5v spike using DC-DC LM2596T-12/NOPB, Switching Regulator

[jcampen]

I have just built a DC-DC regulator to reduce 26v down to 12v. I have used the simple recommended circuit as outlined for this LM2596T-12/NOPB found on page 9 of this data sheet.

The issue I have is when I turn it on, the output voltage jumps up to 16.5v for a fraction of a second before settling to 11.94 volts requirement. Id say the surge is easy less than 1/4 of a second.

My question is, why does it do this and will it damage the 12v circuits it will be supplying power to? Is there something simple I can add to the circuit to stop this spike?

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

Are you sure this surge is really present across the output? How are you detecting it? If you're using an oscilloscope across the output, try connecting the tip of the probe to the earth point in the circuit and powering it up. If you see a large spike, it is a common mode surge and isn't really appearing across the output.

 

[jcampen]

Kris I'm just using a multi meter set to Volts.

 

[KrisBlueNZ]

A digital multimeter? Auto-ranging or manual ranging? Can you find or borrow an analogue (pointer-type) meter?

 

[jcampen]

Yes both of the ones I have are digital. One is auto-ranging and the other is a just a cheap digital, but with a few preset voltage ranges. I do have a very high end analogue multimeter, but it in storage in Aus. and I am currently based in Malaysia. Why will the analogue type make a difference?

 

[KrisBlueNZ]

An analogue meter won't give you a misleading indication when the voltage changes quickly, assuming the meter movement is properly damped. An auto-ranging digital meter could be misleading. Try setting your auto-ranging meter to manual range hold.

How many conversions per second does it make? How long is the overvoltage spike? Can you estimate it in terms of a number of conversions?

Do you have a digital meter with an bar graph scale along the bottom, like Fluke DMMs have? If so, is the overshoot visible on that scale?

Have you tested the power supply with an without load? What is the load?

 

[jcampen]

Thanks for the tips Kris. I will get a analogue meter and test it with that. Ive put it under load onto a few non sensitive items and it seems to be working fine on those. I measured the voltage and the spike is much less when its under load.