Recommendation around buck converter use


#1

Greetings,

I am looking for an opinions around using a buck converter to power a Photon.

The project I have build uses a 12v 2A power supply. The 12 volts is needed by other devices which is controlled using transistors. To step down the voltage to the Photon I am using an MP1584EN step-down buck converter module. The device is rated to 3A (1.8 typical). The output of the buck converter is connected to VIN.

According to the specs the buck’s output ripple frequency is 340KHz with a voltage of 30mv. Is this enough to effect the operation of the Photon or cause other side affects? My oscilloscope is not working so I am unable to measure actual from the device.

Would it make any different to add a capacitor to the output? I worry at that frequency about possible low resistance issues.

I did notice that my multimeter does read frequency, and verified my outlet is at 60Hz (as a test). The measured frequency of the buck output is negligible, though that may also due to the very low voltage of the ripple.

I haven’t used buck converters in the past so I am not sure if external components are usually needed. The devices purchased weren’t that expensive but have held a very reliable voltage over many days of use and test loads.

Thank you for any feedback.


#2

I’d definetly add some caps.
Although the ripple won’t affect the controller due to its own 5V->3.3V concerter circuitry, but the onboard converter doesn’t like noise very much and with some unfortunate circumstances you might get into some resonance issue between the two converters.
We have seen 5/3.3 converters die of such conditions.

So better donate a couple of caps rather than run the (unlikely) risk.


#4

Recommendation on capacitor size?

Brett


#5

So I noticed one of the units had a hot regulator so I took some voltage readings. The voltage on VIN was 5.12v The voltage on the 3.3v pin was ~5. So perhaps there is something going on there. All the other devices are still at ~3.3. I am getting quite concerned about this setup.

I need to know best practice here, do I continue with the buck converts and add some capacitance on VIN to ground, or dial the buck converter down to 3.3v and power from there, or use something altogether different to power the Photon. I have been having issues with some of these installations, and it seems I may be breaking it down to the source here.

Just took measurements. Pulled device from project and powered with USB cable . The onboard regulator does get hotter than normal.
3.3 pin is at ~4.68 and VIN is ~4.8. So for sure this Photon has a bad regulator. What isn’t known is if it was like this originally or occurred over time. I assuming to continuing to run this photon will kill it.


#6

Definitely sounds like the Photon is damaged with 4.8V entering a 3V3 pin.

Where are you purchasing the MP1584EN modules from?


#7

The are these: http://amzn.to/2t3sDzH

Brett


#8

Hi @ScruffR, any thoughts on capacitor size to use on buck converter output (VIN and GND)?

Brett


#9

I’m more a kind of drop the odd 22nF & 220nF ceramic here and there guy, but @bko might have a better founded view on the topic :wink:
I don’t think you’ll need a bigger electrolytic for buffering, but for getting rid of/reducing the high frequencies the ceramic caps should help.


#10

I have not used this buck regulator before. The datasheet says that the inductor and output capacitor need to vary in value based on the output voltage but yours seem to be fixed. They recommend 22uF for 5V output.

I would try one low-ESR cap at around 22uF and then 0.01uF or similar.


#11

Thank you. Which data sheet were you looking at, the one for the MP1584EN chip?


#12

So one of my worries here is that the regular module has an output capacitor. If I another to the output I will reducing the overall output capacitance. I don’t think that is a good idea .


#13

Unlike resistances, parallel capacitances add up while serial caps reduce the overall capacitance.


#14

Well I feel embarrassed. Not sure how I got that backwards. I know better.


#15

@brettski This place has a ton of high-quality DC to DC converters.

Heres a list of buck regulators: