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Which capacitor is the cause of the problem of my switching power supply circuit

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Dailin

New Member
Joined
Mar 28, 2017
Hi, I'm a Chinese student. I'm using a switching power supply chip LM2575(http://www.kynix.com/uploadfiles/pdf9675/LM2575-12BN.pdf) to drive the load by turning DC voltage 24V to 1.5V. It works well except a problem...

Everytime when the power’s off, the capacitor C2 (maybe?) will still supply power to the load for a short period. Due to this, I cannot turn off the load immediately.

If the problem is not caused by C2, then what’s the cause?

I wonder if there are any solutions to this problem?

Sorry for my poor English, but I really need your suggestions! Thanks in advance!

Here's the schematic:

114637yzwsbfvywwfwxazh.jpg
(R3 is the load)
 

Dolk

I once overclocked an Intel
Joined
Mar 3, 2008
Haha, I would really like to know how you ended up here to ask this question. But regardless I can help out. First though, how come your image was edited? I cannot read Mandarin so I'm not sure what R3 is to represent but I assume it is the Load device.

What is the value of R3? What is the time constant when you combine it with C2? This will first start you off to answering your question. If R3 is too low of a Resistance to create a fast RC time constant, than add additional resistance in parallel to C2.

Also how come you do not have a switch on pin 5? You will need this to properly control power. In real life, removing power from VCC is not ideal, you want a switch to turn on and off the device as you need.
 

RJ88

Disabled
Joined
Mar 24, 2017
what's the value of C1/C2? could try going a little lower, you say when shut it off still powering and you want nothing and it to go instantly dark? could add more resistors to ground? of whatever appropriate size and a smaller value that will bleed it?.