Fairchild FSB44104A, User Manual

Page: 19 / 24

AN-9111 APPLICATION NOTE
© 2015 Fairchild Semiconductor Corporation www.fairchildsemi.com
Rev. 1.1
• 6/26/15
18
0 2 4 6 8 10 12 14 16 18 20
0
10
20
30
40
50
60
70
80
90
100
110
6.8[  F]
Continuous Sinusoidal Current Control
10[  F]
22[  F]
33[

F]
47[  F]
100[  F]
C
BS_min
=(I
Leak
*  t)/  V
BS
Conditions :  V
BS
=0.1 [V], I
Leak
=4.5 [mA]
Minimum Value
Recommend Value
Commercial Capacitance
Boo
tstrap
C
apacita
n
ce
, C
BS
[

F
]
Switching Frequency, F
SW
[kHz]
Figure 23. Capacitance of Bootstrap Capacitor on
Variation of Switching Frequency
Based on switching frequency and recommended ΔV
BS

I
Leak
: circuit current = 10 mA

ΔV
BS
: discharged voltage = 2.0 V (recommended value)

Δt: maximum on pulse width of high-side MOSFET =
2 ms (depends on user system)

More than 2~3 times

20~30 μF.

S
tandard nominal capacitance 22 ~ 35 μ F
6.4.4. Selection of Bootstrap Resistor Considering
Operating
A resistor must be added in series with the bootstrap diode
to slow down the dV
BS
/dt and determine the time to charge
the bootstrap capacitor. If the minimum ON pulse width of
low-side MOSFET or the minimum OFF pulse width of
high-side MOSFET is t
O
; the bootstrap capacitor must be
charged to increase the voltage by ΔV during this period.
Therefore, the value of bootstrap resistance can be
calculated by:
BS
ΔV
) (

 

BS
o BS CC
BS
C
t V V
R
where:
V
CC
= Supply voltage;
V
BS
= Minimum bootstrap voltage;
t
O
= Minimum ON pulse width;
C
BS
= Bootstrap capacitor value; and
ΔV
BS
= Ripple voltage of V
BS
.
Calculation Examples of R
BS
:
V
CC
= 15 V, V
BS
= 13 V (minimum voltage)
t
O
= 200 µs (if carrier frequency is 5 kHz, 1-cylce is 200 µs)
C
BS
= 20 µF (obtained bootstrap capacitor value)
ΔV
BS
= 2 V (recommended value)
 
 

 


 

10
2 20
200 13 15
ΔV
) (
BS
V F
s V
C
t V V
R
BS
o BS CC
BS


If the rising dV
BS
/dt is slowed significantly, it could cause
missing pulses during the startup phase due to insufficient
V
BS
voltage.
Note:
18. The capacitance value can be changed according to the
switching frequency, the capacitor selected, and the
recommended V
BS
voltage of 13.0~18.5 V (from
datasheet). The above result is just a calculation
example. This value can be changed according to the
actual control method and lifetime of the component.