ldopower supplyswitch-mode-power-supplyswitching-regulator
I Designed a 5v,7A power supply from 24v input for my new project. It is giving 4.9v, but have noise issue. When I increasing the load its noise level is also increasing.
I connected 1A load and a rapberry pi at the output. Is it an acceptable level of noise.
I am attaching the schematics and output waveform, Kindly check it and suggest me a solution.
After repairing a returned unit, take a very careful oscillogram across the Schottky with a short 10:1 scope probe (directly connect the tip and use a very short ground wire) and no bandwidth limiting on your good quality (100MHz or better, digital storage preferred) scope.
An example of a short probe, borrowed from elsewhere on EE.SE:
What you're looking for is high voltage (a spike, or ringing) on the leading edge of the voltage waveform. Any time you switch an inductive load, there's the potential for high voltage - unless you carefully look for it (i.e. with a short probe and a good scope) you won't see it. Your layout will have a great influence on whether or not there will be nastyness across the Schottky, so measure to be sure.
If you do end up seeing something ugly, you may need to introduce an RC snubber across the Schottky to keep the diode from avalanching.
A 39000uF cap and a transformer winding is way too much for a filter.
I suspect your power supply is either defective, poorly-designed, or being operated way out of spec. The spikes are happening at a rate of ~85-90 kHz, which could be the switching frequency. The higher-frequency ringing afterward is clearly due to the spikes. If you can zoom in on the spikes with your scope, it might tell you (and us) more. A link to the Amazon page or a datasheet would also be helpful.
Regardless, your options are:
The Photon's suggestion of trying a larger load is also a good one.
Best Answer
Noise seems high although as some have suggested if it works, why fix it? The Pi will have it's own bypassing capacitors that mitigate this noise. You're clearly using a commercial /professional switch chip rather than some Frankenstein 555 timer creation. The chip's data sheet has layout recommendations towards it's end. These are pretty important due to the large switch currents flowing around (and beneath) the chip. Do these recommendations match yours?
I'm assuming that your circuit is lifted straight from the data sheet. So you might expect the same ripple /noise that it gives. I can't read the screen sufficiently on my device to ascertain the noise, but is it ~30mV @ 20 MHz? I'm not sure I believe this as data sheets can offer representative information. The data sheet extract below is from this commercial PSU outputting 5V @ 6A. Ripple is 120mVp-p, higher than your chip's datasheet, but still significantly lower than your results.
If you can't reduce the noise to an acceptable level though layout improvements, you might consider anther stage of LC filtering. You'd only need uH stuff as you'd be looking to smooth frequencies in the order of 100kHz - MHz.
I can't see from your 'scope grab, but do you have 20 MHz bandwidth limiting on? You need to have that set to usefully compare noise readings between PSUs.
I also suggest you watch Dave's video regarding correct oscilloscope probe technique for measuring PSU noise. You really should do it differentially as in the video. It is likely that you'll see improved results with a better probing technique as you should be able to compensate out at lot of extraneous noise.
Personal statement: I never build switch mode power supplies unless I'm specifically trying to learn how. If I ever need a switcher, I always buy off the shelf. There's a lot of analogue /EMI /ground loop theory to absorb and digest to minimise the noise they're notorious for. Once you factor in the components, especially those weird specialist high frequency transformers and a commercially manufactured PCB, the cost differential is small. Plus you can consider any extra cost as an insurance premium for ruling out odd behaviour from an uncertain DIY switcher. Just sayin'.