Datasheet

LT4430
12
4430fc
For more information www.linear.com/LT4430
The second solution is to make a preregulator as shown
in Figure 2b. In this example, the bias supply equals (V
Z1
– V
BE
). Select R2 to bias Zener diode Z1 and to supply
base current to QBS. Resistor R3 (on the order of a few
hundred ohms), in series with Q5’s base, suppresses
possible high frequency oscillations depending on QBS’s
selection. The preregulator circuit has additional value for
fully synchronous converters. Fully synchronous convert
-
ers require gate drivers to control the secondary-side
applicaTions inForMaTion
MOSFETs turn on and turnoff. The gate driver circuitry
requires supply current in the range of 10mA to 100mA
depending on the gate driver supply voltage, MOSFET size
and switching frequency. The preregulator bias supply is
ideal for powering both the LT4430 and the gate driver
circuitry, especially since the gate drivers typically use a
supply voltage between 5V to 12V. The preregulator circuit
finds wide use in fully synchronous forward converters,
push-pull converters and full-bridge converters.
•
4430 F02a
T1
N2
N1
1:N
N = N1 + N2
V
OUT
C
OUT
*OPTIONAL SEE TEXT
•
LT4430
V
BIAS
C1
R1*
D2
D1
••
V
IN
•
•
V
IN
4430 F02b
T1
1:N
V
OUT
LT4430
V
BIAS
C
OUT
C1
R1*
D2
C2
*OPTIONAL SEE TEXT
D1
R3*
R2
QBS
Z1
Figure 2a. Flyback Converter with Tapped Secondary Bias
Figure 2b. Flyback Converter with Preregulator Bias