SLUA110 Datasheet, PDF (5/19 Pages) Texas Instruments – PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

APPLICATION NOTE
The use of a Schottky diode from the PWM output to
ground will correct both situations. Connected with the
anode to ground and cathode to the output, it will prevent
the output voltage from going excessively below ground,
and will also provide a current path. To be effective, the
diode selected should have a forward voltage drop of less
than 0.3 volts at 200 milliamps. Most 1-to-3 amp diodes
exhibit these traits above room temperature. The diode will
conduct during the shaded part of the curve shown in
figure 10 when the voltage goes negative and the current
is positive. The current is allowed to circulate without
adversely effecting the IC performance. Placing the diode
as physically close to the PWM as possible will enhance cir-
cuit performance. Circuit implementation of the complete
drive scheme is shown in the schematic.
Power MOSFET Drive Circuit
U-111
inductance and parasitic capacitance, in addition to the
magnetizing inductance and FET gate capacitance. Cir-
cuit implementation is similar to the previous example.
Transformer Coupled Push-Pull MOSFET Drive Circuit
Figure 11.
Transformer driven circuits also require the use of the
Schottky diodes to prevent a similar set of circumstances
from occurring on the PWM outputs. The ringing below
ground is greatly enhanced by the transformer leakage
Transformer Coupled MOSFET Drive Circuit
Figure 13.
Peak Gate Current and Rise Time Calculations
Several changes occur at the MOSFET gate during the
turn-on period. As the gate threshold voltage is reached,
the effective gate input capacitance goes up by about
fifteen percent, and as the drain current flows, the capaci-
tance will double. The gate-to-source voltage remains fairly
constant while the drain voltage is decreasing. The peak
gate current required to switch the MOSFET during a spec-
ified turn-on time can be approximated with the following
equation.
Several generalizations can be applied to simplify this
equation. First, let Vgth, the gate turn-on threshold, equal
3 volts. Also, assume gm equals the drain current Id
divided by the change in gate threshold voltage, dVgth. For
most applications, dVgth is approximately 2.5 volts for utili-
zation of the FET at 75% of its maximum current rating. In
most off-line power supplies, the gate threshold voltage is
a small percentage of the drain voltage and can be elimi-
nated from the last part of the equation. The formulas to
determine peak drive current and turn-on time using the
FET parameters now simplify to:
D1 .D2: UC3611 Schottky Diode Array
Figure 12.
Switching times in the order of 50 nanoseconds are attain-
able with a peak gate current of approximately 1 .0 amps in
many practical designs. Higher drive currents are obtain-
able using most Unitrode current mode PWMs which can
source and sink up to 1.5 amps peak (UC1825). Driver ICs
with similar output totem poles (UC1707) are recom-
mended for paralleled MOSFET high speed applications.
SEE APPLICATION NOTE U-118
3-110