Circuit controls inrush current in ac-operated power supplies
Inrush current control circuit for ac operated power supplies.
Peter Demchenko, Vilnius, Lithuania; Edited by Paul Rako and Fran Granville -- EDN, October 6, 2011
Large power supplies that operate from ac wall voltage have large input-filter capacitors. You must limit the inrush current to those capacitors. Otherwise, the supply may trip the ac circuit breaker, or you may damage the rectifier, filer chokes, or PCB (printed-circuit-board) traces. The circuit in this Design Idea inserts a current-limiting resistor in the capacitor’s charging path. It senses when the capacitor has charged to a minimum threshold voltage. It then uses a TRIAC (triode alternating current) to short the resistor. Monitoring the capacitor voltage is preferable to monitoring the input current, thus avoiding large load currents during operation that can cause inrush limiting.
This circuit uses a dynistor to detect
the minimum threshold voltage across
the filter capacitor. Once called a
Shockley diode, a dynistor is an asymmetric
thyristor with an alternating
P+ and N− structure in its anode. In
this break-over diode, avalanche current
triggers a gateless thyristor. The
unidirectional dynistor differs from
the bidirectional STMicroelectronics Trisil, the Bourns TISP (thyristor-surge-protector),
and the Littelfuse SIDACtor (silicon-diode-
alternating-current) devices.
Dynistors are somewhat rare, but you
can still obtain a DB3 type with a 32V
break-over voltage. These devices
sense capacitor voltages greater than
38V. You can use PNP/NPN transistor
pairs or a low-power thyristor with a
zener diode to emulate a dynistor of
any voltage.
A dynistor makes a 700-Hz oscillator
when the capacitor reaches the threshold
voltage. You can easily transmit this ac
signal across an isolation boundary comprising
a transformer or capacitors. If you
are unsure of the isolation rating of your
signal transformer, you can capacitively
couple the transformer (Figure 1). If you use optional capacitors C2 and C3,
their rating voltage should exceed 800V.
Power resistor R3 limits inrush current
and should have a power rating greater
than 2 to 10W. Resistors with integrated
thermofuses are preferable. Use the ZCD
(zero-crossing detector) to synchronize
R3’s shorting event with the transition
of the ac voltage through 0V. Parts such
as Fairchild’s
MOC3062M zero-crossing phototriac-driver
optocoupler serve this function.
Dynistor D1, capacitor C1, resistor
R1, and transformer T2 comprise
an oscillator. It begins working
when the value of E exceeds the
value of the dynistor’s break-over
voltage. The oscillator provides
current pulses greater than 20 mA,
enough to trigger many types of
TRIACs and consuming less than
1.5 mA dc. Because the frequency
of pulses is approximately 700 Hz,
transformer T2 is small. Resistor
R2 limits the discharge current
through dynistor D1. If the transformer
has adequate dc resistance,
you can omit R2. Choose a TRIAC
with a gate-trigger current lower
than 20 mA. You may not need the snubber network comprising RS and CS
if the leakage inductance of T1 and the
inductance of the ac lines are low.
You can adapt the circuit for capacitor
voltages greater than 45V (Figure
2). Feed the dynistor oscillator through
a voltage divider comprising
R4 and R5. This divider
consumes 10 to 20 mA but
keeps the oscillator’s frequency
close to 700 Hz.
To avoid dc-current draw,
you can use or simulate a
higher-voltage dynistor.
This circuit dispenses with
the isolation transformer
and uses capacitors C2 and
C3. Replacing R1 in Figure
1 with R11 and R12 helps
reduce current injection
into earth ground and audio
interference due to 700-Hz
oscillations.Talkback
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Since most applicat ions of large power supplies feed a known load, it seems that a timer could do ass well. It would be simpler anx less expensive aswell.
william Ketel - 2011-22-10 07:30:35 PDT -
Why not just Use an NTC Power Thermistor also know as an Inrush Current Limiter to reduce the amount of inrush current during the switch on phase?
James Borsh - 2011-18-10 12:01:21 PDT -
I like that is as back in 80’s including same problems.
In simple words we can to represent a dynistor as PNP and NPN transistors with 3 resistors. The collector of PNP transistor connected with base of NPN transistor, collector of NPN connected to base of PNP. The first resistor placed in parallel with base emitter of PNP, the third one in parallel with base emitter of NPN, and the second one is connected between bases of PNP and NPN transistors.
There is a ramp generator with capacitor discharge through the dynistor and its frequency is not synchronized with wall frequency and TRIAC firing could be not stable.
Opto-coupler MOC3062M has enough isolation rating to be connected without an additional transformer and capacitors, so C2 and C3 could be shorted and 300 Ohm resistor could be trimmed to limit maximum current through LED.
Vladimir Doubovis - 2011-13-10 17:04:23 PDT -
So what the heck is a Dynistor? Is it something like a tunnel diode? Wikipedia draws a blank. None of my usual semiconductor suppliers list them.
Douglas Butler - 2011-11-10 14:10:54 PDT -
So, somewhere in Lithuania is a bin full of Dynistors. Right next to the 60 Hz transformers. Oughta be good for building guitar amps.
James Van Damme - 2011-11-10 12:42:01 PDT
