Thoughts/advice on using thermistors with voltage doublers for current inrush?

[Deke609]

My PSUD model of the power supply of my Kaiju rebuild shows that the voltage doubler caps briefly draw a ton of current at turn on. I've doubled the capacitance of the doublers to reduce ripple even lower, but the increased capacitance causes even greater inrush current.  I'm a bit concerned by this and want to explore using a current limiter. Thermistors seem to be the go-to device. 

But before I explore thermistors further, I'd like to pick the brains of the BH experts as to possible pros and cons. Are there any reasons not to use thermistors with a voltage doubler?  Are some thermistors "noiser" than others and to be avoided?  This is new territory for me and I'd appreciate any pointers that people may have.

many thanks, Derek

[Paul Birkeland]

How much is the inrush current?

[Deke609]

The initial spike at turn on is around 18A.

many thanks, Derek

[Paul Birkeland]

A UF4007 can handle a peak current of 30A briefly.

The power transformer is also going to have a ton of voltage drop if you try to pull 18A through it, though PSUD doesn't take that into account super well. 

A CL-90 between hot and the power transformer primary will slow this down, but it wouldn't seem to be explicitly necessary based on the information you're providing.

-PB

[Deke609]

Thanks PB. I'll be using some beefier SiC Schottky rectifier diodes rated for 42A non-repetitive surge current, and for 21A of repetitive surge current: FFSP05120A.  I'm more concerned about the life of the doubler caps. I'll be using some long life Rubycon BXW 220u 400V caps. Datasheet attached. The datasheet doesn't mention max instantaneous ripple current -- maybe that doesn't exist? (But I thought I read about that somewhere when I was selecting caps a few months ago). According to PSUD, peak current drops to below 2A after 1500 ms, so maybe I don't need to worry about the doubler caps heating up.  Dunno. I'm totally ignorant here.

Good point about voltage sag @ 18A draw. PSUD shows a modest sag, but nowhere near as low as I would expect based on my limited experience.

Thanks for the suggestion of the CL90.  Is there a particular brand you would recommend? I may install it just out of an abudance of caution.

cheers and many thanks, Derek

[Paul Birkeland]

IMO you're miles away from needing a precharge circuit.  You are using 4x220u/400V caps for the doubler filter node?  (since you said more capacitance than stock)

[Deke609]

Yeah, 4 caps: 2 pairs of paralleled 220u's.

many thanks, Derek

[Deke609]

A follow-up on this. The caps are rated for 1.2A rms of ripple current at 120Hz.  Since I'll have two in parallel, I assume that they will split the current - perhaps not equally, but hopefully not entirely lopsidedly.  PSUD shows peak current at the doubler caps after 1500 ms of approx. 1.85A - so approx 1.3A rms.  So as long as one caps takes at least 100mA of that, I should be good after the turn-on surge. What I'm concerned about (out of ignorance) is that first 1500 ms when current bounces pretty high.  Is there no cause for concern?

many thanks again, Derek

[Paul Birkeland]

What is the actual RMS current displayed on the data table for each of the doubler caps in PSUD?  I would doubt that it's 1.3A RMS.  That's a lot of ripple current for a high voltage supply.

If you believe the current bounces that high, multiply that AC current value times the DCR of the transformer.  That's instantaneous loss, and I bet it is several times as high as the transformer voltage rating. 

[Deke609]

Huh. PSUD shows about 465mA rms.  I just took the peak value and multiplied by 0.707.  Guessing it's not a sine waveform?  But whatever the reason, that's a relief. I was a little surprised by the the high current peaks -- but I'm surprised by so much of this stuff, so didn't think to question my RMS calculation.

cheers and thanks, derek

[Paul Birkeland]

It is most definitely not a sine waveform!  There's not really a negative peak

[Deke609]

Right! It's recitfied. 

So to cap this off: no reason to worry about inrush current?

cheers and thanks, Derek

[Paul Birkeland]

You should always worry about everything, but the next step should be to quantify what you're worried about, then reexamine your paranoia.

[Deke609]

... then reexamine your paranoia.

You're freaking me out, man! How did you know I was paranoid?!!!   

Many thanks again - I can get back to putting the amp together now. Finally finished the metal work (more than 300 holes!), so now on to the fun stuff.

cheers and thanks, Derek

[oguinn]

You should always worry about everything, but the next step should be to quantify what you're worried about, then reexamine your paranoia.

Somebody ought to put that on a t-shirt