How to determine the control method of resonant single-phase full-bridge inverter?

With the continuous development of electronic power device design technology, inverters are widely used in various designs, among which series resonant full-bridge inverters are used more frequently. This article compares and discusses pulse control methods such as pulse density and pulse frequency in series resonant half-bridge transformers, especially focusing on frequency modulation and pulse width.

Basic structural analysis

The basic schematic diagram of the series resonant inverter is shown in Figure 1. It includes a DC voltage source, an inverter bridge composed of switches S1 to S4, and a series resonant load composed of R, L, and C. Among them, switches S1 to S4 can use power semiconductor devices with self-shutdown capabilities such as IGBT, SIT, MOSFET, and SITH. The inverter is a single-phase full-bridge circuit, and its control method is that the driving signals of the two switch tubes in the same bridge arm are complementary, and the two diagonal switches are turned on and off at the same time.

Control method of series resonant inverter

PAM method

There is not only one method of amplitude modulation control. We can adjust the DC voltage source output (inverter input) voltage Ud (the method of phase shift voltage regulation circuit can be used, or the filter circuit composed of chopper voltage regulation circuit plus inductor and capacitor can be used to achieve the purpose of adjusting the output power. That is, the output power of the inverter is adjusted by the input voltage, and the phase control between current and voltage is completed by the phase-locked loop (PLL) to ensure a larger power factor output. The advantage of this method is that the control is simple and easy, and the disadvantage is that the circuit structure is complex and the volume is large.

Pulse Frequency Modulation (PFM) Method

The pulse frequency modulation method is to change the operating frequency of the inverter, thereby changing the load output impedance to achieve the purpose of adjusting the output power.

From the impedance characteristics of the series resonant load

It can be seen that the impedance of the series resonant load changes with the change of the inverter operating frequency (f). For a constant output voltage, the greater the deviation between the operating frequency and the load resonant frequency, the higher the output impedance, and therefore the smaller the output power, and vice versa. The main disadvantage of the pulse frequency modulation method is that the operating frequency changes continuously during the power regulation process, causing the skin depth to change accordingly. In some applications such as surface quenching, the change in skin depth will have a greater impact on the heat treatment effect, which is not allowed in demanding applications. However, since the pulse frequency modulation method is very simple to implement, it can be considered in the following situations:

1) If the load does not have strict restrictions on the operating frequency range, the frequency must be tracked, but the phase difference can exist without being in a resonant operating state.

2) If the Q value of the load is high, or the power adjustment range is not very large, a smaller frequency deviation can meet the power adjustment requirements.

Pulse Density Modulation (PDM) Method

The pulse density modulation method controls the output power by controlling the pulse density, which is actually controlling the time of feeding energy to the load. Its control principle is shown in Figure 2.

The basic idea of ​​this control method is: suppose there are N power regulation units in total, in M ​​of them the inverter outputs power to the load; in the remaining NM units the inverter stops working, and the load energy gradually decays in the form of natural oscillation. The output pulse density is M/N, so the output power is related to the pulse density. Therefore, the output power can be changed by changing the pulse density.

The main advantages of the pulse density modulation method are: the output frequency remains basically unchanged, the switching loss is relatively small, it is easy to implement digital control, and it is more suitable for open-loop working environments.

The main disadvantage of the pulse density modulation method is that the frequency of the inverter output power is not completely equal to the natural resonant frequency of the load, and the working stability is poor in situations where a power closed loop is required. Since the working frequency must be re-locked each time the system recovers from the natural attenuation oscillation state to the output power state, the system may be out of control. Therefore, in situations where the power closed loop or temperature closed loop is required, the working stability is not good. Another disadvantage is that the power regulation characteristics are not ideal, and the power regulation method is stepped.

Resonant Pulse Width Modulation (PWM) Method

In Figure 3, the resonant pulse width modulation changes the output voltage value by changing the phase difference between the driving signals of the two pairs of switch tubes to achieve the purpose of adjusting the power. That is, in the control circuit, the driving signals of the two bridge arm switches (S1, S2) and (S3, S4) that were originally in phase are staggered by a phase angle, so that a zero voltage value is inserted between the output positive and negative alternating voltages. In this way, the effective value of the output voltage can be changed by changing the phase angle, and finally the purpose of adjusting the output power is achieved.

The advantage of this control method is that the power supply always works in a resonant state with a high power factor. However, there are problems with reverse recovery of the anti-parallel diode, small load problems, and soft switching implementation problems. In this way, diodes D2 and D3 are naturally turned off at zero crossing, D1 and D4 are not conducting, and there are no problems caused by diode reverse recovery. S1 and S4 switch at zero current (ZCS), and S2 and S3 are turned off at high current. ZVS can be achieved by connecting capacitors in parallel to S2 and S3. This method is suitable for high-frequency power supplies and devices with serious reverse recovery problems of built-in anti-parallel diodes, such as MOSFET. It can avoid current spikes and increased device losses caused by diode reverse recovery.

To ensure that the leading edge of the trigger signal of the leading arm is in phase with the current signal, the angular frequency is

From the above analysis, we can know that no matter it is the up-frequency PWM or the down-frequency PWM, they have a common feature, that is, while adjusting the output voltage pulse width, they also change the load's operating frequency. Therefore, it is called the pulse width plus frequency modulation method.

Conclusion

This article mainly analyzes the modulation of pulse width and frequency in detail, and gives some commonly used control methods of power and frequency of series resonant single-phase full-bridge inverters. This allows engineers to choose the control method suitable for different occasions based on the load.