Research on series resonant inverter based on time-sharing and phase control

0 Introduction

The induction heating power supply uses the principle of electromagnetic induction to generate eddy currents in the heated workpiece through the induction coil to heat the workpiece. Induction heating has many advantages such as fast heating speed, high heating efficiency, easy temperature control, and easy automation. Therefore, it has been widely used in modern industrial production, and induction heating technology has become increasingly mature. In view of the advantages of IGBT such as small switching loss, fast on-off speed, high operating frequency, large component capacity and gradually decreasing cost, IGBT is selected as the power switch tube. However, the switching loss of IGBT, especially the turn-off loss caused by the tail current under high-frequency switching working state, is very large, which limits the increase of operating frequency. At present, the switching frequency of IGBT can operate at a frequency of 100 kHz in the zero current switching (ZCS) state. The inverter output frequency can be increased by 2 times by using the frequency doubling method, but an additional resonant circuit is required, and the frequency increase is limited, and the commutation conditions of the device are also poor. The control method of IGBT parallel time-sharing can increase the switching frequency of the inverter.

The power regulation methods of induction heating power supply can be divided into two categories: inverter power regulation and DC power regulation. The main methods of inverter power regulation are: pulse frequency modulation (PFM), pulse density modulation (PDM), pulse width modulation (PWM), pulse uniform modulation (PSM), etc. DC power regulation usually uses DC chopping or phase-controlled rectification to change the input DC voltage of the inverter, thereby converting the power regulation of the inverter into regulation of the DC voltage. Each power regulation method has its own advantages and disadvantages. Compared with these power regulation methods, phase power regulation has the advantages of simple control circuit and drive pulse, wide stable working range, fast response speed and strong adaptability.

From the perspective of equipment cost, volume and conversion efficiency, a load series resonant inverter with four IGBTs in parallel is designed, and a new strategy of IGBT time-sharing-phase composite control is adopted to achieve inverter quadruple frequency output and output power regulation, and a systematic theoretical analysis and circuit simulation are carried out. The feasibility of this solution is verified through simulation.

1 Circuit Structure

Figure 1 shows the main circuit structure of the quadruple frequency inverter. The power supply adopts an AC/DC/AC structure. The input is pulsating DC voltage obtained by three-phase uncontrolled rectification, and then a smooth DC voltage is obtained by filtering link C0, which is sent to a load series resonant single-phase full-bridge inverter to generate high-frequency voltage and current on the induction coil. Each bridge arm of the inverter circuit is composed of 4 IGBT switching devices in parallel, CD is a DC blocking capacitor; T is a high-frequency transformer for load matching; R and L are the equivalent inductance and resistance of the induction coil; and the compensation capacitor C forms the secondary side resonant tank circuit of the transformer.

2 Analysis of control strategy

The traditional inverter works in such a way that the IGBTs in parallel in each bridge arm work simultaneously in each switching cycle. Under certain heat dissipation conditions, in order to increase the output frequency, the IGBT must increase the current rating, and the current sharing of parallel devices is also a problem, and the increase in output frequency is also limited. Time-sharing control of the IGBTs in each bridge arm of the inverter can avoid these shortcomings and achieve an increase in output frequency. Its working principle is shown in Figure 2.

As can be seen from Figure 2, Q1a~Q4a constitute the first group of inverter bridges, Q1b~Q4b constitute the second group of inverter bridges, Q1c~Q4c constitute the third group of inverter bridges, and Q1d~Q4d constitute the fourth group of inverter bridges. The four groups of inverter bridges are turned on for a resonant cycle. In this way, if the switching frequency allowed by the IGBT is f0, the output frequency of the power supply is 4f0. At the same time, the phase power regulation method is adopted to adjust the output power by adjusting the conduction width of the switch tube to adjust the lag angle ψ of the output current and voltage. By detecting the zero point of the load current, the conduction time of the switch tube is adjusted to make it lead the current by an angle ψ, ψ is adjustable from 0 to 90°. According to P=UIcosψ, it can be known that changing ψ can achieve the purpose of power regulation. The specific working process analysis of the inverter is shown in Figure 3.

Assume that C1~C4 are the CE junction capacitances of the IGBT. In the initial state, D1a, D4a are turned on, the load resonant current i is negative, and reversely charges C0. Its equivalent circuit is shown in Figure 3(a).

(1) t0-t1: At t0, the current i reverses, ●1a, Q4a is turned on at zero current and zero voltage (ZVZCS), and the load resonant current i is positive. Its equivalent circuit is shown in Figure 3(b). The load resonant current i flows from a to b, and the resonant load is provided with energy by the power supply UD.

The voltage differential equation of the load circuit is listed as:

Initial conditions: uC=-Ucm, i=0

Where: UD is the inverter input voltage; R is the load circuit equivalent resistance; L is the load equivalent inductance; i is the load circuit current.

Solving this differential equation yields:

(2) t1-t2: At t1, UC = UC1, i = i1. Q1a and Q4a are turned off at zero voltage (ZVS). The load resonant current i is positive, and its equivalent circuit is shown in Figure 3(c). The inductor L and C, C1-C4 resonate together; C3 and C2 discharge; C1 and C4 charge.

The voltage differential equation of the load circuit is listed as:

Where: Ca=C+C1.

Initial conditions: UC=UC1, i=i1

Solving this differential equation yields: