Single-phase full-bridge series resonant inverter control method used

Amplitude control is achieved by adjusting the DC voltage source (inverter input) voltage Ud (Phase voltage circuit can be used, you can add a chopper circuit composed of inductance and capacitance filter circuit to adjust the output power to achieve the purpose is the output power by the sine wave inverter input voltage regulation, the phase-locked loop (PLL) to complete the phase between the current and voltage control, in order to ensure a large output power factor.

This method has the advantage of simple control, the disadvantage is that the circuit complex, larger.

3.2 pulse frequency modulation (PFM) method

Pulse frequency modulation method is achieved by changing the operating frequency of the inverter, thereby changing the output impedance of the load in order to achieve the purpose of regulating the output power.

From the impedance characteristic of the series resonant load

Impedance characteristics

Shows that the impedance of the series resonant load inverter working with the frequency (f) and change. For a constant output voltage, when the operating frequency and the larger the load resonant frequency deviation, the higher the output impedance, the output power is smaller, and vice versa.

Pulse frequency modulation major drawback is the operating frequency of the power changing the adjustment process, also will cause the skin depth is changed, in some applications such as surface hardening, the change of the skin depth will have a greater effect of heat treatment impact, which in demanding applications is not allowed. However, due to pulse frequency modulation method is very simple, it can be considered in the following cases to use it:

1) If the load on the operating frequency range is not strictly limited, then the frequency must be tracked, but the phase difference in the resonant state does not exist.

2) If the load is higher Q values, or power adjustment range is not large, a smaller frequency deviation can achieve the power adjustment requirements.

3.3 pulse density modulation (PDM) method

Pulse density modulation method by controlling the pulse density, in fact, is the control of the energy supplied to the load time to control the output power. The control principle shown in Figure 2.

Control schematics

This control method is the basic idea is: Assuming a total of N power adjustment unit, in which the M power adjustment unit in the inverter output power to the load; while the remaining N-M units within the dc to ac power inverter is stopped, load energy in the form of natural oscillations gradually decay. Output pulse density of M / N, so that the output power of the pulse density just linked up. Therefore, the density can be changed by changing the pulse output power.

Pulse density modulation method main advantages are: output frequency is essentially the same, switching losses are relatively small, easy to implement digital control, more suitable for open-loop applications.

Pulse density modulation method main disadvantages are: the output power of the inverter frequency is not exactly equal to the natural resonant frequency of the load, in the case of closed loop power is needed, the work stability is poor. Because each oscillation state recover from the natural decay to the output power of the state to re-lock the operating frequency, then the system may be out of control. Therefore, the temperature of the power loop or closed loop case, the stability of the work is not good. Another drawback is its power regulation characteristics are not ideal, was a step way power adjustment.

3.4 resonant pulse width modulation (PWM) method

In Figure 3, the resonance pulse width modulation is achieved by changing the two pairs of the switch drive signal to change the phase difference between the output voltage in order to achieve the purpose of regulating the power. Manipulation of the control circuit is in phase with the original two-side switches (S1, S2), (S3, S4) shifted between a drive signal phase angle, such that the output voltage of the alternating positive and negative voltage is inserted between a zero value, so long as the change in phase angle can change the output voltage rms, and ultimately achieve the purpose of regulating the output power.