AN-6069 Datasheet, PDF (5/12 Pages) Fairchild Semiconductor – Application Review and Comparative Evaluation of Low-Side Gate Drivers

AN-6069
with magnetizing inductance L_MAG. In both cases, the DC
blocking capacitor C_C is large enough so that its voltage is
approximately constant.
Figure 8. Simplified Pulse Transformer Circuit
In Figure 9, the circuit is modified so that the resistor is
replaced by the gate-to-source terminal of a MOSFET
located on the high side of a bridge circuit.
+Bulk
VDD
IDR
T1
NP:NS
IG
+
VOUT
CC
IMAG
+
+
VP
VS
IN
-
LMAG
-
-
Figure 9. Simplified Gate Drive Transformer Circuit
0(a) shows the operational waveforms for the pulse
transformer circuit, while 0(b) shows operation in a gate
drive application.
APPLICATION NOTE
applied directly to the primary winding of T1, the
transformer would saturate and not be able to transmit useful
information. To prevent this, coupling capacitor C_C is
inserted in series with the primary winding to block the DC
voltage while passing the AC portion of the V_OUT signal.
Transformers designed for pulse and gate drive applications
usually specify a voltage-time product the device can
withstand without saturating the transformer.
In many cases, the same transformer could be used as either
a pulse transformer operation or a gate drive transformer. In
0, the major difference between the two applications is in the
current waveforms. With a constant drive voltage and
magnetizing inductance, L_MAG, the magnetizing current I_MAG
is the same in both circuits. In the pulse transformer
waveforms shown in 0(a), the resistor current IR follows the
secondary voltage V_S, and the driver supplies a current that
is the sum of these two components. In the MOSFET gate
drive waveforms shown in 0(b),
the gate current IG is positive pulses at turn on and negative
pulses at turn off. As in the first example, the driver
supplies a current that is the sum of these two components,
but the waveform has a larger RMS value due to the high-
current pulses.
It is important to examine the direction of current flow
between driver and transformer for the examples of 0. When
V_OUT swings high as shown Figure 11(a), one might expect
the driver to immediately source current. However, the
magnetizing current is negative and, if the load current is not
larger than the magnetizing current, the driver must sink
current until IDR goes positive. The opposite situation exists
in Figure 11(b), when V_OUT goes from high to low and the
driver must source current when expected to operate as a
current sink. Figure 11(c) shows additional diodes providing
a current path if the driver cannot sink current when V_OUT is
high or source current when V_OUT is low, as found in drivers
with a bipolar output stage.
VOUT
IDR
VOUT
IDR
Pulse Transformer
(a)
Gate Transformer
(a)
(b)
Figure 10. (a) Pulse Transformer Waveforms and
(b) Gate Drive Transformer Waveforms
The output of the driver swings from 0V to V_DD producing a
DC component equal to V_DD x duty cycle. If this voltage is
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Rev. 1.0.3 • 1/6/10
5
(b)
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