Can I use a resistor before a bridge rectifier to lower inrush current?

Question

I have been looking for ways to limit inrush current in a power supply circuit I am designing. In LTspice, with no current limiting, I see a spike of around 24A before my caps are charged.

Looking around, I see NTC inrush current limiters as a common solution as well as some ideas with inductors and resistors inline with the capacitor, each with its own advantages and disadvantages.

My idea though was to put a resistor before the bridge rectifier in my circuit, so the resistor would always provide some resistance before the capacitors got charged. Playing around again in LTspice, this drops my inrush current to about 4A (and changes with the resistor value obviously).

Is this ever done? Are there any major good or bad reasons to do this? I suspect no one does this due to decreased power efficiency, but is that the only reason?

Answer 1

There are good and complicated solutions, relay, FET, and simpler, thermistor, but it still may be possible to use the simplest, fixed resistor, without too much dissipation for your application.

If you get an inrush, does it matter? Well, only if it breaks something. So what could break? Supply fuse, transformer, or diode rectifiers.

Fuses have 'T' rated versions that take a long time to blow, for just this application. The transformer is a heavy lump of copper, that's not going to fail.

Read the specification of your diodes carefully. You may be surprised at how much the 'single cycle surge current' is. In a 1N40xx (cheapo workhorse mains diode), the continuous current is 1A, the surge is 30A. For 1N54xx series, the figures are 3A and 200A. This is specifically to allow them to survive inrush. You may find you already have enough stray resistance in your circuit to limit the current to the safe surge value. If not, you maybe won't need much more, and it still may give you acceptable efficiency. If it doesn't, then try smarter solutions.

Answer 2

Using a resistive element to limit inrush current is done.

However, placing a permanent resistor in the AC (or DC ) lines is very inefficient as it is always in-circuit.

The use of a Negative temperature coefficient (NTC) thermistors is often done instead as it provides a level of resistance during the precharge event that is then reduced due to heating. This however does impose a "cooldown" period to facilitate the NTC returning to room temperature resistance in the event of a power loss or power cycle.

An established method is to use a form of soft-start

http://www.ti.com/lit/an/slva156/slva156.pdf

^{simulate this circuit – Schematic created using CircuitLab}

There is equally monolithic chips that provide the same functionality.

http://www.onsemi.com/pub_link/Collateral/NCP330-D.PDF

Answer 3

Adding resistors will create a permanent power dissipation. This is usually undesired. It will also create fluctuating input voltages caused by changes in current draw (voltage drop at resistor).

Answer 4

A quite simple solution (I'm surprised it's not mentioned already) is to add a small coil after the rectifier:

^{simulate this circuit – Schematic created using CircuitLab}

A coil doesn't add much active resistance and doesn't practically affect the efficiency under "steady state" operation. The current spike is suppressed by the inductance which limits the current rising slope by $$L=\frac{dI}{dt}$$

Answer 5

There are fancier ways of limiting inrush current than the simple resistor ,Some of which JonRB has stated. If you are using a resistor there are a few more things to watch out for .Firstly the surge power dissipation of the resistor can be incredible .I have seen Christchurch design engineers blow up such resistors .Its really important to get specifications for peak power dissipation .Melf are good that way but some manufacturers are vauge.Secondly the average power dissipation is dependant on the RMS current not the average current .RMS current can be and often is significantly more than the average .I have seen people unwittingly undersize these resistors . The horrible part about this is that the resistors take some time to fail open circuit .The way the resistor fails is also important .Some resistors will go on fire which is bad .Some are flame retardant and go out ,some dont .If you still want to use a resistor test it under fault conditions .