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Introduction
IGBTs are commonly used in inverter systems, which are used for adjustable motor drives, solar inverters and UPS. Inverters can be single-phase or three-phase, which is basically an extension of the H-bridge. Figure 1 illustrated one of the six gate driver optocouplers from Fairchild Semiconductor driving the IGBT module for the implementation of the three-phase inverter system. In the case of a three-phase inverter, the use of bootstrap circuit can help to reduce the number of isolated control supplies from four (3 power supplies on the high-side + 1 on the low-side) to just one (low-side only).
Bootstrap Circuit Operation
The figure 2 shows a pair of gate driver optocouplers driving the IGBTs. The bootstrap circuit consists of a capacitor (CBOOT), a diode (DBOOT), and a surge limiting resistor (RBOOT), and it is utilized to supply the necessary power to the gate drivers which in turn drives the high-side IGBT.
When the high-side IGBT is turned off and the low-side IGBT is turned on, the power source, VSOURCE, charges CBOOT, through RBOOT and DBOOT. The charging current path is highlighted in red.
The CBOOT charge subsequently acts as a power source to the gate driver optocoupler, and drives the high-side IGBT via the discharge current path, highlighted in blue.
In the subsequent sections, a step by step calculation of the component values of the bootstrap circuit is presented. To simplify the calculation, bypass capacitors of the optocouplers will be assumed to be much smaller than CBOOT. Hence, they will not be taken into consideration.
Calculation for...