Problem with Inrush Current on Power Supply

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jhsa
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Problem with Inrush Current on Power Supply

Post by jhsa » Thu Jan 24, 2019 3:03 pm

Hi guys, this is not related to RC but I believe I need some help :)
The picture shows a power supply I am building, using DC-DC Converters as I need isolated input and output grounds.
When not specified, the electrolytic caps values are 220uF.
The converters are the Mornsun B1212S (in 12V and out 12V, 83mA max) and the B1209S (in 12V, and out 9V, 111mA max).
The problem I am having is that the Q2 Mosfet sometimes blow up. The reverse protection Mosfet also blows up sometimes.
These Mosfets have a max Vgs of + - 20V and a current of about 4A max.
I expect a max current drain of about 700mA.
The circuit is supplied with 12V, I have the over voltage protection because the DC-DC converters max voltage is 13.2V
Resistors R10 and R11 help bringing the trigger voltage close to 13.2V. I used a trimmer to find the correct values.
the protection works good. But if I turn the power ON and OFF a few times one or other mosfet will blow up and become a low resistance between all its pins. Even if i remove the protection circuit, leaving only the reverse Polarity Mosfet (Q1) in the circuit, and connect the 12V to the converters by touching the power Bus using a little wire, I can see some spark when connection is made, and after a few times, the Mosfet blows up :(
So, I guess I might be seeing some High (inrush?) current when closing the circuit.
my question is, what can I do to deal with this?
Higher current Mosfets? Slow down the Mosfet turn ON time? How?
I think hese Mosfets should be able to handle 700mA, as their max is around 4A, right?
But of course if the initial current is much higher, then I understand that they could blow up :(

In case you are wondering why i want to build this, it is to power my self made guitar pedalboard. this will allow me to power all effect pedals with isolated grounds and avoid ground loops. The ground between pedals will be shared only by the signal jack connector..

Thanks in advance for your help..

João

EDIT: Before I decided to use the over voltage protection circuit, I had a 12V regulator(7812) with heatsink. If I fed it with 15V it felt quite hot to touch. perhaps it shouldn't?? If powered by 12V only, it didn't become so hot, but obviously the output voltage was lower than 12V. Still ok for the converters because they need a minimum of about 10.5V to work. But the BS1209S were outputting about 8V only, instead of 9V..

The thing is, with the 12V regulator, I naver had Q1 blowing up.. So, why that? the regulator starts slower?
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PS with over Voltage Protection.png
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Ozzy
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Re: Problem with Inrush Current on Power Supply

Post by Ozzy » Wed May 08, 2019 4:15 pm

My first guess is the issue might be when turning off, the C1, C17, might discharge back through to the main supply and somehow damage some parts. Kinda like a race condition, maybe try inserting a diode right before the C1/C17 caps to see if this fixes the issue, will have to compensate for the 0.6v drop.

If this is a personal project, why all the protection (reverse/OVP)?

Also your 12V linear regulator has a spec for headroom, its the minimum required input voltage above the output voltage, if you can set the input voltage to this voltage, minimum heating in the linear regulator.

About 30 years ago I did a pedal supply for my roommate, I went all linear with lots of caps and shielding to keep the noise to a minimum, he still uses it today. Good luck.
Last edited by Ozzy on Wed May 08, 2019 10:54 pm, edited 1 time in total.

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Ozzy
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Re: Problem with Inrush Current on Power Supply

Post by Ozzy » Wed May 08, 2019 10:53 pm

A quick lookup of a 78xx regulator dropout voltage graph (look up your part numbers datasheet). They also make low dropout (LDO) regulators, I have had good luck with Micrel brand.
Depending on the output current and regulator temp, you can find the dropout voltage (FYI, this is the minimum voltage the regulator needs to regulate its output voltage. If you go higher on input voltage, this is just extra heat the regulator has to dissipate.
78xx Regulator dropout voltage.jpg

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Ozzy
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Re: Problem with Inrush Current on Power Supply

Post by Ozzy » Thu May 09, 2019 8:45 pm

More thinking, I smell smoke :)

Q1 has a benign function. take out Q1 temporarily to troubleshoot the Q2 problem.

Q2 fet is 30V.

Test A---Reduce the input voltage to min input voltage.
Then turn on and turn off.
This will reduce the Vd stress on the Q2.
Does Q2 blow?

Without a scope capture of the transient at turn off or turn on, can't say what is causing the failure.

TEST B are both Q1 and Q2 getting hot?
What is the rating of the 12V zener D1?
Is that blowing also?

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jhsa
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Re: Problem with Inrush Current on Power Supply

Post by jhsa » Thu May 09, 2019 11:16 pm

Hey, I had forgotten about this thread :)
I ended up giving up on the isolated switching regulators, connected all grounds again and left the 78M09 in there for each output. Added some PTC for protection, and that is it.
Due to the isolated switching regulators max input voltage limitation, I also made a circuit using a couple mosfets that disconnects the power if the voltage exceeds 13.2V. So, I have learnt something :)
Thanks for your comments anyway, I'm sure I will come back to these regulators sometime in the future. :)

João
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