IGBT Gate Drivers in High-Frequency Induction Cookers (3)

Gate driver circuits for IGBT power switches
Three types of driver circuits, using discrete transistors (Fig. 5), gate driver optocouplers (Fig. 6) or gate driver transformers (Fig. 7) can be used to drive the power switches in the induction cooker. There are several issues associated with high-frequency gate drivers: parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver, all of which are involved when selecting an appropriate driver circuit.

Typically, the switching frequency of an induction cooker is between 25 kHz and 40 kHz. In order to rapidly 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 charges and discharges the gate capacitor of the IGBT, a relatively high peak gate current may be needed for proper operation. A higher peak current is also desirable to increase the charging and discharging rates during turn-on and turn-off, to help reduce the switching losses of the IGBT. Due to this, managing the power dissipation within the gate drive circuit becomes increasingly important as the switching speeds are increased.
Discrete gate drivers are constructed using bipolar transistors, and NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver, while simultaneously incorporating necessary operational and protective functions such as under voltage lockout (UVLO), is not as space efficient as using integrated circuits. Moreover most discrete transistor driver designs do not provide sufficient safety isolation or noise immunity.
Two methods of providing electrical isolation are pulse transformers and gate driver optocouplers. The pulse transformer is a traditional and simple solution, which, however, suffers from the potential for core saturation in a reasonably-sized transformer, resulting in reduced efficiency. A pulse transformer can only transmit AC signals, and most designs have a limited duty cycle ranging up to 50 percent due to the transformer volt-second relationship.
An additional capacitor and zener diode on the transformer secondary can be added to permit a higher duty cycle. However, this increases the circuit board size and parasitic inductances, which, in turn, increases power losses in the driver circuit.
The gate driver optocoupler IC integrates an LED light source and optical receiver for safety isolation, with transistors to provide sufficient drive current, and protection functions such as UVLO or desaturation detection.
Gate driver ICs are easy to design with, and will save PCB board space. 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, as with any ICs, power dissipation is a major concern.
For the single-switch resonant converter, the designer has the option of the discrete gate driver, gate transformer or gate driver optocoupler topologies. As discussed previously, the quasiconverter resonant voltage can be higher than the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker designs, the discrete gate driver circuit is used as there is no upper power switch, and both the controller and 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, and not provide any isolation. As shown, the pulse transformer galvanic isolation solution becomes increasingly complicated for duty cycle switching above 50 percent. 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 many of the problems that are associated with transformer or discrete transistor drivers.

In this article, the halfbridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen. The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The required size of the gate drive transformer consumes board space, and requires additional work, cost and board space to achieve switching duty cycles above 50 percent. Finally, the use of gate drive optocoupler ICs saves board space through high level feature integration while providing high voltage safety isolation and noise immunity all in one package.


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