A design method for high frequency switching power supply

　　In recent years, with the development of electronic technology, switching power supplies are increasingly used in postal and telecommunications, transportation facilities, instruments and meters, industrial facilities, and household appliances. With the continuous advancement of science and technology, the demand for high-power power supplies is also increasing. At the same time, a large number of integrated circuits, ultra-large-scale integrated circuits and other electronic communication equipment are increasing, requiring the development trend of power supplies to be miniaturized and lightweight. Usually, the size and weight of filter inductors, capacitors and transformers are relatively large, so miniaturization and lightweighting are mainly achieved by reducing their size.

　　We can increase the operating frequency by reducing the number of turns of the transformer winding and the size of the core, but when the switching frequency is increased, the switching loss will increase and the circuit efficiency will be seriously reduced. In response to these problems, soft switching technology has emerged. It uses auxiliary commutation methods based on resonance to solve the switching loss and switching noise problems in the circuit, so that the switching power supply can operate at high frequency and high efficiency. Since the 1970s, domestic and foreign countries have been continuously studying high-frequency soft switching technology, which is now relatively mature. The following design takes a 2KW power supply as an example.

　　1. Design content and methods

　　1.1 Selection of main circuit type

　　The type of conversion circuit is mainly selected according to technical conditions such as load requirements and given power supply voltage. Among several commonly used conversion circuits, the voltage borne by the power switch tube of the half-bridge and full-bridge conversion circuits is half that of the push-pull conversion circuit, and the mains voltage is higher, so the push-pull conversion circuit is not selected. When the half-bridge conversion circuit and the full-bridge conversion circuit output the same power, the power switch tube of the half-bridge conversion circuit bears twice the working current, which is not easy to select, and the output power is smaller than that of the full-bridge, so the full-bridge conversion circuit is used.

　　The switch elements of the traditional full-bridge conversion circuit are turned on or off under the control of the gate under the conditions of high voltage or large current. During the switching process, the voltage and current are not zero, and overlap occurs, resulting in switching losses. The switching loss rises sharply with the increase of the switching frequency, which reduces the circuit efficiency and hinders the increase of the switching frequency. On the basis of the phase-shift control technology, the output capacitor of the power tube and the leakage inductance of the output transformer are used as resonant elements to make the four switch tubes of the full-bridge converter turn on at zero voltage in turn to achieve constant frequency soft switching. Due to the reduction of switching process losses, the conversion efficiency can reach 80%-90%, and there will be no excessive switching stress. Therefore, the phase-shift control full-bridge zero voltage switching pulse width modulation (PSC FB ZVS-PWM) conversion circuit is selected.

　　The phase-shifted full-bridge converter circuit is one of the most widely used soft-switching circuits. It is characterized by a simple circuit. Compared with the traditional hard-switching circuit, it does not add auxiliary switches and other components. The principle is shown in Figure 1. It is mainly composed of four identical power tubes and a high-frequency transformer. E is the input DC voltage, T1~T4 are switch tubes, D1~D4 are internal diodes, and C1~C4 are the output capacitors of the switch. Taking the first bridge arm as an example, the transformer leakage inductance and the power output capacitor C1 are resonated. During the process of releasing the leakage inductance energy to the capacitor C1, the voltage on the capacitor gradually drops to zero, and the internal diode D1 is turned on, creating the ZVS condition of T1.

Figure 1 Schematic diagram of phase-shift controlled full-bridge converter circuit

　　1.2 Control method

　　The control mode refers to the way in which the converter control circuit controls the main circuit to achieve the purpose of automatic control and meet the requirements of automatic voltage or current stabilization. The traditional PWM type electronic switch has voltage and current at the same time when turning on and off the switch, and the loss is relatively large. The zero voltage switch-pulse width modulation converter (ZVS-PWM) is an electronic switch that conducts when the voltage at both ends is zero and turns off when the current is zero. The ideal value of the turn-on and turn-off loss is zero. Here, the typical UC3875 phase-shift control full-bridge zero voltage switch-pulse width modulation conversion circuit is selected.

　　1.2.1 UC3875 control chip

　　UC3875 is a special chip launched by UNITRODE in the United States for phase shift control solutions. UC3875 can shift the phase of the full-bridge switch to achieve fixed-frequency pulse width modulation control. UC3875 has 20-pin and 28-pin packages. Here we take the 20-pin package as an example to introduce the device.

　　1.2.1.1 Internal structure block diagram and pin functions

　　The internal structure block diagram is shown below:

Figure 2 UC3875 internal structure block diagram

　　The pin functions are as follows: Pin 1 (Vref), reference voltage; Pin 2 (E/A OUT), inverting output of the error amplifier; Pin 3 (E/A-) inverting input of the error amplifier; Pin 4 (E/A+) non-inverting input of the error amplifier; Pin 5 (C/S+) current detection; Pin 6 (SOFRSTART) soft start; Pin 7 (DELAY SET C/D) output delay control; Pin 8 (OUT D) output D; Pin 9 (OUT C) output C; Pin 10 (Vcc) power supply voltage; Pin 11 (Vin) chip power supply; Pin 12 (PWR GND) power ground; Pin 13 (OUTB) output B; Pin 14 (OUTA) output A; Pin 15 (DELAY SETA/B) output delay control; Pin 16 (FREQ SET) frequency setting terminal; Pin 17 (CLOCK/SYNC) clock/synchronization; Pin 18 (SLOPE) steepness; Pin 19 (ramp wave) Pin 20 (signal ground).

　　1.2.1.2 Working of UC3875

　　Pin 1 outputs +5V reference voltage, which can be used as the power supply for other components of internal or external circuits. Pin 2 is used as the voltage feedback control terminal. When the output signal of the pin reaches a certain value, the internal RS trigger and gate circuit make the output of C inverted with the output of A, that is, the output signals of A and C are phase-shifted by 180 degrees; similarly, when the output signal of pin 2 is lower than 1V, the internal RS trigger and gate circuit make the output of C in phase with the output of A, that is, the output signals of A and C are phase-shifted by 0 degrees. It can be seen that by controlling the output of pin 2, the phase between A and C can be controlled to change between 0 and 180 degrees. The working principles of B and D are similar to those of A and C. Pin 3 is used as the inverting input terminal of the error amplifier, and the output power supply voltage is usually detected by a voltage divider resistor. Pin 4 is used as the non-inverting input terminal of the error amplifier, connected to the reference voltage of pin 1, and the output power supply voltage of pin 3 is detected. Pin 5 is used as the current detection terminal, and its reference is set to an internal fixed 2.5V (by voltage divider). When the voltage exceeds 2.5V, the output is turned off, and the soft start pin 6 is reset to achieve overcurrent protection. Pins 7 and 15 are used as output delay control terminals. The dead zone is set by setting the current between the pins and the ground. It is added between the driving pulses of the two tubes in the same bridge arm to achieve the transient time when zero voltage is turned on. Pins 8, 9, 13, and 14 are used as output terminals to drive MOSFETs and transformers. Pin 10 is used as the power supply voltage terminal to provide the required power for the output stage. Pin 11 is used as the chip power supply to provide power for the digital and analog circuit parts inside the chip. There is an undervoltage lockout circuit inside, with an opening threshold of 10.75V and a closing threshold of 9.25V. There is a hysteresis of 1.5V between opening and closing, which can effectively prevent the circuit from jumping when working near the threshold voltage. Pin 16 is used as the frequency setting terminal, and an external resistor and capacitor are required to set the oscillation frequency. When pin 17 is used as an output, it provides a clock signal; as an input, it provides a synchronization point. Pin 18 is used as a steepness terminal, and an external resistor is required to generate a ramp wave. Pin 19 is used as a ramp wave terminal, and an external capacitor is required to ground. Pin 20 is the signal ground and is the reference for all voltages.

　　1.2.2 Control Circuit

　　The main parts of the control circuit schematic are shown in Figure 3.

Figure 3 Control circuit schematic

　　The core of UC3875 is the phase modulator. Its 13-pin B output signal is inverted with the 14-pin A output signal, and the 9-pin C output signal is inverted with the 8-pin D output signal. These four drive signals are driven by the drive transformer after current expansion ~ MOS tube. The characteristics of phase control are reflected in the fact that the four output ends of UC3875 have the same drive pulse to drive the A/B and C/D half-bridges respectively. The active time is controlled by phase shifting, so that the four switches of the full bridge are turned on in turn. There is a dead zone before each output stage is turned on, and the dead zone time can be adjusted. During the dead zone time, it is ensured that the output capacitor of the next power switch device is discharged, providing voltage turn-on conditions for the switch device to be turned on. Therefore, the resonant switch action time of each pair of output stages (A/B, C/D) can be controlled separately. In the full-bridge conversion topology mode, the advantages of phase shift control are most fully reflected. UC3875 can work in both voltage mode and current mode, and has over-current shutdown to achieve fast protection of faults. Figure 4 shows the control waveform of the phase shift control full-bridge circuit.

Figure 4 Control waveform of phase-shift control full-bridge circuit

　　The control method of the phase-shift control full-bridge circuit has the following characteristics:

　　(1) Within the same switching cycle Ts, the on-time of each switch is slightly less than Ts/2, while the off-time is slightly greater than Ts/2.

　　(2) The upper and lower switches in the same half-bridge cannot be in the on state at the same time. There is a certain dead time between each switch being turned off and the other switch being turned on.

　　(3) Comparing the switching function waveforms of the two pairs of switches T1, T2 and T3, T4 that are diagonally opposite to each other, the waveform of T1 leads T2 by 0 to Ts/2 time, while the waveform of T3 leads T4 by 0 to Ts/2 time. Therefore, T1 and T3 are called leading bridge arms, while T2 and T4 are called lagging bridge arms.

　　2. Conclusion

　　This article introduces a 2KW phase-shifted full-bridge conversion (PSC FB ZVS-PWM) soft-switching power supply with UC3875 chip as the control circuit. Since the switch tube operates under ZVS conditions, high frequency can be achieved, and the control is simple and the performance is reliable. It is suitable for high-power occasions. And it can maintain constant frequency operation, so there will be no high voltage and high current at the same time, reducing the stress on the switch and achieving high efficiency. The size of the power supply is greatly reduced.