Abstract: To address the problem that the output power of a conventional thyristor parallel resonant intermediate frequency power supply cannot reach the rated power during the melting period, a trigger control circuit for adjusting the power angle φ of the DC/AC inverter is designed. Combined with the original AC/DC phase-controlled double closed-loop control circuit, the intermediate frequency melting power supply can achieve efficient control. Keywords: intermediate frequency power supply; power factor angle φ adjustment; turn-off time control 1 Overview A conventional intermediate frequency power supply is composed of an AC/DC controllable rectifier and a single-phase DC/AC current-type parallel resonant inverter. Its normal operation during the induction heating melting process is shown in Figure 1, which is based on the premise that the current iH in the load circuit leads its voltage uH. The lead trigger time of the thyristor in the inverter circuit should be greater than the turn-off time of the thyristor, i.e. t>(γ＋δ)/ω (1) Where: γ is the thyristor commutation overlap angle; δ is the recovery angle; ω is the angular frequency of the intermediate frequency power supply. Let β be the lead trigger angle. To ensure safe commutation, a safety margin angle θ should be considered. Then β = γ + δ + θ (2). The angle φ that the fundamental frequency of the load current iH leads its voltage uH is called the load lead power factor angle. As can be seen from Figure 1(b), φ = γ/2 + δ + θ (3). When the medium frequency power supply is used to melt metal, the materials being melted are mostly ferromagnetic materials. The resonant angular frequency ω of the load circuit increases with the furnace temperature. As can be seen from equation (2), this will lead to a decrease in the lead trigger time t = β/ω = (γ + δ + θ)/ω, and will also make the lead power factor angle φ smaller. If the commutation overlap angle γ and θ remain unchanged, this means that the turn-off recovery angle δ of the thyristor decreases, which may lead to inverter failure. It can be seen that when the actual recovery turn-off time decreases, in order to ensure the safe operation of the power supply, the trigger angle β or the lead power factor angle φ should be adjusted in time. 2. Principle of High-Efficiency Control of Medium-Frequency Power Supply When a medium-frequency power supply is used for smelting, its ideal operating condition should be to maintain a large or constant power output during the smelting period to rapidly increase the furnace temperature, reduce heat loss, shorten smelting time, and improve unit output and efficiency. However, in the actual metal smelting process, the magnetic permeability and conductivity of the smelted material change with temperature, causing a change in the equivalent resistance RH of the load. This means that the maximum designed output power (i.e., Pdmax = Udmax Idmax) is not reached for most of the smelting process. In fact, as can be seen from the main circuit block diagram in Figure 1(a), to achieve constant power output, it is only necessary to match the equivalent DC resistance Rd (Rd = Ud/Id) with the impedance of the medium-frequency load circuit. That is, when RH changes, a certain method is used to keep Rd constant, so that the medium-frequency output power will not change with RH. According to the relationship between Rd, RH and φ of the parallel resonant intermediate frequency power supply, Rd≈0.81cos2φRH (4), it can be seen that when the equivalent resistance RH of the load circuit changes, as long as the power angle φ is adjusted, Rd can remain unchanged, thereby achieving high efficiency and energy saving. 3 Reference of Thyristor Turn-Off Time (TOT) Control Circuit Medium frequency power supply products represented by German AEG and British RADYNE all adopt the TOT (turnoff time) timing control method. Its feature is that the trigger angle is adjusted in time according to the difference between the standard given TOT and the actual TOT, so as to accurately control the turn-off recovery time of the inverter thyristor. As mentioned above, whether from the perspective of safe operation requirements or the requirement to ensure constant power output, it is desirable to adjust the trigger angle (i.e., the lead power factor angle φ). For this reason, we reference the "lead trigger pulse forming circuit" of the "TOT" timing control method from reference [2] to meet the requirements of constant power output of high efficiency intermediate frequency melting power supply for φ angle adjustment. Figure 2 shows the block diagram and waveform of the "leading trigger pulse forming circuit" in the TOT control method. This circuit consists of the intermediate frequency load circuit voltage uH and the capacitor branch current signal and their conversion circuit, an XOR gate U1A, a comparator B, a JK flip-flop U3A, and a ramp generation circuit. Its core function is to ensure that during the TOT time before uH crosses zero, the comparator B generates a falling edge, causing the JK flip-flop to flip and output a leading trigger pulse from the Q and Q terminals. The inverting input of comparator B is connected to the ramp voltage signal uc2; while the non-inverting input is connected to the φ angle adjustment signal uc1. The trigger pulse position is determined by comparing uc1 and uc2 (intersection point). Figure 3 shows the control idea and strategy for the φ angle . Conventional parallel resonant current-type intermediate frequency power supplies generally design their control circuits according to the following idea: in the initial stage of heating, the trigger angle is fixed at a certain φmin, and the intermediate frequency voltage uH is increased by adjusting the control angle α of the rectifier bridge; while in the later stage of heating, the melting is completed by maintaining the maximum DC output power Pdmax = UdmaxIdmax. However, due to the variation in RH, Pdmax is not reached for most of the melting time, resulting in a long melting cycle, high heat loss, and low efficiency. Therefore, the control process during the initial heating phase can be kept unchanged, while in the later heating phase, a control method of adjusting φ can be used to keep Rd constant, maintaining maximum power output and transforming the inefficient medium-frequency power supply into a highly efficient one. The control circuit for adjusting the φ angle is shown in Figure 3. In the figure, ① is a comparator used to control the "on-off" state of the field-effect transistor Q1; ② is the φ angle adjuster; ③ is an adder; ④ is a limiting circuit; and ⑤ is a lead-triggered pulse forming circuit. Figure 4 shows the curves of uHf (the feedback value of the medium-frequency furnace coil voltage), ud, and uc1 during the φ angle adjustment process. Before the system is put into operation, uH* is at its maximum value (which can be determined based on the withstand voltage of the capacitors and inverter thyristors in the medium-frequency load circuit), and the maximum value uc1max and minimum value uc1min of uc1 correspond to φmin and φmax. In stage I, the DC voltage ud has not yet reached its maximum value, and the magnitude of uH is entirely adjusted by the original rectifier bridge control angle α. At this time, ud is less than the comparator ① setting value ub1, comparator ① outputs a high level, the field-effect transistor Q1 conducts, the φ angle regulator ② does not function, the output of ③ is the maximum value, and the output of ④ is the maximum limit value of uc1, uc1max (φmin). In stage II, the DC voltage ud has reached its maximum value, comparator ① flips, causing the field-effect transistor Q1 to turn off, the φ angle regulator starts working, and automatically adjusts. If the φ angle is greater than φmax during the adjustment process, then the output of ④ will limit the amplitude. 5 Conclusion The high-efficiency medium-frequency melting power supply control circuit designed in this paper has the following characteristics: —High circuit integration and strong anti-interference capability, suitable for medium-frequency induction melting with a frequency of 1000Hz～2500Hz; —The input signal is taken from the original detection circuit and control circuit, eliminating the need for an additional detection circuit; —The angle adjustment circuit can replace the original inverter trigger and can serve as a backup for the original inverter trigger, enabling efficient control of the medium-frequency power supply and improving operational reliability.