AN1894 Datasheet, PDF (1/14 Pages) STMicroelectronics – BUCK-BOOST CONVERTER REFERENCE BOARD

AN1894
®
- APPLICATION NOTE
VIPower: VIPer12A NON ISOLATED BUCK AND
BUCK-BOOST CONVERTER REFERENCE BOARD
P. LIDAK - R. HAUSER
ABSTRACT
Presented circuit can be used to produce a single, non isolated positive or negative voltage. It is
dedicated for building an auxiliary power supply based on the VIPer12AS monolithic device with rather
low output power required.
1. INTRODUCTION
There are some applications, where an off-line power supply without isolation between input and output
can be tolerated and rather low output current is required. In this case the converter should be simple
and low cost. These requirements can be satisfied by a step-down converter based on monolithic device
VIPer12AS that incorporates the PWM controller together with the Vertical power MOSFET switch in a
SO8 package. The presented power supply has two variants. The first is a buck (step-down) converter
with a positive output voltage referenced to the common ground. The second one with negative output
voltage is a buck-boost converter. The presented reference board incorporates both variants by different
assembly options.
2. CIRCUIT DESCRIPTION
2.1 Buck Converter +15V/100mA, +5V/60mA or 20mA (Variant 1)
2.1.1 Operating Conditions
Input Voltage range
Input Voltage Frequency range
Main Output
Second Output (through linear regulator)
Total Maximum Output Power
90-264 VAC
50/60 Hz
15V / 100mA
5V / 60 or 20mA
1.6W
2.1.2 Circuit Operation
The total schematic of the power supply can be seen in Figure 1. The output of the converter is not
isolated from input. For this reason the reference ground is common for an input and output connection
terminal. The input capacitor C1 is charged from line via one way rectifier consisting of diodes D1 and
D2. Two diodes in series are used for EMI reasons to sustain burst pulses of 2kV. The capacitor C1
together with capacitor C2 and inductor L1 forms an EMI filter. The DC voltage at C2 is then applied to
the VIPer12 that works as a high side switch. It means the IC and corresponding supply and feedback
loop circuitry is floating. The IC supply circuit consists of the high voltage diode D4, ceramic capacitor C7
low voltage D7 and capacitor C4. The voltage feedback loop is provided via zener diode D3, resistor R3
and capacitor C6.
The diode D7 between capacitor C7 and C4 ensures the proper start-up of the converter. Thanks to this
diode the feedback loop circuit is separated from supply circuit. The internal start-up current source of
the VIPer12 charges the IC supply capacitor C4 to a specified start-up threshold voltage of about 16V.
March 2004
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