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Old 11th August 2014, 08:19 AM   #1
BigE is offline BigE  Canada
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Default Soft start circuit design and other psu issues

I do not see how the soft start circuit can be made into a slow charge for a large capacitor bank in a high powered class A amp, as either the load on switching is VERY high, or the dissipation in the bypass resistors is very high. without the class A load, it is no problem, with the load, it is extreme. I think one would have to also switch the load after a duration..... which means more relays....

If any interest, I can post an LTspice sim using inductors for transformers.....
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Old 11th August 2014, 09:19 AM   #2
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In another thread, Mark Johnson pointed out the following document and made, amongst others, the following comments. See section 5.2 In-Rush Control. Perhaps this is instructive?

http://www.ti.com/lit/an/snaa057b/snaa057b.pdf

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The NSC application note snaa057b is instructive, because it contains actual oscilloscope waveforms of an actual linear power supply during soft-start. Have a look at Figure 5 in particular; it shows the filter capacitor voltage vs time.

Notice that the extra resistance in the transformer primary (the "soft start") only participates for the first 500 milliseconds after switch-on. If the mains frequency is 50Hz and if a full wave bridge rectifier is used, there will be 50 waveform crests during those 500 milliseconds. You will get 50 diode current-pulses during soft start. Fifty! That turns out to be quite a lot.

As I'm sure you've seen in simulation, the frightening aspect of filter-capacitor-charge-up occurs on the first five to ten waveform crests. These are the ones whose diode current-pulses are truly enormous. After about the tenth current pulse (about 100 msec after startup), the current pulses are no longer frighteningly huge. NSC's current limiter prevents these first ten current-pulses from being enormous. And the next ninety pulses thereafter, too.

So NSC has managed to put together a 9-element soft start circuit which quite successfully limits current to both the transformer and to the rectifier/capacitor assembly. They didn't need two separate soft start circuits with wildly different activation times. An ICL thermistor would prolong the startup event over an even longer time than 500 msec , see the GE data sheet. You can form your own opinion about whether this is preferable or not.

By the way, inquisitive readers might enjoy printing Figure 5 at high magnification, then measuring asymptotes, slopes, curvatures, breakpoints, and the like. You may be able to extract the total series resistance of the experimental setup (Mains Rthevenin impedance + xformer wire resistance + xformer core losses + PCB track resistance + Capacitor ESR + diode tangency (dV/dI) + etc).
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Old 11th August 2014, 01:40 PM   #3
BigE is offline BigE  Canada
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Thank you!

I will try to digest that.
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Old 11th August 2014, 04:20 PM   #4
dmbox is offline dmbox  United States
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The most elegant and least complicated "soft start" design is to use a NTC device from Amatherm.

Inrush Current Limiters - Thermistors | Ametherm
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Old 12th August 2014, 08:17 AM   #5
AndrewT is offline AndrewT  Scotland
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Then add on a bypass that activates soon after the NTC has done it's job. Typically 100ms to 300ms after power on.
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Old 12th August 2014, 01:21 PM   #6
BigE is offline BigE  Canada
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Thanks gents,

I don't have issues with soft start, it is the slow charge of the cap bank that is of concern. I am using two of Tea Bags boards, wired for a center tapped transformer, and the C in the CRC is 44,000uF. That's 88,000uF per rail, per channel.
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Old 12th August 2014, 01:46 PM   #7
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Using NTCs after the secondary windings will allow slow charging of the capacitor bank, they aren't just for the primary side. A convenient spot would between the rectifiers and the caps on the rail side. As Andrew suggests, you can bypass the NTCs after a few hundred ms. As long as it has the current capability, you could run that relay from the mains soft start as the required timing is similar.
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Old 12th August 2014, 02:18 PM   #8
AndrewT is offline AndrewT  Scotland
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I reckon the slow charge should be a much longer delay until activating the bypass.
Many seconds even tens of seconds.
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Old 12th August 2014, 03:55 PM   #9
BigE is offline BigE  Canada
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Indeed, and it is during that time that the resistors have to dissipate a LOT of heat. Especially when filling a fairly large capacitor bank.
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Old 13th August 2014, 08:51 AM   #10
AndrewT is offline AndrewT  Scotland
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That's where NTCs offer a big advantage.
A correctly selected NTC (or series of NTCs if the required is not available) will not overheat.
There is another advantage to NTC limited current.
As the charge voltage builds up, the NTC has heated during that short period. The resistance is now slightly less and in part compensates for the reduced charging voltage (charging voltage = transformer emf - voltage on the capacitor).
This gives closer to constant current charging and is the shortest time that the capacitor can be charged at that current limit.

A fixed resistor gives a non constant current charge. Theoretically the capacitor never actually achieves full charge. In practice >90% of full charge is achieved in ~5*RC time constant and in 10*RC time constant it is effectively fully charged.
An NTC does it faster than this.
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