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A New Approach To PFC Inrush Protection

IP.com Disclosure Number: IPCOM000030130D
Publication Date: 2004-Jul-29
Document File: 4 page(s) / 106K

Publishing Venue

The IP.com Prior Art Database

Abstract

There are currently two common approaches to PFC inrush current protection. Such protection caters only for the first instance of power application at the switching on of the AC power. Faced with higher customer demand and expectations towards higher stability in power supply systems, a new concept is conceived that might fulfil the demand of an "available at all times" protection for the newer and improved generation of PFC units which themselves will be designed to be glitch-tolerant.

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A NEW APPROACH TO PFC INRUSH PROTECTION

Linus Liu

February 2003

ABSTRACT

There are currently two common approaches to PFC inrush current protection. Such protection caters only for the first instance of power application at the switching on of the AC power. Faced with higher customer demand and expectations towards higher stability in power supply systems, a new concept is conceived that might fulfil the demand of an “available at all times” protection for the newer and improved generation of PFC units which themselves will be designed to be glitch-tolerant.

INTRODUCTION

The PFC circuit serves to generate a regulated, high voltage DC output while regulating the power factor of the power drawn from the input such that the current will be proportional to the input voltage at any particular instant. However, due to the fact that there is a large “bulk” capacitor across the output, high peak currents are drawn at the point of application of power to the input. This is known as “inrush” current. In order to prevent damage to the circuitry, a PFC converter usually requires an inrush protection circuit.

In general, current protection methods only function at initial power application. Subsequent protection is not provided since most PFC converters self-protect by latching off or by entering into a non-operation mode (protection mode) during a power irregularity.

However, current market requirements increasingly demand power supply devices to be up and running at all times and to be able to recover as fast possible after a power interruption, rather than sitting in a “latched” mode which requires user intervention to reset the system.

REVIEW OF CURRENT PRACTISES

The two common methods of current limiting are the resistive method and the SCR (TRIAC) method.

1.      The resistive method uses a resistor of an appropriate rating connected in series with the main power connection to the PFC circuit. This resistor is removed (i.e. shorted with a relay contact) after the output capacitor is charged. Usually the relay drive circuit requires a low power supply from the PFC circuit itself to close the relay contact after the initial inrush has passed and the PFC converter has commenced operation. The PFC at this instance commences its own current limiting functions such as a “soft-start” or “Peak-Current detection”. This is an approach that causes power dissipation for a short duration.

2.      The SCR or TRIAC approach limits the inrush current by progressively varying the phase of the AC line voltage at which the SCR/TRIAC is switched on during start up. The instantaneous line voltage at which the SCR is activated is incrementally higher at each subsequent cycle ensuring that the difference between the line voltage and the output bulk capacitor voltage is small enough to result in negligible inrush current . This kind of circuit is considered to be non-power dissipating. However, the triggering circuit itself consumes power and the SCR has to b...