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 Bootstrap Components:

Bootstrap Capacitor

To size the bootstrap capacitor, CBOOT, we need to ensure that it stores enough charge to supply the power consumption of the gate driver and the necessary gate charge to turn the high-side IGBT “on”. Thus, the total charge required during this period is shown by equation (1):

QTOTAL = Qg +(Idd+ Ilk) x Ton                                                                                                                                 (1)

Qg = Gate charge of the IGBT.

Idd = Optocoupler gate driver supply current.

Ilk = Total leakage current.

Ton = High-side IGBT turned on period.

The next factor affecting the size of the capacitor is the maximum voltage ripple allowed, ΔVBOOT. The main consideration for ΔVBOOT is the minimum gate voltage, VGEmin, which is required to drive the IGBT effectively. If insufficient gate voltage is applied to the IGBT, the IGBT will go out of saturation and into the linear operation, where IGBT power loss is high.

All Fairchild Semiconductor gate driver optocouplers have under-voltage detection function that prevents the application of insufficient gate voltage to the IGBT, when its supply voltage drops below Under-Voltage Lockout Threshold, VUVLO. The min VGEmin should be set above the VUVLO.

The boundary condition for the voltage ripple, ΔVBOOT,   is therefore set by the following equation:

ΔVBOOT  ≤ VSOURCE - VF - VGEmin                                                                                                                (2)

VSOURCE = Optocoupler gate driver supply voltage at the low-side

VF = Bootstrap diode forward voltage

VGEmin = Minimum gate to emitter voltage of the IGBT for efficient operation.

Fairchild Semiconductor’s optocouplers have low IDD, across the specified operating temp (-40°C to +100°C) and supply voltage, Vdd (15 V to 30 V), as well as rail-to-rail output switching capability. All these characteristics enable a bootstrap circuit with a smaller value of CBOOT. In contrast, for those gate driver optocouplers with a bipolar output stage, the saturation voltage of the output stage during Ton, will have to be taken into consideration. Hence, the boundary condition is:

ΔVBOOT  ≤ VSOURCE - VCESAT - VF - VGEmin                                                                                                      (3)                                          VCESAT = the saturation voltage of the bipolar output stage gate driver during Ton

The minimum capacitor value is:

CBOOT  =  QToTAL / ΔVBOOT                                                                                                                                   (4)

Bootstrap Resistor

The bootstrap resistor limits the CBOOT charging current, and prevents the overcharging of the capacitor. The value of RBOOT, however, must not be too high as this will increase the required charging time. Based on the assumption that the CBOOT is fully charged after 4 time constant, the RBOOT is bounded by:

RBOOT  ≤ TOFF / (4 * CBOOT)                                                                                                                                              (5)

TOFF = High-side IGBT turned off period (low side freewheeling diode or IGBT turn on)

Bootstrap Diode

The main function of the bootstrap diode is to block the power rail voltage, when the high-side IGBT is turned on. The diode must have the following electrical characteristics:

-Blocking voltage must be higher than the power rail voltage.

-Current rating must be able to meet the product of total required charge and frequency.

- Fast recovery time to minimize the leakage current.

-Low forward voltage drop to optimize the charging potential across CBOOT.

Precautions in CBOOT  Design

Due to the inductive nature of the load and parasitic elements, the Vss of the gate driver optocoupler will be pulled below ground. So when the high-side IGBT turns off, as illustrated in Figure 3, the freewheeling diode of the low-side IGBT will begin to conduct.

The effective voltage across the bootstrap circuitry (CBOOT , RBOOT and DBOOT) becomes:

VSOURCE + VCE(FW)                                                                                                                                                                (6)

VCE(FW) = Negative voltage when the freewheeling diode turns on. The magnitude of the VCE(FW )

will depend on the parasitic inductance, as well as the load current.

As the CBOOT will be charging towards the max value VDD + VCE(FW) , the overvoltage appears on the gate of the high-side IGBT, when the high-side gate driver (rail-to-rail output ) is turned on. Hence, to ensure that excessive VGE is not applied, the size of the various bootstrap components has to be optimized to prevent overcharging. Additional preventive measure can be taken by placing a zener diode across CBOOT.

On the other hand, when the low-side IGBT is turned on, as illustrated in figure 4, the effective voltage across the bootstrap circuitry (CBOOT , RBOOT and DBOOT) becomes:

VSOURCE - VCE(ON)                                                                                                                                                  (7)

VCE(ON) =The collector emitter voltage when the low-side IGBT is turned on.

This will impose an additional restriction on ΔVBOOT, and equation (2) and (3) become:

ΔVBOOT  ≤ VSOURCE - VF - VGEmin- VCE(ON)                        (for rail-to-rail MOS output gate driver)(8)

ΔVBOOT  ≤ VSOURCE - VCESAT - VF - VGEmin -VCE(ON)        (for bipolar output gate driver)                  (9)

In this case, care has to be taken to ensure that the gate driver optocoupler’s voltage supply is still high enough to switch on the IGBT efficiently.

Conclusion

Bootstrap circuits provide an effective method of supplying power to the high side gate driver in many power switching applications. However, various operational constraints need to be taken into account in the bootstrap design and component selection.