Gate Driver Optocouplers in Induction Cooker (3)

Gate Driver Circuits for IGBT Power Switches
Three types driver circuits, the discrete transistor circuit (Figure 5), gate driver optocoupler (Figure 6) and gate driver transformer (Figure 7) can be used to drive the power switches in induction cooker application. There are several issues associated with high-frequency gate drivers; the parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver.

Typically, the switching frequency of an induction cooker is between 25kHz to 40kHz. In order to rapidly charge turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charge and discharge the gate capacitor of the IGBT, a higher peak gate current may be needed for proper operation. Due to this, the power dissipation within the gate drive circuit is important to manage the increase switching speed. The higher peak current is also desirable to increase the charging and discharging during turn on and off as it will help reduce the switching losses of the IGBT.
The discrete gate drivers are constructed using the bipolar transistors. NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver and other functions or protection operation like Under Voltage Lockout (UVLO) is not as space efficient as using integrated ICs. Moreover discrete transistor drivers do not provide sufficient safety isolation or noise immunity.
Two types of isolation method are discussed in this article; pulse transformer and gate driver optocoupler. The pulse transformer is a traditional and simple solution which suffers from saturation limitation for a given transformer size that can reduce efficiency. Normally, a transformer can only transmit AC information and have a limited duty cycle of up to 50% due to the transformer volt-second relationship. Additional capacitor and zener diode on the secondary size can be added to allow a higher duty cycle. However, this increase the design board size and parasitic inductances which in turn increases power losses in the driver circuit.
Gate driver optocoupler ic is an integration of LED for safety isolation, transistors to provide drive current and protection functions like UVLO or Desaturation Detector. Gate driver ICs are easy to design and will save PCB board space in the application. Due to the integrated design, the drive circuitry can be located very close to the power switch which not only saves PCB space but also improves the overall noise immunity of the system. However, like any integrated ICs, power dissipation is main concern observed by designers.
For the single switch resonant converter, designer has the option of the discrete gate driver topology, gate transformer or gate driver optocoupler. As discussed in the previous section, the quasi-converter resonant voltage can be higher compared to the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker design, the discrete gate driver circuit is used as there is no upper power switch and both controller and the power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high side discrete solution would increase the component count while not providing any isolation. As shown, the pulse transformer galvanic isolation solution increases in complexity for duty cycle switching above 50%. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves much of the problems that are associated with transformer driver or transistor discrete solution as mentioned earlier.

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