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參數(shù)資料
| 型號(hào)： | HIP2500 |
| 廠商： | Intersil Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數(shù)： | 5/8頁 |
| 文件大?。?/td> | 81K |
| 代理商： | HIP2500 |
4-5
Relying on the lower freewheeling or body diode to provide
refreshing without physically turning on the lower switch may
fail to properly refresh the bootstrap capacitor under certain
circumstances. This can happen when the load current is
either zero or in a direction as to flow into the upper free-
wheel diode or body diode into the high voltage bus. Unless
the lower power switch is turned on each cycle for the short
refresh period, the bootstrap capacitor may not get
refreshed. The conditions which can lead to this situation
often occur in motor controls where the motor is coasting in
one direction or the other at extremely light loads. Also
controllers with a tendency to “over-modulate” can cause a
refresh failure.
Users, anxious to get their circuit up and running quickly, will
find that a ceramic bootstrap capacitor of approximately
0.1
μ
F to 0.15
μ
F will be sufficient to drive most small to
medium size MOSFETs. The leadless surface-mount capac-
itors minimize series inductance and enhance rapid refresh-
ing of this capacitor.
The bootstrap diode should be a small signal high voltage
type capable of blocking full DC bus voltage plus the V
CC
voltage. The recovery charge of the diode should be small
so that when it recovers it will not appreciably discharge the
bootstrap capacitor. Leakage current is usually not a con-
cern, since the recovered charge of the diode will be much
more significant than the leakage current over the PWM
cycle. A 1000V signal diode, such as industry standard
1N5622, is preferable to a lower voltage diode, since its junc-
tion capacitance and recovered charge will be smaller. Also
a higher voltage diode will have a low reverse leakage cur-
rent when operated at half of its rated blocking voltage. Stan-
dard small signal diodes such as the 1N4000 series should
be avoided. No rule of thumb will work in all situations, so it
is usually better to take a more detailed look at all the factors
which affect the bootstrap capacitor size.
The required value of capacitance depends on the V
CC
volt-
age, the switching frequency, the HIP2500 high side supply
current requirement, and the amount of equivalent gate
capacitance or gate charge required to fully charge the gate.
The gate charge requirement is generally included on most
MOSFET data sheets. Figure 7 shows a curve for a Intersil
IRF450 MOSFET. By designing the bootstrap circuit to sup-
ply the total required gate charge shown on the MOSFET
data sheet, the designer has included the effects of the Miller
capacitance.
As previously mentioned, the layout of the bootstrap circuit
should be compact so as to minimize the series inductance
of the bootstrap circuit. Excessive inductance will interfere
with rapidly charging the bootstrap capacitor during the time
provided by the minimum off-time of the upper switch. The
time allotted for turning off the upper switch is under full con-
trol of the circuit designer. However, maximum switching fre-
quency and duty cycle requirements often forces the
designer to live with “off-times” of less than 1 microsecond.
As the allotted refresh time is forced lower and lower, the
need for a short refresh loop becomes crucial.
A Real Example
A more exact sizing of the bootstrap capacitor than indicated
by Equation 1 takes into account the upper bias supply cur-
rent to the HIP2500 and leakage and recovery effects of the
bootstrap diode. Obviously PWM frequency will affect the
size requirement of the bootstrap capacitor too, so it would
be valuable to include PWM frequency as well. If we define
the following terms:
I
DR
I
QBS
Q
rr
Q
G
f
PWM
= PWM operating frequency
V
BS1
= C
BS
voltage immediately after refresh
V
BS2
= C
BS
voltage immediately before refresh
C
BS
= Bootstrap capacitance
then it will be possible to calculate a value for the bootstrap
capacitor as shown in Equation 2:
= Bootstrap diode reverse leakage current
= Upper supply quiescent current
= Bootstrap diode reverse recovered charge
= Turn-on Gate charge transferred
As an example, suppose we wish to drive an IRF450 to 15V
allowing a droop of 0.5V over the PWM cycle (i.e., V
BS1
-V
BS2
)
with a 16nC recovery charge and a leakage current of 2
μ
A for
the bootstrap diode with a maximum bias current, I
QBS
, of
400
μ
A. The gate charge, Q
G
, of 120nC required to drive the
IRF450 was read from the data sheet (see Figure 7). The
desired PWM switching frequency will be 20kHz. Using Equation
2, one would need a bootstrap capacitance of at least 0.31
μ
F.
Since 0.33
μ
F is the next larger standard capacitance available, a
ceramic capacitor of this value will be chosen.
The length of the refresh time required to charge the boot-
strap capacitor still needs to be evaluated. The refresh loop
is comprised of the bootstrap capacitor, the bootstrap diode,
stray circuit board resistance (the designer has laid out his
circuit to minimize this and stray inductance) and the
V
G
20
16
12
8
4
0
0
25
50
75
100
125
Q
G
TOTAL GATE CHARGE (nC)
I
D
= 13A
V
DS
= 400V
V
DS
= 250V
V
DS
= 100V
FIGURE 7. GATE CHARGE vs VOLTAGE
CBS
QG
--------------------------------–
Qrr
+
(
-------------------------------------
)
+
+
=
(EQ. 2)
Application Note 9010
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