Abstract This paper introduces the working principle of a series resonant power supply that achieves frequency and amplitude modulation output using sinusoidal pulse width modulation (SPWM) technology. The power supply employs a half-bridge inverter circuit composed of IGBT modules, which features IGBT drive, overvoltage, overcurrent, overheat protection, and fault lockout functions. Keywords: Sinusoidal pulse width modulation, IGBT module, fault lockout, power supply 1 Introduction The application of sinusoidal pulse width modulation and variable frequency speed control technology in industrial control is becoming increasingly widespread. Many power testing instruments require high power and high performance to meet the testing requirements of power equipment. Currently, most high-power switching power supplies on the market use MOSFET semiconductor field-effect transistors and bipolar power transistors as their core power devices, neither of which can meet the requirements of small size, high frequency, and high efficiency. MOSFET field-effect transistors have the characteristics of fast switching speed and voltage-type control, but their on-state resistance is high, making it difficult to meet the requirements of high voltage and high current; although bipolar power transistors can meet the requirements of high withstand voltage and high current, they lack fast switching speed and are current-controlled devices, requiring a large power drive. The Insulated Gate Bipolar Transistor (IGBT) integrates a MOSFET and a bipolar transistor, offering advantages such as voltage-type control, high input impedance, low drive power, fast switching speed, high operating frequency, and large capacity. An inverter power supply using high-performance IGBTs as the switching inverter element and employing frequency conversion amplitude modulation (VAM) technology boasts advantages such as high efficiency, reliable performance, and small size. 2. Working Principle This power supply utilizes high-frequency inverter technology, a digital signal generator, sinusoidal pulse width modulation (PWM) and VAM, timing-controlled power-on, and series resonant output. The power supply features high efficiency, high output power, and small size. Its overall principle block diagram is shown in Figure 1. A sine wave generated by the digital signal generator is modulated by a 25kHz triangular modulation wave to obtain a sinusoidal pulse width modulation (PWM) wave, which drives the IGBT inverter element via a drive circuit. Changing the frequency of the sine wave allows for frequency and amplitude modulation output. The inverter output is a series resonant output, filtering out the high-frequency carrier signal to obtain the desired sinusoidal signal. The timing control circuit controls the power source to power on slowly, ensuring a stable current during power-on and avoiding surges caused by non-zero-crossing switching. The control circuit also incorporates a fault-locking function; in the event of a power failure, the lockout function prevents the IGBTs from being turned on. When a fault occurs, the IGBTs are locked open, and the large-capacity filter capacitor stores a significant amount of energy. Therefore, the power supply section has fault protection features that automatically cut off the power supply and automatically discharge. The entire unit is designed with comprehensive protection functions including dual overcurrent, overvoltage, and overheat protection. 3. Control and Drive Circuit The control circuit refers to the main control circuit, including the generation of a sinusoidal pulse width modulation wave, duty cycle adjustment, and fault-locking circuitry. The frequency of the sinusoidal modulation wave in the control circuit can be adjusted according to the actual application. The drive circuit uses the Mitsubishi EXB840 dedicated IGBT drive module, which can drive IGBTs up to 150A/600V and 75A/1200V. The internal drive circuit of this module ensures a signal delay of ≤1μs, making it suitable for switching operations up to 40kHz. When using this module, it is important to note that the IGBT gate-emitter loop wiring must be less than 1M, and the gate-emitter drive wiring should use stranded wire. The EXB840 drive circuit is shown in Figure 2. 4. Inverter and Buffer Circuit This power supply uses a half-bridge series resonant inverter circuit, and the main circuit principle is shown in Figure 3. In high-power IGBT resonant inverter circuits, the structural design of the main circuit is crucial. Due to the parasitic inductance of the leads in the circuit, the surge voltage Ldi/dt induced on the inductor during IGBT switching cannot be ignored. Since this power supply uses a half-bridge inverter circuit, it will generate a larger di/dt compared to a full-bridge circuit. Properly designed overvoltage protection, i.e., a buffer circuit, is essential for the normal operation of the IGBT. Improper design of the buffer circuit will increase its losses, leading to severe overheating, damage to components, and hindering long-term operation. The process is as follows: When VT2 is turned on, as the current rises, the stray inductance Lm causes Uab to drop to Vcc - Ldi/dt. At this time, the buffer capacitor C1, which was charged to Vcc in the previous operating cycle, discharges through the anti-parallel diode VD1, VT2, and buffer resistor R2. In the buffer circuit, the instantaneous conduction current ID1 flowing through the anti-parallel diode VD1 is the sum of the stray inductance current IL and the current IC flowing through the buffer capacitor C1. That is, ID1 = IL + IC. Therefore, IL and di/dt are much smaller compared to a bufferless circuit. When VT1 is turned off, due to the stray inductance Lm, Uce rises rapidly and exceeds the bus voltage Vcc. At this time, the buffer diode VD1 is forward biased, and the energy stored in Lm (LmI2/2) is transferred to the buffer circuit. The buffer circuit absorbs the stored energy, preventing a significant rise in Uce. 5. Calculation and Selection of Buffer Components Where: f—switching frequency; Rtr—switching current rise time; IO—maximum switching current; Ucep—transient voltage peak value. In selecting components for the buffer circuit, capacitors with high voltage ratings should be chosen, diodes should preferably be high-performance fast recovery diodes, and resistors should be non-inductive. 6. Conclusion This power supply has been successfully applied to high-power power testing instruments. Compared with traditional methods, it not only offers higher measurement accuracy but also improves work efficiency, increases work safety, and reduces labor intensity.