Datasheet
SLUS542F − OCTOBER 2003 − REVISED JULY 2009
24
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ADDITIONAL APPLICATION INFORMATION
Figure 9 shows an active clamp forward converter with high-side clamp utilizing a P-channel auxiliary switch.
Detailed analysis and design examples of active clamp converters are published in the references listed at the
end of this datasheet.
Gate Drive Implementations
Both topologies can make use of either the high-side or the low-side clamp arrangement. Depending on the
choice of the clamp circuit, the gate drive requirements of the auxiliary switch are different.
C
CLAMP
Q
AUX
Q
MAIN
+V
IN
Figure 10. High-Side N-Channel (UCC2893/4)
12
P
C
CLAMP
Q
AUX
Q
MAIN
+V
IN
Figure 11. Low-Side P-Channel (UCC2891/2)
12AUX
Interfacing with a high side N-channel clamp switch is achievable by using high side gate drive integrated circuits
or through a gate drive transformer. When a transformer is used, special attention must be paid to the fact that
the clamp switch is operated by the complementary waveform of the main power switch. Since the operating
duty cycle of the converter can vary between 0 and D
MAX
, the gate drive transformer must be able to drive the
auxiliary switch with any duty cycle from 1−D
MAX
to near 1.
The low side P-channel gate drive circuit involves a level shifter using a capacitor and a diode which ensures
that the gate drive amplitude of the auxiliary switch is independent of the actual duty cycle of the converter.
Detailed analysis and design examples of these and many similar gate drive solutions are given in reference [6].
Bootstrap Biasing
Many converters use a bootstrap circuit to generate its own bias power during steady state operation. The
popularity of this solutions is justified by the simplicity and high efficiency of the circuit. Usually, bias power is
derived from the main transformer by adding a dedicated, additional winding to the structure. Using a flyback
converter as shown in Figure 12, a bootstrap winding provides a quasi-regulated bias voltage for the primary
side control circuits. The voltage on the VDD pin is equal to the output voltage times the turns ratio between
the output and the bootstrap windings in the transformer. Since the output is regulated, the bias rail is regulated
as well.