Inrush protection of a PCB power transformer - diyAudio
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Old 23rd July 2013, 12:22 PM   #1
JFace is offline JFace  Canada
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Default Inrush protection of a PCB power transformer

I'm designing a power supply for a one tube direct box using a pcb mount transformer. Because this is a small signal box, I want to filter the DC as much as possible. With the added capacitor banks, I am simulating inrush currents in excess of 2 amps on the secondary, which is normally rated for 40mA.

In this situation, would you:
1) Not worry about it, the transformer can handle it
2) Put an NTC somewhere on the line
3) Make a simple soft start
4) Something else

I haven't found much information regarding tiny transformer protection, but they need love, too.
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Old 23rd July 2013, 01:10 PM   #2
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Could you elaborate on the amount of capacitance and secondary voltage?

Sidenote 1: If ripple free DC is what you're after, why not use a regulator and and do with less capacitance?
Sidenote 2: If going the NTC route, always bypass it after a few seconds. First of al, there's always some resistance present which also varies depending on the current being drawn and second it will reduce the heat buildup inside your enclosure.

Last edited by funk1980; 23rd July 2013 at 01:15 PM.
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Old 23rd July 2013, 01:18 PM   #3
FoMoCo is offline FoMoCo  United States
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If you've got a lot of capacitance, you're going to get repetitive peaks as the capacitors charge. This is normal for a transformer to operate this way. However, if your simulation does not include the impedance of the transformer (and it's more complex than adding primary and secondary impedance) your simulation is useless.

On thing to keep in mind is that a lot of capacitance isn't the best route. In fact, the induced noise from the charging spikes can be worse. A better bet is to design for about a volt of ripple and follow with a 78xx series (or something with a better PSRR) regulator. Some here will also recommend a cap multiplier. I don't, but there's nothing wrong with that approach.

Without more information, nothing specific can be offered for advice.
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Old 23rd July 2013, 01:23 PM   #4
JFace is offline JFace  Canada
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I am bridge rectifying it and using CRCRC filtering...47u-220u-220u

200VAC to eventually 250VDC

I wasn't confident that the transformer could supply the current needed for a regulator.
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Old 23rd July 2013, 01:29 PM   #5
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That's a lot of capacitance! Specially for a one-tube small signal solution. I'd go with a LM317 regulator in Maida configuration for it to handle the high voltage. Super tight DC with a fraction of the capacitance (saves a lot of room too). A simple zero crossing SS relay is all the 'soft-start' you'll ever need.

As far as current for the regulator; the current is largely determined by the load, not the regulator itself. I use a 30mA transformer with the above configuration in a 2x 12AX7 preamp without issue.

Last edited by funk1980; 23rd July 2013 at 01:46 PM.
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Old 23rd July 2013, 03:08 PM   #6
FoMoCo is offline FoMoCo  United States
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Now that I know that it's a high voltage power supply, 1 V of ripple is not a good choice. 5 to 10 would be better. Funk's suggestion is a good option to consider.
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Old 23rd July 2013, 03:14 PM   #7
JFace is offline JFace  Canada
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Ok, I was sold on Maida except I have the same problem I started with. The regulator requires a reservoir capacitor before the input. Even with a single 22uF feeding a soft start load, I'm getting an inrush of 1.5A. (I'm using PSU Designer II)

Is this acceptable for a short period? Is this inaccurate?
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Old 23rd July 2013, 03:34 PM   #8
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Quote:
Originally Posted by JFace View Post
Ok, I was sold on Maida except I have the same problem I started with. The regulator requires a reservoir capacitor before the input. Even with a single 22uF feeding a soft start load, I'm getting an inrush of 1.5A. (I'm using PSU Designer II)

Is this acceptable for a short period? Is this inaccurate?
Well, with a zero crossing SS relay (sharp has some nice ones), you'll prevent switch-on on the top of the sinus peak where capacitor inrush is highest. This will charge the caps in a more controlled fashion. Transformer primary inrush will be larger but negligible with these small transformers

Second, I don't believe PSUDII is taking the DC resistance of the transformer into account, which also limits the max amount of current that can flow on startup.

But if you want to be absolutely sure, you could add e.g. a 15 ohm 5W resistor just after bridge rectifier. Voltage drop will me minimal (0,6V @ 40mA). Add another 100nF film cap in front of the resistor to shunt any possible diode noise to ground, use UF400X diodes and you'll have an awesome PSU.

Last edited by funk1980; 23rd July 2013 at 03:37 PM.
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Old 23rd July 2013, 03:39 PM   #9
JFace is offline JFace  Canada
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Quote:
Originally Posted by funk1980 View Post
Well, with a zero crossing SS relay (sharp has some nice ones), you'll prevent switch-on on the top of the sinus peak where capacitor inrush is highest. This will charge the caps in a more controlled fashion. Transformer primary inrush will be larger but negligible with these small transformers

Second, I don't believe PSUDII is taking the DC resistance of the transformer into account, which also limits the max amount of current that can flow on startup.

But if you want to be absolutely sure, you could add e.g. a 15 ohm 5W resistor just after bridge rectifier. Voltage drop will me minimal (0,6V @ 40mA). Add another 100nF film cap in front of the resistor to shunt any possible diode noise to ground, use UF400X diodes and you'll have an awesome PSU.
I appreciate the advice. Can you recommend a Maida implementation for a starting point? I've only been able to find the original version, and user tomchr's 21st Century Maida Regulator.
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Old 23rd July 2013, 04:02 PM   #10
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Below is a design I've successfully build for my 2x12AX7 preamp at 320VDC (250VAC input). I;ve gone a bit overboard with the caps as well . The HTSUPPLY labels is where the SSR goes. You might have to juggle some values to get the right output voltage. The LM317 also needs a minimum load to get it to regulate, but a look at the datasheet will give you an idea how to scale the values of e.g. R105/R106.
Use a suitable heatsink for the LM317 and MJE340.
You'll also see another 100nF cap (C102). I believe it's good practice to bypass big electrolytics with a smaller film (of ceramic) cap.

Tomchr's design is awesome, but more advanced for my needs and some components are harder to come by then the old trusted LM317. The 'original' Maida configuration has proven itself over the years.

maida.png

Last edited by funk1980; 23rd July 2013 at 04:13 PM.
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