Introduction The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a unipolar voltage-controlled device. It not only has self-turn-off capability but also advantages such as low drive power and fast turn-off speed, making it a commonly used switching device in switching power supplies. MOSFET-controlled switching power supplies have advantages such as small size, light weight, high efficiency, and low cost, making them suitable for instrument power supplies. This paper presents a design example of a small-power regulated power supply with a wide-range continuously adjustable (0-45V) voltage range controlled by MOSFETs. Overall Structure and Main Circuit Figure 1 shows the overall block diagram of this power supply. The working principle is as follows: Figure 1 Principle Block Diagram. The full-bridge rectifier circuit rectifies the 220V mains voltage into a non-adjustable DC voltage Ud = 1.2U, approximately 198V. The voltages on the two equivalent filter capacitors are both above 99V. After inversion by the DC/AC converter, a 20kHz AC voltage with adjustable pulse width is output. This voltage is then fed into the rectifier and filter circuit via the two secondary windings of the high-frequency transformer, dividing the positive and negative half-cycles, to output the DC voltage. The magnitude of the DC output voltage of this power supply is controlled by the output pulse width of the PWM generator. The main circuit is shown in Figure 2. The key components for DC-AC conversion in the main circuit are power MOSFETs VT1 and VT2. When VT1 is on and VT2 is off, the current in the circuit flows from the positive terminal of capacitor C1 to D1-S1 of VT1, and then back to the negative terminal of capacitor C1 through the primary side of the transformer, forming a loop, and uAB is a positive voltage. The induced voltage on the secondary side of the transformer is positive at the same terminal, VD1 conducts, and the output U0 is positive at the top and negative at the bottom. When VT2 is on and VT1 is off, the same conclusion can be drawn: U0 is positive at the top and negative at the bottom. The magnitude of U0 depends on the conduction time of VT1 and VT2 by the control circuit. Control Circuit The function of the control circuit is to realize PWM waveform synthesis and isolated drive of the controllable DC/AC converter. PWM Waveform Generation: The power supply design of this circuit is a ±15V DC regulated power supply with a three-terminal integrated voltage regulator as its core. (1) PWM Control Principle The pulse width PWM waveform is generated using the powerful TL494 fixed-frequency modulation chip, which has 16 pins. The internal and external circuits are shown in Figure 3. Figure 3 Internal and external circuits of TL494 When pin 13 of the TL494 chip is low, pins 8 and 11 work synchronously and output single-ended signal; when pin 13 is high, pins 8 and 11 work in push-pull mode and output dual signal. This circuit adopts the latter working mode. The highest operating frequency of the chip is 300kHz. The actual operating frequency is determined by the resistor and capacitor connected to pins 5 and 6. Its oscillation frequency is calculated as f = 1.1P(RTCT). The oscillation frequency selected in this design is 20kHz. The sawtooth wave is sent to the inverting inputs of comparators 1 and 2 inside the chip. The sawtooth wave and the output of the on-chip error amplifier are compared in PWM comparator 2, while the dead time control level and the sawtooth wave are compared in dead time comparator 1. The outputs of the two are rectangular waves of a certain width. They are sent to the OR gate circuit at the same time. After being divided by the frequency divider, they are then controlled by the corresponding gate circuit to alternately turn on the internal transistors, so that pins 8 and 11 output PWM waveforms with a phase difference of 180°. Its working waveform is shown in Figure 4. Figure 4 Working waveform The inverting terminal (pin 2) of error amplifier 1 is connected to the adjustable given voltage Ug. Changing Ug can change the voltage value of pin 3, thereby changing the width of the output waveform of PWM comparator 2, so that U0 can be continuously adjusted from 0 to 45V. (2) Dead time control In order to ensure that there is enough time interval between switching devices VT1 and VT2 when one transistor is turned off and the other transistor is turned on, and to prevent DC side short circuit caused by direct connection of power switching elements, the dead time of the two switching devices is controlled by pin 4. The dead-time voltage reference value is provided by the internal reference source pin 14 connected in series with capacitor C5, and the minimum dead-time value Toff (min) is jointly determined by R5 grounding. Additionally, when the input power is first turned on, R5 and C5 form a soft starter. Since the voltage across the capacitor cannot change abruptly, at the moment of startup, the dead-time control terminal 4 is at the same potential as the internal reference voltage 14, which is a high level. The dead-time comparator 1 also outputs a high level, blocking the two transistors at the output terminal. As the capacitor voltage continues to rise, the potential at terminal 4 gradually decreases, and the two transistors gradually turn on, ensuring that the output voltage of the power supply does not change abruptly, thus achieving soft starting. During normal operation, the voltage across R5 is approximately 0. At this time, the conduction time of the main circuit switching elements (which determines the output voltage value during normal operation) is determined by comparing the given voltage Ug connected to the inverting input of error amplifier 1 and the feedback voltage Uf connected to the non-inverting input. The isolation and drive circuits VT1 and VT2 are driven by a dedicated integrated drive module IR2110. The isolation drive circuit is shown in Figure 5: Figure 5 IR2110 drive module and external wiring circuit Overvoltage and overcurrent protection To modify the load curve, protect the safe operation of the MOSFET, prevent overvoltage, and reduce du/dt, an overvoltage absorption circuit consisting of a resistor, fast diode, and capacitor is connected in parallel between D1 and S1 of the MOSFET. The overcurrent signal is detected from the main circuit and sent from pin 16 to the non-inverting input of error amplifier 2. Pin 15 serves as the comparison reference. When an overcurrent occurs, the voltage at pin 16 rises, and the output pin 3 of comparator 2 goes high, blocking the pulse width signal. Conclusion This power supply uses commonly used integrated circuits and power modules with high reliability in industrial environments as much as possible, prioritizing ease of implementation and maintenance, and aiming for practicality. Experimental verification shows that this circuit has strong anti-interference capability, stable output voltage, reliable operation, and an output current of up to 15A. It is well-suited for use as a DC power supply for instruments and devices and has good potential for widespread application. (Source: Power Transmission and Distribution Equipment Network)