FAIRCHILD Bootstrap Circuit Design and Application Guide for High-Voltage Gate-Drive IC Data Handbook

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Application Note AN-6076 Design and Application Guide of Bootstrap Circuit for High-Voltage Gate-Drive IC

1. Introduction unique level-shift design. To maintain high efficiency and manageable power dissipation, the level-shifters should not

The purpose of this paper is to demonstrate a systematic

draw any current during the on-time of the main switch.

approach to design high-performance bootstrap gate drive

A widely used technique for these applications is called

circuits for high-frequency, high-power, and high-efficiency

pulsed latch level translators, shown in Figure 1.

switching applications using a power MOSFET and IGBT. It should be of interest to power electronics engineers at all levels of experience. In the most of switching applications, efficiency focuses on switching losses that are mainly depen-

dent on switching speed. Therefore, the switching character-

istics are very important in most of the high-power switching

PULSE GENERATOR

applications presented in this paper. One of the most widely IN RR

NOISE CANCELLER

used methods to supply power to the high-side gate drive cir- cuitry of the high-voltage gate-drive IC is the bootstrap VS power supply. This bootstrap power supply technique has the advantage of being simple and low cost. However, it has

some limitations, on time of duty-cycle is limited by the Figure 1. Level-Shifter in High-Side Drive IC

requirement to refresh the charge in the bootstrap capacitor

and serious problems occur when the negative voltage is pre- 2.2 Bootstrap Drive Circuit Operation

sented at the source of the switching device. The most popu-

The bootstrap circuit is useful in a high-voltage gate driver

lar bootstrap circuit solutions are analyzed; including the

and operates as follows. When the VS goes below the IC

effects of parasitic elements, the bootstrap resistor, and

supply voltage VDD or is pulled down to ground (the low-

capacitor; on the charge of the floating supply application.

side switch is turned on and the high-side switch is turned off), the bootstrap capacitor, CBOOT, charges through the 2. High-Speed Gate-Driver Circuitry bootstrap resistor, RBOOT, and bootstrap diode, DBOOT, from the VDD power supply, as shown in Figure 2. This is pro-

Sh oo t-t hr ou gh ur re nt 2.1 Bootstrap Gate-Drive Technique vided by VBS when VS is pulled to a higher voltage by the co pe ns at ed at dr iv er

high-side switch, the VBS supply floats and the bootstrap

The focus of this topic is the bootstrap gate-drive circuit

diode reverses bias and blocks the rail voltage (the low-side

requirements of the power MOSFET and IGBT in various

switch is turned off and high-side switch is turned on) from

switching-mode power-conversion applications. Where

the IC supply voltage, VDD.

input voltage levels prohibit the use of direct-gate drive cir-

cuits for high-side N-channel power MOSFET or IGBT, the RBOOT DBOOT

DC SUPPLY

principle of bootstrap gate-drive technique can be consid-

Bootstrap charge current path

ered. This method is utilized as a gate drive and accompany- Bootstrap discharge current path

ing bias circuit, both referenced to the source of the main

VDD HO Q1

switching device. Both the driver and bias circuit swing

ILOAD

between the two input voltage rails together with the source CBOOT

of the device. However, the driver and its floating bias can

be implemented by low-voltage circuit elements since the Q2RG2

COM LO

input voltage is never applied across their components. The driver and the ground referenced control signal are linked by a level shift circuit that must tolerate the high-voltage differ- ence and considerable capacitive switching currents between

Figure 2. Bootstrap Power Supply Circuit

the floating high-side and ground-referenced low-side cir- cuits. The high-voltage gate-drive ICs are differentiated by

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