Abstract: This paper introduces a design scheme for a 600V input multi-output high-frequency switching power supply. The main circuit adopts a dual-transistor flyback power converter, and the control circuit uses current-mode PWM control technology. Keywords: dual-transistor flyback converter, current-mode PWM control, emitter follower, complementary bipolar drive, optocoupler isolation, feedback
1 Introduction With the continuous development of modern industry, higher requirements have been placed on the performance, efficiency, and weight of power supply devices. Conventional linear power supplies can no longer meet the requirements. Switching power supplies adopt high-frequency pulse width modulation control technology and have the characteristics of good performance, high efficiency, small size, light weight, low noise, and wide voltage regulation range. This paper introduces a high-frequency switching power supply with a wide input and multiple outputs that is used as an auxiliary power supply in industrial high-power power supplies.
2. Main Circuit
Main Circuit Selection: The main circuit adopts a dual-transistor flyback power main circuit. Firstly, based on the design requirements of multiple outputs, a flyback configuration is chosen. One advantage of flyback is its ability to easily obtain multiple outputs from a single power supply. Secondly, the input voltage is relatively high at 600V. If a single-transistor configuration were used, the MOSFET would withstand 3-4 times the input voltage VIN at the moment of turn-off, placing high voltage requirements on the MOSFET and consequently increasing its cost. Using a dual-transistor configuration avoids this problem. In Figure 1, diodes D19 and D31 act as clamps, feeding excess energy back to the power supply and clamping the voltage at VIN at the moment of MOSFET turn-off. This reduces the requirements for components and is easier to implement. During operation, a control chip generates PWM, and the drive circuit excites the MOSFET in PWM mode. The switching transistors turn on and off synchronously, thereby converting the input voltage VIN into a high-frequency square wave AC voltage. The transformer design is one of the key aspects of the entire circuit. When designing a transformer, the primary inductance should not be too large, and an air gap should be added to the magnetic core. Otherwise, the current rise rate will be small, the conduction time will be short, and the current rise value will be small, resulting in the circuit's inability to transfer the required power. Simultaneously, the transient magnetic saturation effect of the transformer must be carefully considered during design. Under transient load conditions, when the input voltage is high and the load current is low, if the load current suddenly increases, the control circuit will immediately widen the output pulse width to provide supplementary power. In this way, both the input voltage and pulse width become maximum simultaneously. Even if it is only for a brief period, the transformer will saturate, causing runaway and malfunction. This necessitates that transformers be designed for high input voltage and wide pulse width.
3. Control Circuit
(1) PWM pulse generation and closed-loop voltage regulation
To achieve good voltage regulation, load regulation, and system stability, this design uses the current-mode control chip UC3844, which has a maximum duty cycle of 50%, a startup voltage of 16V, and features overvoltage protection and undervoltage lockout. Its basic operating process is as follows:
First, the output voltage is stepped down by the feedback network, and then this feedback voltage is sent to the error amplifier to compare with the reference voltage, generating an error voltage signal. The pulse width modulation section picks up this error voltage and compares it with the current of the power transformer, converting it into an appropriate duty cycle to control the number of power pulse modulations in the output section. In Figure 2, R16 and C14 determine the operating frequency of the system. C11, C12, and R5 connected between pin 1 of the error amplifier output and pin 2 of the feedback serve as compensation. D2, D3, C5, C6, and R6 enable the UC3844 to achieve soft start, the time of which is determined by C5 and C6. The starting resistor R3 is determined by the IC's starting current. The block diagram of the UC3844 is shown in Figure 3.
(2) Drive circuit
The choice of drive type is crucial. First, the rise and fall times of the MOSFET switching voltage depend entirely on the speed at which the drive circuit removes the gate charge. Therefore, a low-impedance, high-current drive is selected to enable faster MOSFET switching and reduce switching losses. Second, it is essential to avoid overshoot current when the control voltage changes between on and off. Based on the above, an emitter follower complementary bipolar drive is chosen. Then, a high-frequency pulse transformer drives both MOSFETs for isolation. The specific circuit is shown in Figure 4.
In Figure 4, R153 and R154 provide current limiting protection for the IC. The double anti-series Zener diode is used to protect the power MOSFET from excessive gate-source voltage. The Zener diode has a Zener voltage of +12V. R155 and R156 are used to reduce the drain-source impedance.
(3) Feedback circuit
For switching power supplies, isolation and anti-interference technologies are crucial. The main advantages of optocouplers are unidirectional signal transmission, complete electrical isolation between the input and output, strong anti-interference capability, long lifespan, and high transmission efficiency. Therefore, this design uses optocoupler isolation as the feedback loop. This circuit compares the output voltage (after voltage division) with a reference voltage formed by a TL431. The current change of the diode-transistor in the optocoupler TLP521-1 controls the feedback pin of the UC3844, thereby changing the PWM width to stabilize the output voltage. Parameter determination: For example, based on experience, first select If's current as 0.4mA, R126 as 330Ω, R125 as 10KΩ, Rr2 as a 1KΩ adjustable resistor, U3 as 15V, and Ur as the 431 reference voltage of 2.5V. Then, R124 = (U3/Vr - 1)R125 = 50KΩ.
4. Conclusion This power supply is designed using the current-mode PWM control chip UC3844, which has fast dynamic response, good frequency response characteristics, high precision, wide voltage regulation range, low ripple, and system stability, and can well meet the requirements of industrial power supplies.
Key technical specifications:
Input: DC 400-630V
Output: 5V/0.8A, 15V/0.4A, 15V/0.5A, 15V/0.6A, 20V/2A
Power efficiency: 85%
Transformer turns ratio: Primary/Secondary 100/3 4 5 5 6
References
[1] Zhang Zhansong, Cai Xuansan. Principles and Design of Switching Power Supplies. Beijing: Electronic Industry Press, 1998.
[2] Raymond A. Mack, Jr., translated by Xie Yunxiang et al. Introduction to Switching Power Supplies. Beijing: Posts & Telecom Press, 2007.
