IR2177 International Rectifier Corp., IR2177 Datasheet - Page 18

• Manufacturer Part Number: IR2177
• Description: Phase Current Sensor Ic For Ac Motor Control
• Manufacturer: International Rectifier Corp.

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
namely ∆V
V
I
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:
The minimum size of bootstrap capacitor is then:
Some important considerations
a) Voltage ripple
There are three different cases making the
bootstrap circuit get conductive (see Figure 20)
18
Q
LK_CAP
BS
IGBT displaying relevant V
TOT
In this case we have the lowest value for V
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
− Floating section quiescent current (I
− Floating section leakage current (I
− Bootstrap diode leakage current (I
− Charge required by the internal level shifters
− Bootstrap capacitor leakage current (I
− High side on time (T
decrease are:
=
I
(Q
LOAD
is only relevant when using an electrolytic
Q
LS
LS
); typical 20nC
< 0; the load current flows in the low side
BS
+
, the influencing factors contributing to
(
V
I
QBS
BS
C
=
BOOT
+
V
+
CC
I
min
LK
−
HON
+
=
V
I
CEon
F
).
LK
∆
Q
−
V
TOT
_
DIODE
V
BS
CEon
+
LK
LK_DIODE
I
LK
QBS
)
_
LK_CAP
CAP
);
)
);
⋅
T
);
BS
BS
HON
,
.
b) Bootstrap Resistor
A resistor (R
bootstrap diode (see Figure 20) to limit the current
when the bootstrap capacitor is initially charged. We
suggest not exceeding some Ohms (typically 5,
maximum 10 Ohms) to avoid increasing the V
time-constant. The minimum on time for charging
the bootstrap capacitor or for refreshing its charge
must be verified against this time-constant.
c) Bootstrap Capacitor
For high T
capacitor is used, its ESR must be considered. This
parasitic resistance develops a voltage divider with
R
charge of bootstrap capacitor. The voltage step and
the related speed (dV
general rule, ESR should meet the following
constraint:
Parallel combination of small ceramic and large
electrolytic
compromise, the first acting as fast charge tank for
the gate charge only and limiting the dV
reducing the equivalent resistance while the second
keeps the V
d) Bootstrap Diode
The diode must have a BV> 600V (or 1200V
depending on application) and a fast recovery time
(t
back from the bootstrap capacitor to V
rr
on and V
freewheeling diode
boot
high side freewheeling diode get forwarded
biased
In this case we have the highest value for V
Turning on the high side IGBT, I
and V
< 100 ns) to minimize the amount of charge fed
generating a voltage step on V
I
I
LOAD
LOAD
S
is pulled up.
CE
> 0; the load current flows through the
= 0; the IGBT is not loaded while being
HON
BS
can be neglected
capacitors
ESR
voltage drop inside the desired ∆V
V
boot
designs where an electrolytic tank
BS
) is placed in series with the
V
ESR
IR2277S/IR2177S(PbF)
+
=
BS
R
V
BS
=
BOOT
CC
/dt) should be limited. As a
V
CC
−
is
V
⋅
V
−
F
CC
normally
V
+
F
V
≤
FP
LOAD
3
V
CC
BS
www.irf.com
flows into it
supply.
at the first
the
BS
/dt by
best
BS
BS
BS
.
.