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Old 19th May 2016, 07:50 PM   #1
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Default Inrush current limiter for DC

Here is my problem:
Have an SMPS (700W, +/- 75V rails) powering my audio amp.
Both channels (2x300W) are powered by the same SMPS.
Amp boards have in total 4 x 2700uF on them.
https://www.digikey.com/product-deta...435-ND/1699522

Apparently this is too much for the SMPS to charge when I turn this
thing on. Usually it will turn the red LED on (failure to start), and
then I need to power it off, wait 30s, and power it on again.
After 2nd power on, it will start with no problems (caps are already (partially) charged I guess).

Any ideas how way to fix it, without messing with SMPS itself?
Add NTCs to the amp rails?
Or some other simple inrush current limiter can be used?
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Old 19th May 2016, 07:57 PM   #2
Mooly is offline Mooly  United Kingdom
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Use smaller reservoir caps. Honestly.

The SMPS running at high frequency needs correspondingly smaller caps to achieve the same amplitude ripple when compared to a conventional 50/60Hz transformer supply.

Adding NTC's to the rails will destroy the regulation as seen at the output end.
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Old 19th May 2016, 08:07 PM   #3
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This is unregulated SMPS.

To remove the boards from heatsinks and replace caps, will require lots of work - removal of all transistors
from heatsinks (16 in total);, it will be pain in the neck. The chassis is kind of crowded
On the other hand - plugging some little board to the rails between spms and amp would be relatively easy..

Quote:
Originally Posted by Mooly View Post
Use smaller reservoir caps. Honestly.

The SMPS running at high frequency needs correspondingly smaller caps to achieve the same amplitude ripple when compared to a conventional 50/60Hz transformer supply.

Adding NTC's to the rails will destroy the regulation as seen at the output end.

Last edited by minek123; 19th May 2016 at 08:15 PM.
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Old 19th May 2016, 08:16 PM   #4
Mooly is offline Mooly  United Kingdom
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That won't make any difference to what you are trying to achieve. The high loading at start up will be causing the switching transformer to saturate, and that is detected as a fault or overload condition.

I would at least try it and see what value cap you need to work down to for it to start reliably. Depending on the frequency, a 100 or 220uf cap could be performing the same job as a 10,000uf cap in a traditional PSU. Of course ripple current in the cap is another matter at this power level, and that is something you need to look at, maybe splitting the final value into several parallel caps.
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Old 19th May 2016, 08:18 PM   #5
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Thanks Mooly, I guess that's the way to go.
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Old 19th May 2016, 08:28 PM   #6
Mooly is offline Mooly  United Kingdom
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Well I would certainly try it and see where it gets you. Good luck and let us know how it works out
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Old 21st May 2016, 02:36 AM   #7
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A SMPS will only tolerate a overload current for a short time on power up.
This is to charge its own output capacitors.
It assumes anything longer is a fault condition and rightly shuts down.

As said by others you don't need huge capacitors at the amplifier end.
The switching frequency is probably many 10's kilohertz compared to 50Hz for a linear supply.
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Old 21st May 2016, 03:55 AM   #8
wwenze is offline wwenze  Singapore
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Quote:
On the other hand - plugging some little board to the rails between spms and amp would be relatively easy..
Add a soft start module then.
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Old 21st May 2016, 07:31 AM   #9
Pafi is offline Pafi  Hungary
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If the SMPS can't charge 4x2700 uF, then it's likely to be unable to serve the high impulse current needs of the amp either. Reducing caps can be dangerous, ripple current capability must be preserved! Should have linked the SMPS and amps, not the caps. AC inrush current limiting may help also, depending on the SMPS.

Last edited by Pafi; 21st May 2016 at 07:33 AM.
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Old 21st May 2016, 07:43 AM   #10
Pafi is offline Pafi  Hungary
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Mooly!
Quote:
Use smaller reservoir caps. Honestly.

The SMPS running at high frequency needs correspondingly smaller caps to achieve the same amplitude ripple when compared to a conventional 50/60Hz transformer supply.
If the only AC current flowing on the caps were at the switching freq, then you were right. But the load current can have any freq, therefore low impedance must be ensured at any freq. Since the PSU is not regulated, and probably not have synchronous rectifier, its ability to maintain low impedance is limited.
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