I need to reduce the inrush current of a 24Vdc to 5Vdc linear regulator supply.

Thread Starter

Mozago

Joined Jun 14, 2017
10
Hello all,

Here's my schematic so far:


The circuit's input is a 24V DC carrier with a 2.176 MHz frequency that needs to be filtered out. Steady state current is about 700mA when the supply has its load.

I need the inrush current to be at about 400 mA or less. Here's what I measure with the circuit as pictured above:


That first spike is about 980mA and the second spike is about 660mA. A bit higher than I need.

My understanding is that the first spike is my inrush before the regulator turns on (input caps) and the second spike is after the regulator turns on (output caps).

If I remove the 1uF capacitor on my 5V output I get the following:


Only the initial current spike 980mA remains.

Here's a closer picture of that first spike if it helps:


Any advice someone can offer me to reduce this inrush current would be appreciated.

Thanks!
 

Thread Starter

Mozago

Joined Jun 14, 2017
10
Are the values of those capacitors shown correctly?

Is it the current or the ringing that is hurting you?
I took a moment to double-check and yes, those capacitor values are correct.

And it is the current that is hurting me. I need to lower it to 400mA or less to meet particular design requirements. If reducing the ringing could help accomplish that it would be a happy bonus.
 

DickCappels

Joined Aug 21, 2008
7,955
I am surprised to see so many amps being drawn for so many microseconds with such small capacitors. Has the inrush current always been there?

If I may ask, what is the purpose of the transistor-diode circuit?

A simple solution might be to put a resistor in series with the LM317 such that the LM317 drops out of regulation when the current exceeds your maximum allowable current. A way to do that and have the limiting point adjustable is to use a lower value resistor and an adjustable pre-regulator.
 

Thread Starter

Mozago

Joined Jun 14, 2017
10
One option would be to include a series current-limiting resistor which gets switched out after a short time delay.
Or you might find a suitable NTC inrush limiter.
The device I'm designing this power supply for can end up in hot conditions up to 150 °F. I don't think I can use and NTC Inrush Limiter because they work like a thermistor.
 

Thread Starter

Mozago

Joined Jun 14, 2017
10
I am surprised to see so many amps being drawn for so many microseconds with such small capacitors. Has the inrush current always been there?

If I may ask, what is the purpose of the transistor-diode circuit?

A simple solution might be to put a resistor in series with the LM317 such that the LM317 drops out of regulation when the current exceeds your maximum allowable current. A way to do that and have the limiting point adjustable is to use a lower value resistor and an adjustable pre-regulator.
Yes, the inrush has always been there.

The transistor diode circuit is off of TI's datasheet for the LM317. It's a slow start circuit.

I don't think I can put a resistor in series with the LM317 because it has to accept a 10-30V input. The resistor may throw this off. I should have mentioned this requirement in the original post.
 

DickCappels

Joined Aug 21, 2008
7,955
I suspect that there is something going on between the risetime of the 24V power supply and the resonant frequency of your filter. Can you measure the risetime of the power supply?

Maybe the answer is finding a different resonant frequency for your filter, and maybe even adding a little resistance too.

Edit: By different resonant frequency I mean a much lower resonant frequency.
 
Last edited:

Thread Starter

Mozago

Joined Jun 14, 2017
10
I suspect that there is something going on between the risetime of the 24V power supply and the resonant frequency of your filter. Can you measure the risetime of the power supply?

Maybe the answer is finding a different resonant frequency for your filter, and maybe even adding a little resistance too.

Edit: By different resonant frequency I mean a much lower resonant frequency.
It looks like this turned out to be the issue. I was able to get inrush current lowered to an acceptable number by changing the values in the filter. In my case going from a 10uH inductor to a 47uH and adding a second 470pF cap at the voltage input worked.

Thanks!
 
Top