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Old 3rd November 2004, 04:47 PM   #1
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Default Active Inrush Current Limiter

I was looking for a cheaper (than a relay), less space consuming way of active current inrush limiting. The circuit below will add minimally to the effective ESR of the main filter capacitor, the resistor in the cap's ground path limits the initial current, eventually the IRFP044's gate is charged and it shorts the resistor. I haven't drawn in the discharge path for the IRFP044 gate, but it is important. Most of this was discussed in On-Semi apnote 1542 with regard to SMPS converters, incandescent bulbs etc.

Here's the basics:
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Old 3rd November 2004, 05:40 PM   #2
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Are you aware of that most of the current is for magnetizing the transformer core if we talk about toroids? The main problem is that the fuses goes in the house and the fuse in the amp will be unnecessarily large.

Both the rectifier diodes and the caps itself usually can take this inrush current. If you are talking about a switch mode power supply, it's a different thing though.
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Old 3rd November 2004, 05:56 PM   #3
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Quote:
Originally posted by peranders
Are you aware of that most of the current is for magnetizing the transformer core if we talk about toroids? The main problem is that the fuses goes in the house and the fuse in the amp will be unnecessarily large.

Both the rectifier diodes and the caps itself usually can take this inrush current. If you are talking about a switch mode power supply, it's a different thing though.

I have recently demonstrated to myself that my diodes can't load 36,000uF -- these are MBR20100CT's -- 20 amp, 100 V Schottky's, not MUR860's. Without the limiter the current in the first couple of pulses out of the rectifier is over 100 amps. Thus the project.

I've pointed out that in the 400 watt Sorensen supply the current is actively limited with a resistor/relay combination.

R8 -- the second current sense resistor is 0.001 Ohm, not 0.001k.
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Old 3rd November 2004, 06:04 PM   #4
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Are you sure about the diodes? Normally you calculate 5-10 times the current under short time.

From the datasheet I read max peak current 300 A! So I'll guess they will handle this without any circuits. If you haven't had any problems, don't try to fix it.
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Old 3rd November 2004, 06:11 PM   #5
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No, I have popped the diodes. I agree with you -- of course, the U.S. Army slogan used to be: "If it isn't broken, fix it until it is !"
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Old 3rd November 2004, 08:50 PM   #6
Steven is offline Steven  Netherlands
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I tend to agree with peranders that at least also the transformer will draw a big inrush current (especially toroids). If you solve that problem first by inrush current limiting on the primary side, you might have solved the problem of popping diodes at the secondary side at the same time (for free).

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Old 3rd November 2004, 09:43 PM   #7
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fwiw, the Sorensen supply uses a relay and resistor on the primary side --
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Old 3rd November 2004, 09:52 PM   #8
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Quote:
Originally posted by jackinnj
No, I have popped the diodes. I agree with you -- of course, the U.S. Army slogan used to be: "If it isn't broken, fix it until it is !"
Are you sure you broke the diodes just because of this? 36000 uF isn't extremely much and I'll gather your voltage isn't particular high either.
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Old 3rd November 2004, 09:57 PM   #9
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Let me make sure i understand.

A Toroid transformer will drawn large amounts of current at start up even with nothing connected to the secondary sides???

I thought the caps would be the main current draw at start up?


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Old 3rd November 2004, 11:06 PM   #10
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Quote:
Originally posted by Zero Cool
Let me make sure i understand.

A Toroid transformer will drawn large amounts of current at start up even with nothing connected to the secondary sides???
Yes. Someone I knew had equipment to measure the currents, and measured over 70 A. Please remember, that this is a short burst - the first two cycles AFAIR.

Quote:
Originally posted by Zero Cool
I thought the caps would be the main current draw at start up?
That comes afterwards.
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