Howto properly filter/smooth stepped up voltage

Thread Starter

mexxle

Joined Oct 23, 2016
7
Hello,
I have a digital radio which is powered by four AA batteries or with an external power supply which provides 6V DC.

I would like to build an adapter to use the radio with a regular USB connector. Since USB only delivers 5V I used a small step-up module (based on MT3608) to step up the voltage to the desired 6V. The radio powers up but it can't detect any radio station. I guess stepping up the voltage induces some high frequency noise which is disturbing the radio somehow.

Checking the power line with a scope I found out, that there is a Vpp of approx. 800mV at 1.2MHz (which is exactly the switching frequency of the MT3608). I assume this is too much for the radio...

My question is, how to filter/smooth this properly. I tried different things using obviously a capacitor, some inductors I had laying around and so on, but nothing really helped. Could you please give me an advice?

Thanks
 

philba

Joined Aug 17, 2017
960
A different approach - what does your radio do with the 6V? Does it regulate it down to 5V? Maybe there is a point you can simply connect 5V.

While I'm at it, have you tried just using 5V straight into the radio (instead of 6)? My reasoning is that it will need to handle a range of voltages as the batteries discharge.

by the way, 800mV of ripple is terrible. your step-up is probably malfunctioning.
 

Thread Starter

mexxle

Joined Oct 23, 2016
7
@bertus: Previoulsy not, but I just tried it with a 220uF Panasonic Low-ESR electrolytic cap. Did not work.

@philba: Until now I didn't open up the radio, but I have tried to use directly 5V and this resulted in bad signal strength. Not as bad as when powering with the 6V stepped up, but still very low signal strength compared to battery usage.

I also tried a different step-up module based on XL6009 with lower ripple at about 500mV, still no radio signal :-(

PS: Powering the radio from my lab bench power supply with 6V works perfectly.
 

philba

Joined Aug 17, 2017
960
500mV ripple is pretty terrible too. A reasonably decent PS should be in the 50mV or less range (actually 50 mV is kind of high). Let me guess, these are chinese PS modules off ebay... Bertus's low ESR question comes into play here - I wonder if replacing the output cap on the PS with a decent low ESR one could fix the ripple.
 

Thread Starter

mexxle

Joined Oct 23, 2016
7
Yes these are cheap modules from china, but shouldn't be the issue also gone (or at least better) when putting a low ESR in parallel at the output?
 

philba

Joined Aug 17, 2017
960
Yes these are cheap modules from china, but shouldn't be the issue also gone (or at least better) when putting a low ESR in parallel at the output?
Well, the cap needs to be close to the regulator. I think a half a volt ripple is kind of hard to get rid of with just a cap. Did you look at it on your scope? Some of the product that Chinese ebay vendors sell has flunked QC and they are selling it off cheap. I buy stuff from them but it's kind of a crap shoot.
 

Thread Starter

mexxle

Joined Oct 23, 2016
7
Just verified again, with no load on the output of my MT3608 step up module I have Vpp up to 1.22V

@bertus: Thanks for the schematic, since I don't have 1uH inductors laying around I'm ordering some and check this once they arrive.
 

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