IR22771S/IR21771S(PbF)
14
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2 Sizing tips
2.1 Bootstrap supply
The V
BS1,2,3
voltage provides the supply to the high
side drivers circuitry of the IR22771S/IR21771S.
V
BS
supply sit on top of the V
S
voltage and so it
must be floating.
The bootstrap method to generate V
BS
supply can
be used with IR22771S/IR21771S current sensors.
The bootstrap supply is formed by a diode and a
capacitor connected as in Figure 16.
Figure 16: bootstrap supply schematic
This method has the advantage of being simple and
low cost but may force some limitations on duty-
cycle and on-time since they are limited by the
requirement to refresh the charge in the bootstrap
capacitor.
Proper capacitor choice can reduce drastically
these limitations.
Bootstrap capacitor sizing
Given the maximum admitted voltage drop for V
BS
,
namely V
BS
, the influencing factors contributing to
V
BS
decrease are:
Floating section quiescent current (I
QBS
);
Floating section leakage current (I
LK
)
Bootstrap diode leakage current (I
LK_DIODE
);
Charge required by the internal level shifters
(Q
LS
); typical 20nC
Bootstrap capacitor leakage current (I
LK_CAP
);
High side on time (T
HON
).
I
LK_CAP
is only relevant when using an electrolytic
capacitor and can be ignored if other types of
capacitors are used. It is strongly recommend using
at least one low ESR ceramic capacitor (paralleling
electrolytic and low ESR ceramic may result in an
efficient solution).
Then we have:
HON
CAP
LK
DIODE
LK
LK
QBS
LS
TOT
T
I
I
I
I
Q
Q
+
)
(
_
_
The minimum size of bootstrap capacitor is then:
BS
TOT
BOOT
V
Q
C
=
min
Some important considerations
a) Voltage ripple
There are three different cases making the
bootstrap circuit get conductive (see Figure 16)
I
LOAD
< 0; the load current flows in the low side
IGBT displaying relevant V
CEon
CEon
F
CC
BS
V
V
V
V
In this case we have the lowest value for V
BS
.
This represents the worst case for the bootstrap
capacitor sizing. When the IGBT is turned off the
Vs node is pushed up by the load current until the
high side freewheeling diode get forwarded
biased
I
LOAD
= 0; the IGBT is not loaded while being
on and V
CE
can be neglected
F
CC
BS
V
V
V
I
LOAD
> 0; the load current flows through the
freewheeling diode
FP
F
CC
BS
V
V
V
V
+
In this case we have the highest value for V
BS
.
Turning on the high side IGBT, I
LOAD
flows into it
and V
S
is pulled up.
b) Bootstrap Resistor
A resistor (R
boot
) is placed in series with bootstrap
diode (see Figure 16) so to limit the current when
the bootstrap capacitor is initially charged. We
suggest not exceeding some Ohms (typically 5,
maximum 10 Ohm) to avoid increasing the V
BS
time-
constant. The minimum on time for charging the
bootstrap capacitor or for refreshing its charge must
be verified against this time-constant.
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