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Series Application note
Publication Date: September 2016
51
4.3 Note for designing the bootstrap circuit
When each device for bootstrap circuit is designed, it is necessary to consider various conditions such as
temperature characteristics, change by lifetime, variation and so on. Note for designing these devices are listed as
below. For more detail information about driving by the bootstrap circuit, refer the DIPIPM application note
"Bootstrap Circuit Design Manual"
(1) Bootstrap capacitor
BSC employs electrolytic capacitors in general, and recently ceramic capacitor with large capacitance is also
applied to it. Please note that DC bias characteristic is considerably different between electrolytic capacitor and of
ceramic capacitor when applying DC voltage. Its characteristics especially differ with large capacitance type.
Table 4-3-1 shows example of difference between the above two capacitors.
Table 4-3-1 Differences of capacitance characteristics between electrolytic and ceramic capacitors
Electrolytic capacitor
Ceramic capacitor
(large capacitance type)
Temperature
characteristics
(Ta:-20~ 85°C)
•
Aluminum type:
Low temp.: -10% High temp: +10%
•
Conductive polymer aluminum solid type:
Low temp.: -5% High temp: +10%
Different due to temp. characteristics rank
Low temp.: -5%~0%
High temp.: -5%~-10%
(in the case of B,X5R,X7R ranks)
DC bias
characteristics
(Applying DC15V)
Nothing within rating voltage
Different due to temp. characteristics, rating
voltage, package size and so on
-70%~-15%
DC bias characteristic of electrolytic capacitor is no problem, however, it is necessary to note its ripple
capability by repetitive charge and discharge, its ambient temperature which affects the capacitor’s life time
greatly, and so on. These above characteristics are just example data which are quoted from the WEB site, so it is
recommended to inquiry to the capacitor manufacturers about detailed characteristics.
(2) Bootstrap diode
integrates bootstrap diodes for P-side driving supply. This BSD incorporates current limiting resistor
(typ. 20
Ω). The V
F
-I
F
characteristics (including voltage drop by built-in current limiting resistor) are shown in
Fig.4-3-1, 2 and Table 4-3-2.
(a) BSD V
F
-I
F
characteristics
(b) BSD V
F
-I
F
characteristics (Enlarged view)
Fig.4-3-1 Typical V
F
-I
F
curve for bootstrap Diode (For PSS**M(N)C1FT)
(a) BSD V
F
-I
F
characteristics
(b) BSD V
F
-I
F
characteristics (Enlarged view)
Fig.4-3-2 Typical V
F
-I
F
curve for bootstrap Diode (For PSS**M(N)C1F6)
0
5
10
15
20
25
30
35
40
45
50
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
I
F
[m
A]
V
F
[V]
0
100
200
300
400
500
600
700
800
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
I
F
[m
A]
V
F
[V]
