
AN-937 (v.Int)
basically
three ways of developing a gate drive signal that is referenced to a floating point:
1. By means of optically coupled isolators.
2. By means of pulse transformers.
By means of DC to DC chopper circuits with transformer isolation.
7.1 MGDs with optocouplers
Most optocouplers require a separate
supply grounded to the source on the
receiving end of the optical link and
a booster stage at the output, as
shown in Figure 14a. One of the
major difficulties encountered in the
use of optocouplers is their
susceptibility to noise. This is of
particular relevance in applications
where high currents are being
switched rapidly. Because of the
dv/dt seen by the V
EE
pin, the
optocoupler needs to be rated for
high dv/dt, in the order of 10 V/ns.
Figure 15a shows an MGD with
under-voltage lockout and negative
gate bias. When powered with a 19
V floating source, the gate drive
voltage swings between +15V and -
3.9V. D1 and R2 offset the emitter voltage by 3.9V. The switching waveforms shown in Figure 15b are similar to those in Figure
14b except for the negative bias. Q3, D2 and R5 form the under voltage lockout circuit.
The LED D2 is used as low voltage, low current reference diode. Q3 turns on when the voltage at the anode of D2 exceeds the
sum of the forward voltage of LED and the base-emitter voltage of Q3. This enables the operation of the optocoupler. The
tripping point of the under voltage lock-out circuit is 17.5V. The start-up wave forms are shown in Figure 16.
Figure 12.
A pulse shaper. The 555 is used as an illustration of a Schmitt Trigger pulse shaper
INPUT PULSE
T = RC
WITH DIODE
CONNECTED
AS SHOWN
V
H
LOAD
+12V
8
7
2
4
5
6
1
3
IRF7307 OR IRF7507
C
555
8
4
2
3
6
1
R
4.7K
INPUT
Figure 13.
Pulse shaper implemented with an integrator
V
H
LOAD
+12V
8
7
2
4
5
6
1
3
C
INPUT
CA3103
+
-
C
R
V
INPUT
WITH DIODE
CONNECTED
AS SHOWN
SLOPE OFRCV/SEC
To Order
Index