Gate Driver Optocouplers in Induction Cooker (2)

By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of circuit where the resonant inductor, Lr and capacitor Cr are located. From this, we can obtain the equivalent circuit for half-bridge and quasi resonant circuit, shown in Figure 3 and Figure 4. From these equivalent circuits, the operation of the induction cooker, the sizing of the resonant inductor, capacitor and control algorithm can be conceived.

In order to reduce component size, minimize switching losses and reduce audible noise during operation (above 20kHz resonant frequency), induction cooker circuit typically utilizes resonant or soft switching techniques. This circuitsoft switching technique can be subcategorized into two methods: Zero-voltage switching and Zero-current switching.
Zero-voltage switching occurs when the transistor turn-on at zero voltage. Zero-current switching refers to elimination of turn-off switching loss at zero current flow. The voltage or current administered to the switching circuit can be made zero by using the resonance created by an L-C resonant circuit. This topology is named a “resonant converter.” This allows the application to utilize resonant frequency and obtain the benefits mentioned compared to conventional hard switching techniques.
The advantages of half-bridge series resonant are stable switching, and lower cost due to streamlined design. The voltage within the circuit is limited to the level of the input voltage which reduces the voltage stress across IGBT power switch. This in-turn allows the designer to lower the cost by choosing a lower rating IGBT. The disadvantage is that the overall half-bridge control is more complicated, the size of heatsink and PCB area is bigger and insulated gate driver circuits, especially on the upper IGBT (S1 in Figure 1).
The advantage of quasi-resonant converter is that it needs only 1 IGBT power switch which reduces design size PCB and heat sink. The disadvantages are that the quasi resonant switching the high resonant voltage which can be higher than the DC input voltage stressed on to IGBT power switches. This requires a higher cost and blocking voltage power components.


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