AN-9052 Datasheet, PDF (1/4 Pages) Fairchild Semiconductor – Design Guide for Selection of Bootstrap Components

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Application Note AN-9052
Design Guide for Selection of Bootstrap Components
1. Bootstrap Circuit
1.1 Bootstrap Floating Supply
Using a N channel MOSFET as a high side switch requires a
voltage supply referenced at the source of the MOSFET.
One of the most widely used method in supplying power to
the high-side circuitry is the use of the bootstrap floating
supply due to its inherent simplicity and inexpensive fea-
tures. This kind of floating supply is suitable for providing a
gate drive circuitry to directly drive high side switches that
operate up to rail voltages. The basic circuit of the bootstrap
supply, shown in Figure 1, is formed by a diode (Dbs) and a
capacitor (Cbs). But, this type of floating supply has limita-
tions on refreshment of Cbs when duty cycle is very high or
turn-on time is very long. In the case where the gate voltage
is not enough to fully turn-on the MOSFET (Q1), the output
of gate drive IC (HO) should be turned-off to prevent the Q1
from operating in high dissipation mode. The optional gate
resistor (Rg) is used for the purpose of controlling the turn-
on/turn-off time of the Q1, and the bootstrap resistor (Rbs) is
used to limit the current and prevent the bootstrap capacitor
(Cbs) from overcharging.
VCC
Gate Drive IC
VB
iQLS iQBS
HO
Dbs
Rbs
VS
Q1
Rg
Cbs
VS
RVS
X
-
Load
+
VFP
or
Vx
+
Low side Switch(Q2) -
figure 1. Bootstrap Circuit
1.2 Operation of Bootstrap Circuit
The charged capacitor (Cbs) supplies the voltage to the tran-
sistors of the gate drive IC, which is used to turn ON and
OFF the external high side switch (Q1). The bootstrap
capacitor(Cbs) gets charged from the voltage supply (VCC),
through the bootstrap diode (Dbs), when the voltage at node
X (VX) is pulled down to ground or even below ground
level. The bootstrap capacitor needs to be sized properly to
account for the case when Vx is pulled down to ground,
which Vbs is at its lowest level, and cause under voltage
lockout (UVLO) malfunction. Most gate drive ICs have und-
ervoltage detection circuit that prevents from driving an
external switch when Vbs drops below a certain level (speci-
fied in datasheets as VBSUV level). The VBSUV level
depends on the external switch that it is driving. The under-
voltage level for IGBTs are in the 9V~10V range, and for
MOSFETs in the 4V~5V range. In the case where the node
X goes below the ground level, Cbs will be overcharged by
the level in which it goes negative. There are negative tran-
sients at node X caused by the parasitic inductances and peak
forward voltage drop (Vfp) of the body diode at the low side
switch that needs to be considered also. All of the overcharg-
ing affect mentioned above needs to be taken into account in
determining the size of Cbs. Adding resistors Rbs, Rvs, and
using a diode with a low Vfp value are other possible solu-
tions to limit the overcharge effect on Cbs. Let us now look
at the case that causes the Cbs to discharge. Cbs discharges
when Q1 turns-on or node X is floating. The associated dis-
charging factors are gate drive power, leakage current in
each component, and current consumption in the gate drive
IC. From an application point of view, specific conditions
such as the duty cycle of PWM that causes ripple voltages on
Cbs, operation frequency, and the type of modulation at
which Q1 operates needs to be examined to make sure that
Cbs can handle.
1.3 Initial Charging and Refreshment of Boot-
strap Capacitor
Another key parameter in selecting bootstrap components is
initial start-up time. The initial charging time(tch) can be cal-
culated from the following equation:
t_ch
≥
C_bs ×
R_T ×
1
D----
×
ln


V-----_c^---_c^{---–-----V-----}_b^---_s^-_,^-V-_M^----c-_i^-c-_n^{----–----V-----}_f^{---–-----V----}_x^{-}
(1 )
Where,
RT = Rbs + Rvs (with low side switch and no load)
RT = Rbs + Rvs + RL (with loads including equivalent
impedance at node X)
D = duty cycle
In the case where PWM is not used, the load not connected,
and the low side switch turned on the charging time at the
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/10/08
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