Specifications

The circuit attains a periodic steady-state in which the volt-seconds across the
are exactly balanced by the volt-seconds due to
t voltage level
transformer due to the input voltage
the output voltage. For continuous conduction operation (in which the rectifier current
never falls to zero), the output voltage is given by:
Equation 1: Outpu
()
ino
V
ND
V
−
=
1
D
In Equation 1 above, D is the duty ratio, defined as the fraction of the total switching
interval t SFET is , charging the transformer, N is the primary-to-secondary
transformer turns ratio, and is the input voltage.
The gate MOSFET s pulsed by the LM5021 PWM contro hich requires a
supply voltage at its VCC nd 15 ower start up the IC
energy is trickled charged into C105 via R101 and R104 from the input voltage. This
energy is applied to an in rnal linear regulator whos ut is the VIN-pin and whose
output, regulated at about 8V, is the VCC pin. Once the circuit is running the
apacitor C105 is kept charged by a bias winding on the transformer through diode
D102 and resistor R100. The bias voltage is related to the output voltage by the turn
a secondary winding, and is therefore insensitive to changes in
the input vo
t voltage constant by
djusting the duty ratio of the pulses it sends to the gate of the MOSFET.
hat MO on
of the i ller w
pin of between 8 a V to p it. To
te e inp
c
r tio between it and the
ltage.
The output voltage is regulated against changes in the input voltage and the load
current by the voltage reference and error amplifier REF100 and the opto-coupler
U101 which send a correction signal across the primary-secondary isolation barrier to
the LM5021. This signal allows the LM5021 to keep the outpu
a
Other essential functions implemented by the LM5021 are soft-start (via C106),
switching frequency setting (via R108) and MOSFET/ transformer current sensing (via
C107, R109 and R110).
Some of the component values in the circuit are just place-holders. The components
to the left of the bridge rectifier comprise an EMI and transient voltage filter. The
thermistor R116 is an inrush current limiter. The diodes across the primary of the
transformer, and the series capacitor-resistor pairs across the drain of the FET and
across the output rectifier all serve to reduce ringing and spiking in the circuit, and to
reduce EMI. The values of all these components should be determined depending on
the application implementation.
Gx64 APPLICATION NOTE
This document is the sole and exclusive property of WAVECOM. Not to be distributed or divulged without prior written agreement.
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Power Management
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