these improvements and retouching are also The scope of the present invention should be construed as the scope of the present invention, and the scope of the present invention should be defined by the scope of the claims.

Claims

An active clamp forward switching power supply circuit includes a transformer, a first N-channel FET, a first capacitor, a second capacitor, a clamp network, a first diode, and a second diode. 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 is connected to the second diode anode at the same end, the second diode The 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 end of the secondary winding is simultaneously connected with the first diode. The anode and the other end of the second capacitor are connected, and the output is negative; the positive terminal of the input DC power source is simultaneously connected with the cathode of the same name end of the first primary winding and the clamp network, and the first primary winding has a different name end and the first N groove The drain of the MOSFET is 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 input DC power supply. Negative end; first N-channel field The gate of the effect tube 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 opposite end of the first primary winding, and the other end of the first capacitor is The second primary winding is connected to the different name end, wherein the clamping network comprises at least a third capacitor and a second N-channel FET, and the third capacitor is connected in series with the second N-channel FET. The serial connection is one of the following two ways:

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

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

The first N-channel FET is turned on every period, and the second N-channel FET is turned on every cycle when the active clamp forward switching power supply is close to full load; When the active clamped forward switching power supply is lightly loaded, the two N-channel FETs are turned on every m cycles, m is a positive integer, and the active clamped forward switching power supply load is lighter, the value of m is more Big.
An active clamp forward switching power supply circuit according to claim 1, wherein a positive terminal of the input DC power supply is simultaneously connected to a drain of said first N-channel FET, said first Two primary windings are connected at different ends, and a source of the first N-channel field effect transistor is connected to the same end of the first primary winding; the second primary winding has the same name end and the clamp a cathode of the bit network is connected, the first primary winding different 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; the first N-channel FET is The gate 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 same end of the first primary winding The other end of the first capacitor is connected to the same end of the second primary winding.
The active clamp forward switching power supply circuit according to claim 1 or 2, wherein said second N-channel FET Q2 is at an integer m when the switching power supply is lightly loaded. Change back and forth between a value (m+1) that is one greater than the integer.
An active clamp forward switching power supply circuit according to claim 3, wherein a parallel and said first between said drain and source of said second N-channel FET The diodes of the same N-channel FET have the same body diode direction.
An active clamp forward switching power supply circuit according to claim 4, wherein said first primary winding and said second primary winding have the same wire diameter.
An active clamp forward switching power supply circuit according to claim 5, wherein said first primary winding and said second primary winding have opposite physical paths of excitation current .