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Part Manufacturer: ST Microelectronics

Part Description: NON ISOLATED POWER SUPPLIES IN BUCK AND INVERTER CONFIGURATION USING VIPER20 DEVICE

Part Details:

AN1317 - APPLICATION NOTE ® 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, itsassociated high voltage Power MOSFET switch and a full set of other passive components, and providea 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 isolatedflyback configuration but it is also the right solution for different types of not isolated power suppliesapplications where low power (1W to 5W), wide input voltage range and low prices are required. In thiscase, a simple two pins inductor can replace an expensive safety transformer. The basic principle of thistype of supplies is to convert a high voltage source to a low voltage one by the only way of the switchingfrequency and duty cycle management. The applications like home appliances (microwave oven, washing machine, triac drivers...), industrialapplications (motors control,...) do not require galvanic isolation between the mains lines and the lowvoltage load, especially when one of the low voltage outputs must be connected to one of the mainslines. 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 ownnon isolated power supply using the VIPer20. It defines the key components, and highlights thedifferences 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, isgiven fig. 1. The Buck structure is composed here by the on chip Power MOSFET, the inductor L1, thefree 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 highdifference between the input and the output voltages. Its value would be at the maximum equal to thevoltages ratio, when in continuous mode and even less in discontinuous mode. If the switching frequencyis too high, the Power MOSFET conduction time will decrease accordingly, which may result in earlyburst mode operation if lower than the minimum turn on time of the device. In practice, the chosen January 2001 1/23 AN1317 - APPLICATION NOTE switching frequency will be comprised between 20 kHz and 30 kHz, just above audible values. During the start up phase, the VIPer20 is in standby mode and its on chip high voltage current sourcesources a current on the VDD pin until the voltage across the capacitor C2 reaches the VDDon threshold.Then, this current source is turned off and the device starts switching. After a transition phase duringwhich the output voltage grows up, the VDD supply of the VIPer20 is provided by the capacitor C2 andfinally, from the positive output through the diode D2 when the output voltage becomes higher than thecurrent VDD value. Figure 1: 2W typical single output not isolated power supply with Buck topology D1 AC IN 1N4007 D2 BYT01-400V R1 VDD DRAIN 10k - OSC 13V + C2 10uF COMP SOURCE 16V VIPer20 C1 R2 22uF 400V 3.9k C3 C4 10nF 100nF L1

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