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January 2001

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AN1317

NON ISOLATED POWER SUPPLIES IN BUCK AND

INVERTER CONFIGURATION USING VIPer20 DEVICE

A. Bailly - S. Luciano

INTRODUCTION

The VIPer20 is a full integrated switching device. It replaces the conventional PWM driver circuit, its

associated high voltage Power MOSFET switch and a full set of other passive components, and provide

a high level of performance thanks to its current mode structure and standby operation capability.

1. SCOPE

The VIPer20 is initially designed to be used at the primary side of any off line power supplies in isolated

flyback configuration but it is also the right solution for different types of not isolated power supplies

applications where low power (1W to 5W), wide input voltage range and low prices are required. In this

case, a simple two pins inductor can replace an expensive safety transformer. The basic principle of this

type of supplies is to convert a high voltage source to a low voltage one by the only way of the switching

frequency and duty cycle management.

The applications like home appliances (microwave oven, washing machine, triac drivers...), industrial

applications (motors control,...) do not require galvanic isolation between the mains lines and the low

voltage load, especially when one of the low voltage outputs must be connected to one of the mains

lines.

All these applications will take benefits from VIPer20 features:

• Full integrated PWM start up current source and high voltage Power MOSFET, allow to build simple,

robust, cost effective and compact power supplies.

• Built in overtemperature and overcurrent protection provide a safe control in overload conditions.

This application note gives all the elements to enable the designer to start the development of his own

non isolated power supply using the VIPer20. It defines the key components, and highlights the

differences between the Buck and the Inverter (also called Buck-Boost) topologies.

2. NOT ISOLATED TOPOLOGIES

2.1 VIPer20 In Buck Topology

The basic schematic of a VIPer20 in Buck topology delivering 2W typical, with a fixed output voltage, is

given fig. 1. The Buck structure is composed here by the on chip Power MOSFET, the inductor L1, the

free wheeling diode D3, the output filtering capacitor C5 and the output load itself.

In this topology, the VIPer20 switching duty cycle is very low (a few percent) because of the very high

difference between the input and the output voltages. Its value would be at the maximum equal to the

voltages ratio, when in continuous mode and even less in discontinuous mode. If the switching frequency

is too high, the Power MOSFET conduction time will decrease accordingly, which may result in early

burst mode operation if lower than the minimum turn on time of the device. In practice, the chosen