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To power an amplifier, I use a lipo battery 3.7v (4000mah) or two 18650 batteries 3.7v (4000mah) in parallel with a DC/DC boost converter that boost the voltage to 24.2 volts. The circuit performs pretty well.

On the load side (output) of the DC/DC converter I use a 25V 4700uF capacitor which introduce a problem, inrush current (when charging) for a second or so when switched on. Not a healthy situation, want to avoid this.

I read allot about inrush current and there seem many options to fight against it but it is not clear to me which is the best one. I hope it can be simple as putting one 'simple' component into the circuit without loosing too much effort. The capacitor is really helpfull, especially at high volume with much bass.

So I came allong the NTC and PTC thermisistors. The NTC does not fit the situation and could be risky when the device is switched off and on again when the NTC is not cooled down, so it doesn't eliminate the inrush current. The disposed heat (wasted energy) is also something I don't like.

The PTC one is the opposite, but how fast is it? Do it really fight inrush current? Because the PTC heats up when there is much current but I suppose it takes some time to heat up (and therefore draw some current) before it enables protection?

Before I decide to buy a bunch of these PTC thermisistors, I like to hear/learn/know some advise of you, professionals.

Questions I have:

  1. Does a PTC avoid inrush current from the start (how accurate)?
  2. Does a PTC influence the performance of the capacitor (or the whole achievements of the PSU)?
  3. The amperage, is this the maximum current the PTC can handle or is this the maximum amount of current it will use?

The next question is, will it help to divide the capacitance into several capacitors with the same amount in parallel to reduce inrush current? Or at least to reduce the time of the inrush current.

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    \$\begingroup\$ Are you sure the inrush current is a problem? What are you concerned about? Load on the converter, or load on the battery? What's the peak current during inrush (on both sides of the converter)? What's the design current of the converter? \$\endgroup\$
    – marcelm
    Nov 16, 2016 at 23:20
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    \$\begingroup\$ 4700 uF is kind of ridiculous. The easiest thing would be to use a smaller capacitor. Also, what specific problem is the inrush causing? Maybe there is some other way to deal with it. One solution is to use a resistor in series with the capacitor. And bypass the resistor with a MOSFET. Some signal will turn on the MOSFET after the capacitor is charged up most of the way. \$\endgroup\$
    – mkeith
    Nov 16, 2016 at 23:33
  • \$\begingroup\$ The converter is max 2A but is not the problem, it happen also without load, a dip, led dim for one second (fade). It's like a short circuit for a short period of time because the capacitor needs to be charged. I'am concerned about damage. I don't now the inrush current because I am unable to measure it. \$\endgroup\$
    – Codebeat
    Nov 16, 2016 at 23:35
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    \$\begingroup\$ Why is 4700uf ridiculous? It must power an Amp. \$\endgroup\$
    – Codebeat
    Nov 16, 2016 at 23:39
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    \$\begingroup\$ I usually use DC-DC converters that have a "soft start" facility - that way you don't get inrush current. \$\endgroup\$
    – Majenko
    Nov 16, 2016 at 23:51

4 Answers 4

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1) No, a PTC will not help from the start. You are correct that it takes some time to react to the high current before it will trip or limit current. So it is just a matter of whether it will trip fast enough for your needs. Very short current spikes are often not noticeable.

2) Yes, a PTC does influence the performance somewhat. Since it is in series with the input voltage, and it has some resistance. Whether this resistance will cause a problem depends on many things. Mainly, the allowable voltage drop, the max current, and the resistance of the actual PTC selected.

3) Not clear what amperage you are referring to. Datasheets for PTC's normally have several amperages listed.

Dividing the capacitors will not help.

It sounds like what is happening is that when there is high demand for current, the power supply is not able to meet the demand, so the regulator output voltage starts to go down. This is what causes the LED's to dim. It may be that you need a power supply with more capacity to avoid this problem rather than using a large capacitor.

A lot of audio amplifier designs on the web use a step-down transformer/bridge rectifier/filter capacitor for a power supply. Often this type of supply uses really large capacitors to try to smooth out the 100 or 120 Hz ripple. But your supply, since it is a DC-DC converter probably does not need such a large capacitor. I don't think you should automatically reject the idea of using a smaller capacitor.

If your design contains a micro-processor, you can consider using a simple power resistor to limit the inrush current to the capacitor, and use a power MOSFET to bypass the resistor after power on. The microprocessor would turn the MOSFET on during power on (after some delay), and turn it off during power off.

If your design does NOT contain a microprocessor, you still might be able to design a simple circuit to turn the MOSFET off until the capacitor voltage reaches over 20V, say. This would eliminate most of the inrush.

Someone might be able to give a better answer if you included more information in your question. What is the output power of the AMP? Is it stereo or mono? Does it need to drive a 2 Ohm load? 4 Ohms? 8 Ohms? What is the part number for the capacitor? This could be helpful if someone wants to know the series resistance for some calculation.

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  • \$\begingroup\$ Thank you for the nice answer! Had to work and sleep, that's the reason of delay in responding to your answer. If I connect the DC/DC converter to USB hub for example, the HUB will reset and after that everything seems to be 'okay'. But since I use 4A batteries and the maximum load of the is 2A, I don't understand why there is such dip because it should be able to the handle 'load'. 'Load', because nothing is connected to the DC/DC converter except the capacitor. So it has nothing to do with the Amp. I know the 2A is a little weak for the total power of the amp but doesn't use it at normal ... \$\endgroup\$
    – Codebeat
    Nov 17, 2016 at 17:52
  • \$\begingroup\$ ... use. It's a class D Amp and don't use much current when idle or working with a low load. Maybe I want to change the DC/DC converter with a more powerful one or try to use two DC/DC converters in parallel. But it is not about the Amp in the first place, the DC/DC converter can handle it at normal to loud load thanks to this capacitor for low frequencies. I have several units of this DC/DC converter and I will test it tonight without or with a lower capacity capacitor to see what will happen. \$\endgroup\$
    – Codebeat
    Nov 17, 2016 at 17:52
  • \$\begingroup\$ Just disconnect the LS 25v capacitor and inrush problem is gone, so it has defentily something to do with the capacitor. Because the DC/DC outputs 24.2v, maybe the 25v capacitor is too close to it's limits. Could that be the reason? \$\endgroup\$
    – Codebeat
    Nov 17, 2016 at 19:38
  • \$\begingroup\$ Okay, tried several things. Without, no inrush current. With lower capacity 480uF and higher voltage acceptance 35v, still the same problem but shorter period. \$\endgroup\$
    – Codebeat
    Nov 17, 2016 at 20:28
  • \$\begingroup\$ Well, the 25V rating is too close. But that is not related to inrush. The inrush current is basically caused by the capacitor. When a large discharged capacitor is attached to a 25V supply, it will always pull down the supply voltage while it charges up. \$\endgroup\$
    – mkeith
    Nov 17, 2016 at 23:19
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I cannot envision how a PTC can reduce inrush on a converter. The inrush IS the capacitor charging current, so it is virtually instantaneous. It is PRECISELY the first few microseconds of time where you NEED the series resistance to limit the current. That's why NTC thermistors are used: high resistance when cold, lower when hot. If you don't like the residual low resistance afterward, you can bypass it with a lower resistance device (switch) AFTER the caps are fully charged, so the thermistor can immediately cool down and return to the high ohm state. But if you are going to do that, just use a regular resistor. An NTC thermistor is used because you DON'T want the cost of adding that other bypass device, but can live with the small amount of resistance is presents in the circuit.

A PTC thermistor would do nothing in the first moments, THEN increase resistance, which is totally pointless in avoiding inrush.

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  • \$\begingroup\$ I agree with this. A PTC can only limit the tail portion of a fairly long duration inrush event. \$\endgroup\$
    – mkeith
    Nov 17, 2016 at 2:18
  • \$\begingroup\$ Thanks for answering. Okay, explain to me this: en.tdk.eu/tdk-en/373562/tech-library/articles/… \$\endgroup\$
    – Codebeat
    Nov 17, 2016 at 3:25
  • \$\begingroup\$ Did you read this? "Under normal operating conditions, a PTC ICL functions as an ohmic resistor. When the power is switched on and the temperature of the component is the same as the ambient temperature, PTC ICLs have a resistance of between 20 Ω and 500 Ω, depending on the type. This is enough to limit the inrush current peak. Once the DC link capacitors are sufficiently charged, the PTC ICL is bypassed." \$\endgroup\$
    – mkeith
    Nov 17, 2016 at 4:13
  • \$\begingroup\$ @mkeith: Yes, read it, that's why I comment to this answer with that article. \$\endgroup\$
    – Codebeat
    Nov 17, 2016 at 17:32
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    \$\begingroup\$ So in my earlier response I said you could use a plain resistor and a bypass, in that article they are describing the same thing, but using a PTC instead of the simpler resistor. I suppose it might help if there is a rapid duty cycle of the power, because as the PTC heated up, its current limiting effect would INCREASE, which would slow down the capacitor charging, making a self-limiting duty cycle response. The faster someone cycles power, the longer it takes for the unit to fully power up, thereby discouraging the user from flicking the switch on and off so fast... Might not be a bad idea. \$\endgroup\$
    – JRaef
    Nov 17, 2016 at 20:39
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If a NTC is not suitable due to no protection on flicking power switch, you can use mosfet current limiter. Here's an application note from Motorola which does that for you: http://www.bonavolta.ch/hobby/files/MotorolaAN1542.pdf

Google for AN1542 if that mirror disappears, Onsemi didn't bother keeping that little gem from motorola available.

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  • \$\begingroup\$ @Erwinus I have used that circuit with a medical X-ray sensor that I didn't want to die horribly if something goes wrong so I used a fast fuse. It was also hot pluggable and that reduces (but does not completely eliminate) sparking. \$\endgroup\$
    – Barleyman
    Nov 17, 2016 at 18:07
  • \$\begingroup\$ is there a soldered mini board (ready made pcb) that can do this for me? What's the name or partnumber/modelnumber if there is any available? something like this? trottercontrols.com/products/75/inrush-current-limiting-pcb It must be able to handle 5A/6A (example is only 1A), that will be great. The DC/DC converter I had made has been improved now (two in parallel which works outstanding) and can handle 4A max. \$\endgroup\$
    – Codebeat
    Nov 21, 2016 at 20:31
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Other comments about PTC's are missing the point.

PTC's are self protecting resistors. You choose a PTC with a default R sufficient to limit inrush and it needs to be bypassed with a relay/switch after the inrush event is complete.

If you have a crazy user who flips the power switch on/off 100 times the PTC is able to protect itself. In this way a PTC+switch/relay is one of the most robust inrush limiting schemes. Just not the simplest.

https://www.tdk-electronics.tdk.com/download/1537850/65ec22d721749ecc212117e2f632e434/ptc-icl-pb.pdf

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