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:
- Does a PTC avoid inrush current from the start (how accurate)?
- Does a PTC influence the performance of the capacitor (or the whole achievements of the PSU)?
- 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.
Best Answer
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.