Principle and design of phase-shifted full-bridge ZVS converter

    Abstract: The principle of the phase-shifted full-bridge ZVS converter is introduced, and a 3kW phase-shifted full-bridge zero-voltage high-frequency communication switching power supply is successfully developed using the UC3875 controller.

    Keywords: phase-shifted full bridge, zero current switching, zero voltage switching, quasi-resonant

1 Introduction

    The traditional full-bridge PWM converter is suitable for situations where the output voltage is low (such as 5V), high power (such as 1kW), and where the power supply voltage and load current vary greatly. Its characteristic is that the switching frequency is fixed and easy to control. In order to increase the power density of the converter and reduce the volume and weight per unit output power, the switching frequency needs to be increased to the 1MHz level. In order to avoid the loss during the switching process rising sharply as the frequency increases, based on the phase-shift control technology, the output capacitance of the power MOS tube and the leakage inductance of the output transformer are used as resonant components to make the four switching tubes of the full-bridge PWM converter They are turned on at zero voltage in turn to achieve constant frequency soft switching. This technology is called ZVS zero voltage quasi-resonance technology. Since the loss in the switching process is reduced, the overall efficiency of the entire converter can be guaranteed to be over 90%. We developed a zero-voltage quasi-resonant high-frequency switching power supply prototype using Unitrode's UC3875 as the control chip. This article discusses the development process, problems that arise during development and their improvements.

2 Composition of quasi-resonant switching power supply

    ZVS quasi-resonant high-frequency switching power supply is a complete closed-loop system, which includes main circuit, control circuit and CPU communication and protection circuit, as shown in Figure 1.

    It can be seen from Figure 1 that the composition of the quasi-resonant switching power supply is very similar to the structure of the traditional PWM switching power supply. The difference is that it uses soft switching technology in the DC/DC conversion circuit, that is, the quasi-resonant converter (QRC). It is formed by properly adding a resonant inductor and a resonant capacitor on the basis of a PWM switching converter. During operation, the time working in the resonant state only accounts for a part of the switching cycle, and the rest of the time is run in the non-resonant state. So it is called a "quasi-resonant" converter. Quasi-resonant converters are divided into two types, one is zero-current switching (ZCS) and the other is zero-voltage switching (ZVS). The characteristic of zero-current switching quasi-resonant converter is to ensure that the switching tube is disconnected during operation. Before the signal arrives, the current in the tube drops to zero. The characteristic of zero-voltage switching quasi-resonance is to ensure that the voltage at both ends of the operating switching tube has dropped to zero before the turn-on signal arrives.

3 Working principle of zero-voltage quasi-resonant converter

    The main circuit of the full-bridge zero-voltage quasi-resonant converter is shown in Figure 2. Uin is the DC voltage (400V) output by the PFC circuit, S1~S4 are the power switch tubes, and their body diodes are D1~D4. The body capacitances C1~C4 are not shown in the figure, and Lr is the primary series resonant inductance of the transformer T1, ( Including the leakage inductance of the transformer), C is the DC blocking capacitor to prevent the transformer from being saturated due to bias magnetism, and T2 is the current transformer for detection. When the converter overcurrent occurs, the protection circuit cuts off the drive signal to protect the power devices. The secondary voltage of the transformer is rectified by D5, D6 and output LC filter to supply power to the load. Figure 3 shows the waveforms of the transformer's primary voltage UP, secondary voltage US and primary current ip. The ZVS converter can be divided into six operating modes within one cycle, as shown in Table 1. When t
Table 1 Operation mode of ZVS converter in one cycle

4 Duty cycle analysis

    It can be seen from the waveform diagram that due to the leakage inductance of the converter, the primary current has a certain slope in the t1~t3 stage, so the secondary voltage duty cycle (t4-t3)/(t4-t0) is smaller than the primary voltage duty cycle ( t4-t1)/(t4-t0), causing duty cycle loss. The higher the switching frequency, the greater the duty cycle loss.

5 Conditions for achieving ZVS for phase-shifted full-bridge two-arm switch tubes

    As can be seen from Table 1 and Figure 3, S3 and S4 achieve ZVS earlier than S1 and S2 respectively, so S3 and S4 are called the right bridge arms, also known as the leading bridge arms, and S1 and S2 are called the left bridge arms, also known as the lagging arms. It can be seen from Table 1 that S3 and S4 realize ZVS at (t0~t1) and (t4~t5) respectively, and S2 and S1 realize ZVS at (t2~t3) and (t6~t7) respectively. The primary current of the transformer at (t2~t3) and (t6~t7) is smaller than the primary current at (t0~t1) and (t4~t5) respectively, so it is more difficult for the lagging bridge arm to achieve ZVS switching than the leading bridge arm, especially for light It is most obvious when loading.

    From a theoretical analysis, when S1 and S2 implement ZVS switching, the transformer secondary is in the freewheeling stage. During resonance, the resonant inductor releases energy, causing the resonant capacitor voltage to drop to zero, thus achieving ZVS. At this time, the conditions for achieving ZVS are: inductor energy Must be greater than the energy of all capacitors participating in resonance. Right now

    L r I p 2/2>(4C oss /3＋C xfmr )×U 2 in

    In the formula: 4Coss/3 is the equivalent capacitance value considering the nonlinearity of the MOS tube output capacitance, and Cxfmr is the distributed capacitance of the transformer winding. It can be seen from the above formula that the lagging bridge arm achieves ZVS mainly by resonant inductor energy storage. The energy is not large enough at light load, so the lagging bridge arm cannot easily meet the ZVS conditions.

    When S3 and S4 implement ZVS switching, the transformer is in the energy transfer stage. Primary current IP=-Io/n (n is the transformer ratio), primary equivalent inductance Le=Lr+n2LO. Therefore, according to the ZVS condition, the inductor energy must be greater than the energy of all capacitors participating in resonance, and it should be Le(Io/n)2/2>(4Coss/3+Cxfmr)Uin2. Since Le(Io/n)2/2 is quite large, it is easier for the leading bridge arm to meet the ZVS condition even under light load.

6 Phase-shifted full-bridge PWM controller

    The most critical thing about the phase-shifted full-bridge PWM control technology is that the conduction phase of the device can move within the range of 0 to 180°. If the control is not good, especially if the two switching tubes of the left or right bridge arm are turned on at the same time, the device will leading to disastrous consequences. The UC3875 produced by Unitrode can provide 0~100% duty cycle control, and has the necessary protection, decoding and driving functions. It has four groups of driving outputs, and the delay time of each group can be controlled. Its control circuit is shown in Figure 4 shown. E/A+ is connected to a fixed 2.5V voltage (VREF=5V, R5 and R9 are 10kΩ) to serve as a voltage given signal. E/A- is connected to the corresponding output voltage and compared with EA+ to control the phase of OUTA~OUTD and ultimately the output voltage. C/S+ is connected to the control signal (such as primary overcurrent signal, etc.). When the primary overcurrent occurs, C/S+ is greater than 2.5V, and UC3875 stops outputting the drive signal, thus turning off the converter output and preventing disaster accidents. The drive signal is output from OUTA ~ OUTD and expanded by TC4420. The drive transformer drives the S1 ~ S4 MOS tubes. The delay time is determined by the external resistors at pin 7 and pin 15 of UC3875. The actual drive signal timing is shown in Figure 5. .

7 Conclusion

    (1) The control of commutation dead time is very important to achieve zero-voltage switching.

    (2) The power supply of the control part and the output driving part of the UC3875 control circuit should be separated, otherwise the frequency will change during phase shifting.

    (3) In order to achieve ZVS in a wide range, a resonant inductor must be connected to the primary of the transformer. However, the resonant inductor cannot be too large. If the inductor is too large, the duty cycle will be lost, the primary current will be large, the conduction loss will increase, and the inductor will heat up. and other problems, and the efficiency is greatly reduced.

    According to the efficiency requirements of the China Telecommunications Administration for all communication power supplies connected to the network at the end of 1999: the efficiency (from half load to full load) of all communication power supplies greater than 1kW should be greater than 90%. Solving the problem of heat loss of the resonant inductor also solves the efficiency problem. The full-bridge ZVZCSPWM circuit can also be used to make the leading bridge arm realize ZVS and the lagging bridge arm realize ZCS, which can overcome the shortcomings of full-bridge ZVS and achieve an efficiency of more than 93%.