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Low Cost AC Line Transient Protection Circuit

IP.com Disclosure Number: IPCOM000110525D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 2 page(s) / 53K

Publishing Venue

IBM

Related People

Rust, RC: AUTHOR

Abstract

The circuit is used to protect AC line powered circuitry from damage due to voltage transients with microsecond to millisecond duration. Low cost is achieved using a shunt connected varistor to coarsely limit voltage peaks, and a series pass high voltage regulator which further limits output voltage delivered to the protected circuit. DC or rectified AC circuits can be protected by this transient protection circuit.

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Low Cost AC Line Transient Protection Circuit

       The circuit is used to protect AC line powered circuitry
from damage due to voltage transients with microsecond to millisecond
duration.  Low cost is achieved using a shunt connected varistor to
coarsely limit voltage peaks, and a series pass high voltage
regulator which further limits output voltage delivered to the
protected circuit.  DC or rectified AC circuits can be protected by
this transient protection circuit.

      The figure is a schematic of the circuit.  The AC source is
connected to the line filter, which contains series inductors and
shunt capacitors.  VR1, the low cost varistor, is connected in shunt
across the filter's output.

      During a line voltage transient, the low shunt resistance of
the varistor and series resistance in the AC source, line cord, and
line filter form a voltage divider which clamps filter output voltage
peaks to the 500-volt to 900-volt range.

      D1, a diode bridge, rectifies the AC voltage.  C1 and resistor
R1 form a damping network required by the protected circuitry and the
series pass high voltage regulator.

      Q1, a high voltage N-channel MOSFET, is the series pass
element.  In normal operation it is biased on, providing a low
resistance path from the bridge to the protected circuit.  The normal
Q1 gate drive is derived from a winding on the main switching
transformer within the protected circuit.  The gate drive circuit
limits gate-sourc...