Datasheet

UCC2897A
SLUS829D -- AUGUST 2008 -- REVISED JULY 2009
24
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ADDITIONAL APPLICATION INFORMATION
Gate Drive Connection
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].
C
CLAMP
Q
AUX
Q
MAIN
+V
IN
Figure 9. Low-Side P-Channel
AUX
P
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 10, 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 w indings in the transformer. Since the output is regulated, the bias rail is r egulated
as well.