MC34063, sharks, and breadboards (#110)

[naimis]

I've been playing around with building up an SMPS for a project using the MC34063, though I have considerable doubts about the stability of the output voltage on this device.  Anyway, I recently saw some posts either here or on youtube about breadboard-related noise and thought I'd post some related images from my scope.

R1 is on a breadboard, 1/U is on a protoboard.  Same circuit - 9V output.  47uF output cap - the equations turned up a minimum 25uF.  I have to use an obscenely large cap to get that shark fin down to a reasonable level (I *think* I was able to get it down to 100mV p-p with a 1000uF cap, but I wouldn't stake my life on that).


Also it was an excuse for me to learn how to save reference waveforms.

The above was without a load (and I should probably mention this was all done w/ DC coupling and the normal ground clip on the probe).  Here's the same with the design load.


And finally, probably with no load, but with the horizontal scale such that you can actually see/measure the frequency of the sharks:


That's 1.06V p-p and a noise frequency of 354.1 Hz (I really should have made sure the markers were on the freq rather than the p-p measurement, but oh well).

The formula tables said I should use a 391.84pF timing cap, and I ended up using a 390pF cap.  If the signal on pin 3 was any indication, the switching frequency was only ~70KHz rather than the intended 100KHz.  Ah well, it made for an interesting learning experience but I think I'm unlikely to use it in the project I'd intended.

[jahonen]

I think you have guite large ESR on the output capacitor, are you using a conventional electrolytic? For small values, ESR tends to be quite high even for "low-ESR" electrolytics. Try replacing output capacitors with ceramic or oscon/polymer if you want low ESR in a small package or with a small capacitance value. That might be feasible with 9 V output.

Also, 34063 does not have a proper PWM modulator, just a fixed duty cycle oscillator which is gated on when voltage drops below the limit determined by the internal comparator. Thus the erratic switching behavior you see is quite normal. The main merit of 34063 is that it is cheap and ubiquitous, but if you want smoother output, then I suggest that you try something else.

Regards,
Janne

[naimis]

Yeah, Co is a conventional electrolytic.  When I ordered the parts, the output size I had might have been small enough to get a ceramic without spending a ton of money, but that would only be because I had an error in my spreadsheet.  The cheapest thru-hole ceramic cap I can find on mouser in that size (25uF+) is $2, and it goes up pretty rapidly from there.

I'm unsure what the ESR of the cap I'm using is.  I had a pile of 35V 47uF electrolytics which were the best fit I had available, so I used those.  They're Nichicon VR series P/N UVR1V470MDD1CV.  I don't yet have an LCR meter and while the data sheet does give tangent of loss angle, I can't be bothered to do the math