0 Introduction An Insulated Gate Bipolar Transistor (ICBT) is a device composed of a MOSFET and a bipolar transistor. Its input electrode is a MOSFET, and its output electrode is a PNP transistor. Therefore, it combines the advantages of simple and fast driving of MOSFET devices with the large capacitance of bipolar devices. These advantages have led to the increasingly widespread application of IGBTs in modern power electronics technology. Using IGBTs as power switches, the characteristics of frequency multipliers can be used to push the circuit to higher frequencies. By changing the frequency difference between the inner and outer tank circuits, power regulation and load matching can be achieved. In processes such as workpiece quenching and welding, sudden short circuits can occur at the power supply load due to various reasons. This paper studies the circuit characteristics under load short circuit conditions and proposes principles for selecting circuit parameters. 1 Main Circuit Working Principle Analysis The main circuit of the frequency multiplier ICBT high-frequency induction heating power supply is shown in Figure 1. Under steady-state conditions, the circuit, through the alternating conduction of S1-S4 and D1-D4, charges and discharges between the commutation branch and the DC blocking capacitor. The resulting oscillating current flows through the load's AC equivalent resistance RH, forming the positive and negative half-waves of the load current. One working cycle can be divided into several stages as shown in Table 1. 2 Load Short-Circuit Analysis When the load is short-circuited, interference signals enter the inverter bridge, causing a direct short circuit, and the inverter bridge input voltage suddenly drops to zero. At this time, the magnetic energy originally stored in the filter inductor Ld and the electrical energy in the DC blocking capacitor Cd are released to the inverter circuit in the form of short-circuit currents ids and iHs, respectively. The equivalent circuit diagram is shown in Figure 2(a). The short-circuit current is flowing through the IGBT in the bridge is given by the formula: ωs is the oscillation period of the oscillating current; δs is the attenuation coefficient of the oscillating current. This surge current is detected by the detection circuit and causes the protection circuit to act immediately, issuing an overcurrent signal, and the rectifier circuit transitions from the rectification state to the inverter circuit. The equivalent circuit when the inverter bridge is turned off is shown in Figure 2(b). The short-circuit current ids moves to the Cd branch, Cd is charged, the Cd terminal voltage gradually rises, and the short-circuit current decreases. After that, the short-circuit current oscillates and decays in the loop formed by Cd, Ld, and R and C in the absorption circuit until the energy is consumed. ids flows along Ld. Due to the large inertia of the circuit, the current increase is not much and is approximately the working current Ido before the short circuit. Therefore, the amplitude of the surge circuit is Ism=IHsm+Ido (4). To reduce the impact of the surge current generated when the load is short-circuited on the power transistor, two schemes are generally adopted: First, the current magnitude is detected in real time. When it exceeds the protection setting value, the protection circuit immediately acts. This requires the dynamic characteristics of the protection circuit to be very good, including the delay of the detection circuit and the delay of the protection action circuit. It is very difficult to achieve in high-frequency circuits. Second, current limiting components are adopted in the main circuit so that the current rises slowly when the circuit is short-circuited, so that the protection circuit has enough time to respond. In this system, considering the high circuit frequency and large capacity, and the requirement for rapid protection in the event of a short circuit, the above two schemes are combined: when the detection circuit detects an overcurrent, the gate voltage reduction slow turn-off technology is used to enhance the instantaneous overcurrent capability of the power device, and then the protection circuit operates; at the same time, it can be seen from equation (3) that by selecting appropriate values ​​of Cd and LT, the magnitude of the surge current can be reduced to a certain extent. The so-called gate voltage reduction slow turn-off technology refers to the fact that when an IGBT experiences an overcurrent, its gate drive voltage is reduced first, and then it is turned off. This extends the time that the IGBT can withstand the overcurrent and reduces the magnitude of the overcurrent impact on the device. When there is an overcurrent, the on-state voltage drop of the device increases, and the instantaneous heat loss of the tube increases sharply. To prevent thermal damage to the device, the overcurrent time should be short enough, generally <10μs. SHARP's optocoupler PC929 integrates this function with the drive circuit. The internal schematic diagram of the device is shown in Figure 3. As shown in Figure 3, when an overcurrent occurs, pin 9 of PC929 detects an increase in the IGBT on-state voltage drop. The IGBT protector circuit reduces the IGBT drive voltage to limit the amplitude of the IGBT short-circuit surge current. At the same time, the short-circuit signal can be sent to the control circuit, and the IGBT drive signal can be turned off to prevent the device from being damaged by overcurrent. Integrating the protection circuit and the drive circuit can reduce the response time of the protection circuit and reduce external noise interference. The following simulation is used to select appropriate values ​​of Cd and LT to reduce the peak value of the short-circuit surge current. Generally speaking, the instantaneous surge current withstand capability of an IGBT is 2 to 3 times its rated current. Therefore, in this circuit design, the instantaneous current withstand capability of the IGBT is taken as 250A. When the DC voltage Ucd = 500V, in order to prevent the IGBT from burning out when a short circuit occurs, it can be seen from equation (3) that Cd and LT must satisfy equation (5). From the perspective of reducing the short-circuit current, Cd should be as small as possible and LT should be as large as possible. However, if Cd is too small, the circuit has the following disadvantages: the DC blocking effect is not ideal; the sinusoidal distortion of the output voltage is too high, the output voltage decreases, and the heating effect is not ideal; the anti-parallel diode re-conducts, increasing the diode's current capacity, as shown in Figure 4. When LT is too large, the circuit has the following disadvantages: the high-frequency voltage drop across LT is too high, causing the output voltage to decrease; the forward blocking voltage when the transistor is turned off increases; the resonant frequency of the inner tank circuit decreases, and the IGBT and diode undergo secondary conduction, as shown in Figure 5. Figure 6 shows the simulated waveforms when LT/Cd=4, Cd=0.75μF, and LT=3μH. By comparing with Figures 4 and 5, it can be seen that the parameter selection is most suitable at this time. The oscillations in the simulated waveforms are all caused by the parasitic capacitance of the device and the reverse recovery process of the diode. Figure 7 shows the voltage waveform when the transistor is turned off and the load output voltage waveform when LT changes from 3μH to 8μH. It can be seen that the value of LT in this range is relatively suitable. 3 Experimental Results Based on the previous analysis, relevant experiments were conducted, and the waveforms are shown in Figure 8. In this experiment, the IGBT's operating frequency was 50kHz, and the load output frequency was 100kHz. Curve 1 in Figure 8 shows the current waveform flowing through the IGBT. Because the direction of the current transformer is opposite to the actual direction of the current, it is logically opposite to the waveform of the voltage uds across the IGBT shown in curve 2. The experimental waveforms verify the correctness of the previous analysis. 4. Conclusion Using IGBTs as power switches and leveraging the characteristics of frequency multipliers, the circuit can be pushed to higher frequencies. When the load is short-circuited, selecting appropriate values ​​for Cd and LT can reduce the inrush current to a certain extent, thereby better protecting the circuit and ensuring its reliable operation.