6ED family - 2nd generation
Technical Description
Application Note
15
Rev. 1.3, 2014-03-23
AN-EICEDRIVER-6EDL04-1
This pin is on high potential again after transistor T2 is turned off and either T1 or D1 is conducting current. But
now the bootstrap diode D
BS
blocks a reverse current, so that the charges on the capacitor cannot flow back to
the capacitor C
VCC
. The bootstrap diode D
BS
also takes over the blocking voltage between pin VB and VCC. It is
good engineering to choose the same blocking voltage of power transistor T1 and external bootstrap diode. The
voltage of the bootstrap capacitor can now supply the highside gate drive sections.
It is a general design rule for the location of bootstrap capacitors C
BS
, that they must be placed as close as
possible to the IC. Otherwise, parasitic resistors and inductances may lead to voltage spikes, which may trigger
the undervoltage lockout threshold of the individual highside driver section.
The voltage of bootstrap capacitor is approximately
(1)
A current limiting resistor R
Lim
reduces the peak of the pulse current during the turn-on of transistor T2. The
pulse current will occur at each turn-on of transistor T2, so that with increasing switching frequency the
capacitor C
BS
is charged more frequently. Therefore a smaller capacitor is suitable at higher switching
frequencies. The bootstrap capacitor is mainly discharged by two effects: The highside quiescent current and
the gate charge of the transistor to be turned on. The calculation of the bootstrap capacitor results in
(2)
with
i
QBS
being the quiescent current of the highside section,
t
P
the switching period,
Q
G
the total gate charge
and
v
BS
the voltage drop at the bootstrap capacitor within a switching period. An additional margin of 20% is
added for the case of tolerances for the bootstrap capacitor.
Figure 11
Size of the bootstrap capacitor as a function of the switching frequency
f
P
for driving
IKD10N60R according to equ.
with a voltage ripple of 0.1 V
Figure 11 shows the curve corresponding to equ. (2) for a continuous sinusoidal modulation, if the voltage ripple
Δ
v
BS
= 0.1 V. The recommended bootstrap capacitance is therefore in the range up to 4.7
μF for most switching
frequencies. The performance of the integrated bootstrap diode supports the requirement for small bootstrap
capacitances. It is therefore not recommended to exceed a maximum capacitance of C
BS
= 47 µF.
Please note here, that equ. (2) is valid for continuous switching operation according to the switching frequency.
The use of space vector modulations can cause periods up to 60° (electrical), in which no switching of the low
side transistor of a halfbridge occurs and must be considered seperately. This effects the bootstrap capacitor
size, especially for low output current (motor current) frequencies. In this case the variable
t
P
must be set to the
longest period of no switching.
0
1
2
3
4
5
0
5
10
15
20
C
BS
f
P
kHz
µF
