ST ISA006V1 User Manual

6W non-isolated fly-back topology power supply

Features

■ Switch mode general purpose power supply

■ Input: 90 to 264Vac @ 50/60Hz

■ Main Output (regulated): 5V, 500mA @ 50/

60Hz

■ Second output: 15V, 200mA

■ Output power (pick): 5.5W

Description

STEVAL-ISA006V1

with VIPer12AS

Data Brief

This reference design is an auxiliary power supply
based on the VIPer12AS monolithic device and
can be used to produce multiple non isolated
positive or negative voltage outputs.

This solution is based on an offline discontinuous
current mode fly-back converter without isolation
between input and output. The fly-back topology
allows the exploitation of the VIPer12AS current
capability when compared with the buck converter
based power supply. To ensure power supply low
cost, the isolation between input and output is not
provided. This greatly simplifies the transformer
design and production. The VIPer12AS
incorporates the PWM controller with a 60kHz
internal oscillator and the vertical Power MOSFET
switch in a SO-8 package. The presented power
supply has four variants. All the variants have
been incorporated in the presented reference
board through different assembly options.

STEVAL-ISA006V1

ST Components

■ VIPer12AS off-line SMPS primary

■ STPR120A diode, fast recovery trr =25ns 200V

1A SMA

■ STPS1L40A diode, schottky, 40V 1A,

■ TS2431ILT programmable shunt voltage

reference

July 2007 Rev 1 1/5

For further information contact your local STMicroelectronics sales office

www.st.com

5

General circuit description STEVAL-ISA006V1

1 General circuit description

The output of the converter is not isolated from the input. For this reason the reference ground
is common for an input and output connection terminal. The input capacitor C1 is charged from
the mains by single rectification consisting of diodes D1 and D2. Two diodes in series are used
for EMI reasons to sustain burst pulses of 2kV. Capacitor C1 together with capacitor C2 and
inductor L1 form an EMI filter. The DC voltage at C2 is then applied to the transformer primary
winding through the internal Power MOSFET switch of VIPer12 during the switching period ON
time. The snubber circuit consisting of resistor R3 and capacitor C6 reduces the voltage spike
across the transformer primary winding due to the parasitic leakage inductance. Through
rectifiers D4 and D5 and through smoothing capacitors C3 and C4, the power supply provides
two outputs from two transformer windings. The VIPer12AS is supplied by a 15V output voltage
through transistor Q2 and diode D7. The voltage feedback loop senses the 5V output through
resistor divider R5, R7. The control IC U2 compares the resistor divider output voltage to the
internal reference voltage of 2.5V and changes the cathode voltage accordingly to keep 5V
output stable. The FB pin current will decrease the peak primary current to reduce the power
delivered to the outputs.

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