AN-6076 Datasheet, PDF (1/13 Pages) Fairchild Semiconductor – Bootstrap Circuit for High-Voltage Gate-Drive IC

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Application Note AN-6076
Design and Application Guide of Bootstrap Circuit for
High-Voltage Gate-Drive IC
1. Introduction
The purpose of this paper is to demonstrate a systematic
approach to design high-performance bootstrap gate drive
circuits for high-frequency, high-power, and high-efficiency
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
applications presented in this paper. One of the most widely
used methods to supply power to the high-side gate drive cir-
cuitry of the high-voltage gate-drive IC is the bootstrap
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
requirement to refresh the charge in the bootstrap capacitor
and serious problems occur when the negative voltage is pre-
sented at the source of the switching device. The most popu-
lar bootstrap circuit solutions are analyzed; including the
effects of parasitic elements, the bootstrap resistor, and
capacitor; on the charge of the floating supply application.
2. High-Speed Gate-Driver Circuitry
2.1 Bootstrap Gate-Drive Technique
The focus of this topic is the bootstrap gate-drive circuit
requirements of the power MOSFET and IGBT in various
switching-mode power-conversion applications. Where
input voltage levels prohibit the use of direct-gate drive cir-
cuits for high-side N-channel power MOSFET or IGBT, the
principle of bootstrap gate-drive technique can be consid-
ered. This method is utilized as a gate drive and accompany-
ing bias circuit, both referenced to the source of the main
switching device. Both the driver and bias circuit swing
between the two input voltage rails together with the source
of the device. However, the driver and its floating bias can
be implemented by low-voltage circuit elements since the
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
the floating high-side and ground-referenced low-side cir-
cuits. The high-voltage gate-drive ICs are differentiated by
unique level-shift design. To maintain high efficiency and
manageable power dissipation, the level-shifters should not
draw any current during the on-time of the main switch.
A widely used technique for these applications is called
pulsed latch level translators, shown in Figure 1.
VB
UVLO
PULSE GENERATOR
IN
HO
R
NOISE
R
CANCELLER
S
Q
VS
Figure 1. Level-Shifter in High-Side Drive IC
2.2 Bootstrap Drive Circuit Operation
The bootstrap circuit is useful in a high-voltage gate driver
and operates as follows. When the V_S goes below the IC
supply voltage V_DD or is pulled down to ground (the low-
side switch is turned on and the high-side switch is turned
off), the bootstrap capacitor, C_BOOT, charges through the
bootstrap resistor, R_BOOT, and bootstrap diode, D_BOOT, from
the V_DD power supply, as shown in Figure 2. This is pro-
vided by V_BS when V_S is pulled to a higher voltage by the
high-side switch, the V_BS supply floats and the bootstrap
diode reverses bias and blocks the rail voltage (the low-side
switch is turned off and high-side switch is turned on) from
the IC supply voltage, V_DD.
VDD
RBOOT DBOOT
VB
VDD
HO
VS
COM
LO
DC SUPPLY
Bootstrap charge current path
Bootstrap discharge current path
RG1
CBOOT
Q1
ILOAD
LOAD
RG2
Q2
Figure 2. Bootstrap Power Supply Circuit
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
www.fairchildsemi.com