CN101917121A - An Active Clamp Synchronous Rectification Forward Converter - Google Patents

Abstract

The invention discloses an active clamp synchronous rectification forward converter, and belongs to the technical field of electronics. The converter comprises a direct current input power supply, an input capacitor, a main transformer T1, a synchronous rectifier circuit, a filter circuit, a voltage sampling circuit, an optical coupling feedback circuit, a PWM control circuit and an active clamp circuit; the synchronous rectifier circuit consists of a rectifier tube and a driving circuit thereof, and a freewheeling tube and a driving circuit thereof, wherein the driving voltage of the rectifying tube is generated by a secondary auxiliary winding of the main transformer, and has the same phase as that of a first switching tube of the active clamp circuit; and the driving voltage of the freewheeling tube is generated by coupling the driving voltage of a second switching tube in the active clamp circuit, so the driving voltage has the same phase as that of the second switching tube in the active clamp circuit. The synchronous rectifier circuit of the forward converter adopted the fixed driving voltage, has the dead zone time same as the active clamp circuit, and can improve the conversion efficiency and reduce the loss of the active clamp synchronous rectification forward converter.

Description

An Active Clamp Synchronous Rectification Forward Converter

technical field

The invention belongs to the field of electronic technology, and relates to a DC/DC converter of active clamp synchronous rectification, especially a forward converter whose secondary adopts self-driven synchronous rectification.

Background technique

At present, an active clamp synchronous rectification forward converter usually includes a DC input power supply, an input capacitor, a main transformer T1, a synchronous rectification circuit, a filter circuit, a voltage sampling circuit, an optocoupler feedback circuit, a PWM control circuit and an active clamping circuit composition. Its circuit structure is shown in Figure 1: the DC input power supply Vin charges the input capacitor C1, and the discharge current of the input capacitor C1 is input to the primary end of the main transformer T1 with the same name; the primary end of the main transformer T1 with the same name through an active The clamp circuit is connected to ground. The active clamp circuit is composed of NMOS switch tube Q1, PMOS switch tube Q2, clamp capacitor C2, coupling capacitor C3 and clamp diode D1, and the clamp capacitor C2 and PMOS switch tube Q2 are connected in parallel with NMOS switch tube Q1 after being connected in series. The gate of the NMOS switch Q1 is connected to one drive output terminal of the PWM control circuit, the gate of the PMOS switch Q2 is connected to the other drive output terminal of the PWM control circuit through the coupling capacitor C3 on the one hand, and the other gate of the PMOS switch Q2 On the one hand, the clamping diode D1 is grounded, and the sources of the NMOS switch Q1 and the PMOS switch Q2 are grounded. The synchronous rectification circuit is composed of two NMOS switch tubes Q3 (rectifier tube) and Q4 (freewheel tube). The secondary end of the main transformer T1 with the same name is connected to the gate of the NMOS switch tube Q3 and the drain of the NMOS switch tube Q4. The main transformer T1 The secondary opposite terminal is connected to the gate of the NMOS switch Q4 and the drain of the NMOS switch Q3, and the sources of the NMOS switches Q3 and Q4 are grounded. The filter circuit is composed of an inductor L1 and a capacitor C4. One end of the inductor L1 is connected to the secondary terminal of the main transformer T1 with the same name, and the other end is grounded through the capacitor C4; the connection point of the inductor L1 and the capacitor C4 outputs the output voltage Vout of the entire converter, and the output voltage Vout It is sampled by the voltage sampling circuit, coupled and fed back to the PWM control circuit by the optocoupler feedback circuit.

In the aforementioned active clamp synchronous rectification forward converter, the driving signals of the NMOS switch Q1 and the PMOS switch Q2 are in a complementary relationship. When Q1 is turned off, C2, Q2 and the transformer primary excitation inductance and leakage inductance form a resonant circuit, which plays the following roles: 1. Reduce the turn-off withstand voltage of Q1; 2. Help the transformer core reset; 3. Reduce the leakage inductance energy Recycling; 4. Provide an ideal drive signal for synchronous rectification self-drive. In order to prevent Q1 and Q2 from being turned on simultaneously at alternate instants, there is a certain dead time between the driving signals of the two tubes. Since a negative voltage signal is required to drive the PMOS transistor Q2, the Q2 control signal generated by the PWM controller needs to pass through the coupling capacitor C3 and the clamp diode D1 to form a clamp reverse circuit to obtain a negative voltage drive signal to drive Q2. In the transformer secondary, NMOS transistors Q3 and Q4 are used to replace the traditional diode rectification and freewheeling, and the driving signals are respectively taken from the same-named and different-named terminals of the transformer secondary. This is the synchronous self-driven rectification technology. Since the turn-on voltage drop of the NMOS tube is much lower than that of the diode, the loss of the entire converter is reduced. Synchronous rectification technology is the main method to improve power supply efficiency in recent years. The inductor L1 and the capacitor C4 form a filter circuit to obtain a smooth DC output Vout. The voltage sampling is performed on Vout, and then the voltage sampling signal is fed back to the PWM controller through photoelectric coupling isolation, and the latter generates PWM signals to drive Q1 and Q2 according to the feedback voltage signal. This is the complete structure of the commonly used active clamp synchronous rectification self-driven power supply.

The power supply structure shown in Figure 1 does not require an additional synchronous rectification control chip, and has a simple structure and low cost. However, the driving voltage of the rectifier tube and freewheeling tube in this way changes with the input voltage. If the driving voltage is too high, it will cause additional gate drive loss. The pass loss increases. If the driving voltage is too high and exceeds the tolerance of the gate and source terminals of the MOS tube, it will cause the failure of the burnt tube. In addition, due to the existence of the parasitic capacitance between the gate and source of the MOS transistor, this method cannot effectively release the charge on the parasitic capacitance immediately when the MOS transistor is turned off, so that the driving voltage cannot quickly drop to zero, which will cause the rectifier and the continuous The phenomenon of common conduction of flow tubes. All of these factors lead to a decrease in the efficiency of the converter.

In order to solve the problems caused by too high or too low driving voltage of the rectifier tube and the freewheeling tube, Figure 2 shows an improved self-driven active clamp forward converter. The difference between this converter and the converter shown in Figure 1 is that the improved self-driven active clamp forward converter shown in Figure 2 adds two auxiliary windings N1 and N2 on the secondary side of the main transformer T1, respectively Provide driving voltage for the rectifier tube Q3 and the freewheeling tube Q4. By adding two auxiliary windings N1 and N2 to provide driving voltage for the rectifier tube Q3 and freewheeling tube Q4 respectively, and by changing the turn ratio of the auxiliary windings N1 and N2 to flexibly set the driving voltage range, it is possible to avoid using the secondary output to directly drive the rectifier tube and the problems caused by the freewheeling tube. However, in the improved self-driven active clamp forward converter shown in Figure 2, when the rectifier and freewheel are turned off, the gate drive voltage is negative, which may be due to the rectifier and freewheel In addition, there is no dead time between the driving signals of the rectifier tube and the freewheeling tube, and the common conduction of the rectifier tube and the freewheeling tube Still not resolved.

Contents of the invention

In order to solve the loss caused by the drive voltage of the rectifier tube and the freewheeling tube being too high or too low in the synchronous rectification circuit of the existing active clamp synchronous rectification forward converter and the common conduction phenomenon of the rectifier tube and the freewheeling tube Due to the problem of reduced efficiency, the present invention provides an active clamp synchronous rectification forward converter. The rectifier tube and the freewheeling tube of the forward converter synchronous rectification circuit adopt a fixed driving voltage, and there is a dead time between the rectifier tube and the freewheeling tube, which can improve the conversion efficiency of the active clamp synchronous rectification forward converter and Reduce loss.

Technical scheme of the present invention is as follows:

An active clamp synchronous rectification forward converter, as shown in Figure 3, includes a DC input power supply, an input capacitor, a main transformer T1, a synchronous rectification circuit, a filter circuit, a voltage sampling circuit, an optocoupler feedback circuit, and a PWM control circuit and active clamping circuits. The DC input power supply Vin charges the input capacitor C1, and the discharge current of the input capacitor C1 is input to the primary terminal of the main transformer T1; the primary terminal of the main transformer T1 is grounded through an active clamping circuit. The active clamp circuit is composed of NMOS switch tube Q1, PMOS switch tube Q2, clamp capacitor C2, coupling capacitor C3 and clamp diode D1, and the clamp capacitor C2 and PMOS switch tube Q2 are connected in parallel with NMOS switch tube Q1 after being connected in series. The gate of the NMOS switch Q1 is connected to one drive output terminal of the PWM control circuit, the gate of the PMOS switch Q2 is connected to the other drive output terminal of the PWM control circuit through the coupling capacitor C3 on the one hand, and the other gate of the PMOS switch Q2 On the one hand, the clamping diode D1 is grounded, and the sources of the NMOS switch Q1 and the PMOS switch Q2 are grounded. The synchronous rectification circuit is composed of the rectifier tube Q3, the driving circuit of the rectifier tube Q3, the freewheeling tube Q4 and the driving circuit of the freewheeling tube Q4: the secondary main winding of the main transformer T1 is connected to the drain of the freewheeling tube Q4, and the main transformer T1 The different name terminal of the secondary main winding is connected to the drain of the rectifier tube Q3, and the sources of the rectifier tube Q3 and the freewheeling tube Q4 are grounded; the driving circuit of the rectifier tube Q3 is composed of the main transformer T1, the secondary auxiliary winding N1, the capacitor C6, and the diode D2 , diode D3 and triode Q5: the terminal with the same name of the secondary auxiliary winding N1 of the main transformer T1 is connected to the cathode of the diode D2, the anode of the diode D3 and the base of the transistor Q5 through the capacitor C6, and the anode of the diode D2 and the collector of the transistor Q5 are grounded , the cathode of the diode D3 is connected to the emitter of the triode Q5 and the gate of the rectifier Q3. The driving circuit of the freewheeling tube Q4 is composed of capacitor C5, resistor R1, coupling transformer T2, capacitor C7, diode D4, diode D5 and triode Q6: the primary end of the coupling transformer T2 with the same name passes through the resistor R1 and capacitor C5 and then grounded, and the coupling transformer T2 The primary non-identical terminal of the active clamp circuit is connected to the gate of the PMOS switch Q2 in the active clamping circuit; the secondary homonymous terminal of the coupling transformer T2 is connected to the negative pole of the diode D4, the positive pole of the diode D5 and the base of the triode Q6 through the capacitor C7, and the diode D4 The anode of the diode D5 and the collector of the transistor Q6 are grounded, and the cathode of the diode D5 is connected to the emitter of the transistor Q6 and the gate of the freewheeling tube Q4. The filter circuit is composed of an inductor L1 and a capacitor C4. One end of the inductor L1 is connected to the secondary terminal of the main transformer T1 with the same name, and the other end is grounded through the capacitor C4; the connection point of the inductor L1 and the capacitor C4 outputs the output voltage Vout of the entire converter, and the output voltage Vout It is sampled by the voltage sampling circuit, coupled and fed back to the PWM control circuit by the optocoupler feedback circuit.

The operating principle of the active clamp synchronous rectification forward converter provided by the present invention is as follows:

The main switching tube Q1 and the clamping switching tube Q2 are alternately turned on. There must be a certain dead time between the driving signals of the main switching tube Q1 and the clamping switching tube Q2 to prevent the two switching tubes from being jointly conducted. A general active clamp controller has two output drive signals GD1 and GD2, which are used to drive the main switch tube Q1 and clamp switch tube Q2 respectively, and the dead time between the two drive signals can be set by the user .

The driving voltage of the secondary-side synchronous freewheeling transistor Q4 of the present invention is obtained by coupling the driving signal of the primary clamping switch transistor Q2. If the primary PWM drive signal GD1 is directly used to drive the secondary side rectifier tube Q3, and the inverse signal GD2 of this signal is used to drive the secondary side freewheeling tube Q4, there will be several problems: first, the PWM control signal There is a certain delay between the change and the response of the power part; second, the existence of transformer leakage inductance makes there is a delay when transferring energy from the primary side to the secondary side; third, usually the primary and secondary stages of the DC/DC converter The ground is isolated. If the reverse phase signal GD2 is used to directly drive the secondary side freewheeling tube Q4, the primary and secondary sides of the entire converter will no longer be isolated; fourth, there is no dead zone between the rectifier and the freewheeling tube time. However, the above method has some advantages: the PWM output driving range is fixed and the driving ability is strong, which is conducive to optimizing efficiency. As shown in Figure 3: the driving voltage of the freewheeling tube Q4 utilizes the PWM output driving signal on the primary side, and the working principle is as follows:

GD1 and GD2 are two-way drive signals output by the PWM controller, among which GD2 generates a certain delay dead time on the basis of GD1, that is, after GD1 becomes high level, GD2 becomes high level after a dead time period ; Before GD1 becomes low level, a dead zone time GD2 has become low level (see the schematic waveform in Figure 4). After GD2 passes through C3 and D1, the original positive pulse becomes negative pulse G2, which is used to drive the clamp switch tube Q2; G2 is transformed into G4 through a coupling transformer T2. The introduction of T2 has two functions: one is to realize the isolation of the primary and secondary, and the other is to adjust the amplitude of the gate drive voltage G4 of Q4 through the transformation ratio. T2 is an AC coupling transformer. A coupling capacitor C5 must be connected in series in the primary coil, otherwise the primary excitation inductance cannot be reset. The connection of resistor R1 is to reduce the inrush current when the capacitor is charged. The voltage at both ends of the secondary coil of the coupling transformer T2 is positive and negative alternately, and the positive value changes with the duty cycle of the primary driving voltage GD2. In order to keep the positive drive amplitude constant and turn off the negative voltage to zero, a capacitor C7 and a diode D4 are introduced. If the voltage drop of the diode D4 is neglected, the capacitor C7 and the diode D4 can convert the alternating positive and negative secondary voltage of the transformer into an ideal freewheeling tube driving signal changing between the positive maximum value and zero. The role of diode D5 and PNP transistor Q6 is the same as that of diode D3 and PNP transistor Q5. It is to quickly release the charge on the capacitance between the gate and source in the dead time when the MOS tube is turned off, so that the MOS tube can be quickly turned off to prevent common conduction.

The drive voltage of the rectifier MOS transistor Q3 comes from another auxiliary winding coil N1 of the main transformer T1. After entering the steady state, the waveform of the potential difference between S2+ and S2- at both ends of N1 is shown in Fig. 5 . At time t0, the main switch tube Q1 on the primary side is turned off, the transformer coil enters a reset state, and the voltage of the S2+ terminal of the coil N1 relative to the S2- terminal is negative. With the charging and discharging of the clamping capacitor C2 on the primary side, the negative voltage of N1 during the period from t0 to t1 is arc-shaped, with an arc-shaped peak in the middle. At time t0, diode D2 conducts forwardly to charge capacitor C6, and the voltage across C6 is charged to the voltage value of N1 minus the conduction voltage drop of D2, and the voltage polarity of C6 is negative on the left and positive on the right. From t0 to t1, D2 is always on, and the voltage at point A remains unchanged at about -0.7V.

After the time t1, the primary main switch is turned on, the polarity of the transformer coil N1 changes, S2+ is positive, and S2- is 0. The voltage across the capacitor C7 has not had time to change, which is consistent with the previous period, negative on the left and positive on the right. The voltage at point A is positive, which is equal to the superposition of the voltages of S2+ and C7. At this time, D2 is cut off by the reverse bias voltage, and D3 is turned on by the forward bias at this time. The driving voltage G3 of the rectifier tube Q3 is equal to the voltage of point A minus a diode voltage drop. When the rectifier tube Q3 is turned on. Time t2 is the same as time t0, the voltage at point A is -0.7V, the voltage of G3 is still positive, and the reverse bias of diode D3 is cut off. The EB pole current Ieb of the triode Q5 is positive, and Q5 works in an amplified state, and the larger current Iec can discharge the charge of the grid capacitance of Q3. The gate voltage of Q3 just drops to 0, and the rectifier Q3 is turned off. Since the triode Q5 is used to discharge and turn off the Q3, its switching speed is fast and the drive current loss is reduced. The working state after t2 is the same as t0 and starts to work periodically.

In the present invention, the reverse-phase driving voltage of the primary clamping transistor is coupled to the secondary for driving the freewheeling transistor, so the driving voltage of the freewheeling transistor Q4 has the same phase as the driving voltage of the primary clamping transistor Q2. The driving voltage of the rectifier tube Q3 has the same phase as the driving voltage of the primary main switching tube Q1. In this way, the dead time between the rectifier tube Q3 and the freewheeling tube Q4 is the same as that of the primary tubes Q1 and Q2. The dead time can be flexibly set in the PWM control circuit.

To sum up, the present invention has controllable and adjustable dead time and stable rectifier tube and freewheeling tube drive voltage, which can greatly improve the efficiency of the converter.

Description of drawings

Fig. 1 is a circuit structure diagram of an existing active clamp synchronous rectification forward converter.

Fig. 2 is a circuit structure diagram of an existing improved active clamp synchronous rectification forward converter.

Fig. 3 is a circuit structure diagram of the active clamp synchronous rectification forward converter provided by the present invention.

Fig. 4 is a waveform diagram of several driving voltages in the active clamp synchronous rectification forward converter provided by the present invention.

Fig. 5 is a waveform diagram of the drive voltage of the rectifier tube Q3 in the active clamp synchronous rectification forward converter provided by the present invention.

Detailed ways

The technical solution of the present invention has been fully and completely described in the content of the invention, and will not be repeated here.

Claims (1)

1. an active clamp synchronous rectification forward converter comprises direct-current input power supplying, input capacitance, main transformer T1, circuit of synchronous rectification, filter circuit, voltage sampling circuit, optocoupler feedback circuit, pwm control circuit and active clamp circuit;

Direct-current input power supplying Vin charges to input capacitance C1, and the discharging current of input capacitance C1 is input to the end elementary of the same name of main transformer T1; The elementary different name end of main transformer T1 is by an active clamp circuit ground connection; Wherein active clamp circuit is made up of nmos switch pipe Q1, PMOS switching tube Q2, clamp capacitor C2, coupling capacitance C3 and clamp diode D1, clamp capacitor C2 and PMOS switching tube Q2 series connection back are in parallel with nmos switch pipe Q1, the grid of nmos switch pipe Q1 connects a drive output of pwm control circuit, the grid of PMOS switching tube Q2 connects another drive output of pwm control circuit on the one hand by coupling capacitance C3, the grid of PMOS switching tube Q2 passes through clamp diode D1 ground connection, the source ground of nmos switch pipe Q1 and PMOS switching tube Q2 on the other hand;

Circuit of synchronous rectification is made up of the drive circuit of drive circuit, continued flow tube Q4 and the continued flow tube Q4 of rectifying tube Q3, rectifying tube Q3: the drain electrode of the termination continued flow tube Q4 of the same name of T1 level of main transformer main winding, the drain electrode of the different name termination rectifying tube Q3 of T1 level of main transformer main winding, the source ground of rectifying tube Q3, continued flow tube Q4; The drive circuit of rectifying tube Q3 is made up of main transformer T1 the auxiliary winding N1 of level, capacitor C 6, diode D2, diode D3 and triode Q5: the end of the same name of T1 the auxiliary winding N1 of level of main transformer connects the negative pole of diode D2, the positive pole of diode D3 and the base stage of triode Q5 by capacitor C 6, the grounded collector of the positive pole of diode D2 and triode Q5, the negative pole of diode D3 connect the emitter of triode Q5 and the grid of rectifying tube Q3; The drive circuit of continued flow tube Q4 is made up of capacitor C 5, resistance R 1, coupling transformer T2, capacitor C 7, diode D4, diode D5 and triode Q6: the end elementary of the same name of coupling transformer T2 is by resistance R 1, capacitor C 5 back ground connection, the grid of PMOS switching tube Q2 in the elementary different name termination active clamp circuit of coupling transformer T2; The end secondary of the same name of coupling transformer T2 connects the negative pole of diode D4, the positive pole of diode D5 and the base stage of triode Q6 by capacitor C 7, the grounded collector of the positive pole of diode D4 and triode Q6, the negative pole of diode D5 connect the emitter of triode Q6 and the grid of continued flow tube Q4;

Filter circuit is made up of inductance L 1 and capacitor C 4, T1 level end of the same name of a termination main transformer of inductance L 1, and the other end is by capacitor C 4 ground connection; The output voltage V out of the tie point of inductance L 1 and capacitor C 4 output whole converter, output voltage V out through voltage sampling circuit sampling, optocoupler feedback circuit Coupled Feedback to pwm control circuit.

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