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Circuit to Limit Inrush Current in a Boost and Conventional Switcher

IP.com Disclosure Number: IPCOM000115177D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 164K

Publishing Venue

IBM

Related People

Malik, RS: AUTHOR

Abstract

The conventional AC/DC converters use a large electrolytic capacitor to filter the AC ripple. An average DC voltage is developed across the input capacitor. A Boost switcher uses a large input electrolytic capacitor to develop a high voltage compared to an input voltage. Initially, this capacitor is uncharged. At the time of AC line turn on, the voltage across the electrolytic capacitor is zero. A large inrush current will be drawn from the line for awhile before the capacitor is charged to the peak line voltage. This can cause tripping of the circuit breaker besides causing glitches in line voltages and noise generation that could interfere with the other equipment.

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Circuit to Limit Inrush Current in a Boost and Conventional Switcher

      The conventional AC/DC converters use a large electrolytic
capacitor to filter the AC ripple.  An average DC voltage is
developed across the input capacitor.  A Boost switcher uses a large
input electrolytic capacitor to develop a high voltage compared to an
input voltage.  Initially, this capacitor is uncharged.  At the time
of AC line turn on, the voltage across the electrolytic capacitor is
zero.  A large inrush current will be drawn from the line for awhile
before the capacitor is charged to the peak line voltage.  This can
cause tripping of the circuit breaker besides causing glitches in
line voltages and noise generation that could interfere with the
other equipment.

      To avoid this problem, surge resistors or thermisters bypassed
by solid state switches such as R1 and S1 as shown in Fig. 1 are
used.  For low power application, R1 is a Negative Temperature
Coefficient (NTC) is used.  The power dissipation in this resistor
will be significantly high for high power applications.  Also, the
thermister has a very large time constant to revert to high
resistance mode.  It does a very good job at the time of turn on of a
power supply at the start.  However, if the AC goes away and then
comes back in a second or so, the thermister offers a small
resistance and a large inrush current will be drawn by the power
supply.  High power circuits use a large fixed resistor R1 bypassed
by a relay or solid state switch S1.  The use of a solid switch is
preferred over an electromechanical relay due to reliability
problems.  At the time of power supply switch turn on the capacitor
C2 is charged through a high fixed value resistor R1 through L1, CR5
as shown in Fig. 1.  When the voltage across C2 reaches close to the
peak line voltage, switch S1 is turned on.  The current through S1 at
this instant will be very low because the input peak line voltage and
the voltage across C2 are the same.  This switch S1 is always in
series with the line current drawn by the switcher.  It will
dissipate an appreciable amount of power and hurt the efficiency of
the switching power supply.  Also, the circuitry employed to turn on
and turn off the switch S1 is generally very complex.

      To overcome the problem of power dissipation and circuit
complexity, a circuit as shown in Fig. 2 in the dashed portion is
proposed.

      Description of the Circuit - In the new circuit, the inrush
current limit circuit is inserted in series with the electrolytic
capacitor.  Initially, SCR Q2 is off and capacitor C2 is charged
through high resistance R2.  When the voltage across Q2 reaches 10
volt or below, Q2 is turned on.  During normal operation, capacitor
Q2 is charged through Q2 and discharged through diode CR8.  Because
of the high Vout voltage (boost voltage = 400 volt), a much smaller
current flows through Q2 and CR8.  The power dissipation in CR8 and
Q2 will b...