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Surge-Limiting Circuit That Reduces Power Dissipation

IP.com Disclosure Number: IPCOM000101092D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 124K

Publishing Venue

IBM

Related People

Clitheroe, AM: AUTHOR [+4]

Abstract

A surge-limiting circuit is described that offers low power dissipation inside power supplies while giving improved protection against damage under fault conditions compared to existing designs. Reliability improvements are obtained at moderate cost.

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This is the abbreviated version, containing approximately 52% of the total text.

Surge-Limiting Circuit That Reduces Power Dissipation

       A surge-limiting circuit is described that offers low
power dissipation inside power supplies while giving improved
protection against damage under fault conditions compared to existing
designs.  Reliability improvements are obtained at moderate cost.

      The circuit is suitable for applications where internal power
dissipation is large, yet the maximum permitted surge current must be
low, or if the unit has a wide-range power supply that requires
surge- current limiting at high output voltages, it must be capable
of running at low input voltages and hence higher currents.  But the
main advantage of the circuit over prior design is increased
protection against fault conditions.  The circuit has applications in
different areas, ranging from computers and monitors with wide-range
power supplies (90 V - 265 V, for example) to commercial power
supplies.  The reduction in power dissipation by the use of this
circuit reduces the temperature inside the unit and so improves
reliability.  An additional advantage is that the reduced voltage
drop across the surge-limiting circuit will increase the effective
voltage available to the unit, important when operating at low input
voltages.

      Existing circuits to reduce the power dissipation in the surge
resistor do so by using a Negative Temperature Coefficient (NTC)
resistor as the surge limiting component which acts as a high
resistance when the unit is first powered on.  The resistance drops
as the component heats up and so losses during normal operation are
reduced.  The advantage is the simplicity of the circuit, but the
disadvantage is that once the power is turned off, the NTC takes
several seconds to cool down and hence return to a high resistance
state.  Thus, if the unit is powered off and then on again after a
couple of seconds, a common action when wishing to reboot PCs, there
is inadequate protection against the resulting current surge.

      An alternative use is a triac to short out the surge-limiting
resistor.  The triac is powered by a winding from the unit's power
supply transformer so that when the supply is operational, the surge
resistor is switched out of circuit.  This works well as far as
reducing dissipation, but has a disadvantage that for a time after
the mains power is switched off the triac is still turned on, since
the power supply keeps running for a short while.  Should power be
quickly reapplied, there is no surge-limiting resistance and damage
may result.  It may occur if the unit's mains lead is not properly
inserted and is slightly moved, disturbing the power to the unit for
a fraction of a second. The time window during wh...