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Power Supply Inrush Current Limiting Circuit

IP.com Disclosure Number: IPCOM000106906D
Original Publication Date: 1992-Jan-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 3 page(s) / 105K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+2]

Abstract

Described is a circuit designed to restrict in-rush surge current in a power supply during power-on but to have low power dissipation during normal operation. The reduction in inrush current compared to existing solutions leads to increased life of the unit's mains switch. The circuit is economical in components and has been successfully modelled.

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Power Supply Inrush Current Limiting Circuit

       Described is a circuit designed to restrict in-rush surge
current in a power supply during power-on but to have low power
dissipation during normal operation.  The reduction in inrush current
compared to existing solutions leads to increased life of the unit's
mains switch.  The circuit is economical in components and has been
successfully modelled.

      In practically all power supplies, the mains input voltage is
rectified and fed into a large bulk storage capacitor.  At initial
power on, the capacitor is discharged, and so a very large current
would flow unless some additional resistance was added to the circuit
(Fig. 1).  Typically, the in-rush current is limited to around 60
amps and so the value of R1 needs to be around 6 ohms. During normal
operation, the bulk capacitor's charging current flows through R1
causing excessive dissipation.  For an 80 W wide range (90-265 VAC)
power supply, the rms mains current can be 1.2 amps and dissipation
in R1 can reach nearly 10 W which is inefficient.  The fundamental
problem is: a high series resistance is required during power-on, but
a low series resistance is required during normal operation.

      The popular solution is to use a negative temperature
coefficient (NTC) thermistor as the in-rush limiting component.  NTCs
are ideally suited for this role - their resistance when cold is
high, but, as they heat up their resistance drops and power
dissipation is reduced to around 1 W.  The disadvantage of an NTC is
that the thermal time constant is typically several seconds.  If the
power is turned off and then rapidly on again, the NTC resistance
will still be very low, under an ohm, and the in-rush current is
excessive, possibly damaging the mains switch, rectifying bridge,
bulk capacitor and even blowing the unit fuse.  The circuit shown in
Fig. 2 has FET Q2 turned on unless the voltage at node A exceeds a
threshold set by the ratio R4/R5.  If the voltage exceeds this
threshold, Q1 is turned on which turns Q2 off.  Typical values are
Rds(on)=1 ohm and R6=5.6 ohms.  The in-rush current flows through
these two components in parallel initially, and then Q2 rapidly turns
off, leaving R6 as the effective current-limiting element.  During
normal operation, the ratio R4/R5 is set such that...