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Problem with DC-to-DC converter MC34063

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patb:
Hi guys,

I am trying to add to my LM317 based power supply (this is actually a simple kit) an ICL7107 based voltmeter. Here is how it is built at the moment (w/o voltmeter):

1) input 230 VAC
2) transformer to 18 VAC
3) unregulated 24-26 VDC (4700uF capacitor)
4) LM317 based regulator
5) regulated output DC (up to 20V 1A)

Now, I tried to add a little DC-to-DC converter for powering ICL7107-based voltmeter. I had MC34063 in my junk box and I decided to try it. I assembled everything according to the datasheet and connected it in the same place where LM317 module is connected. Everything seemed to work fine, but when I tried to connect something to the regulated output (100mA load, no matter what VDC) I noticed that voltmeter showed higher voltage. After a short investigation I found the cause. DC-to-DC converter is adding a lot of short voltage spikes everywhere in the circuit (more spikes if you connect something to the lm317 output). I assumed that ICL7107 is sensitive to those spikes and shows higher voltage.

Now my question. How to solve this problem. Adding more caps before/after DC-DC converter doesn't help at all. This is the first time when I try to use 34063 chip so maybe I am doing something wrong. Or perhaps this chip is not suitable for this application. Any ideas?

Thanks in advance.

Psi:
can you poste a scope image of there pulses
and maybe a pic of the dc/dc

patb:
Sure.

Basically DC-DC is almost the same as in datasheet (I used 10k/30k instead of 1.2/3.6 R1/R2). I attached two images from scope - the one with more spikes has bigger load. Those show output of LM317.

tecman:
What you are seeing can be the result of a number of things.  Measurement technique (grounding, etc) are a big possibility.  Next the wiring and grounding on the switched are important.  Current paths must be sufficient in critical areas, such as the inductor, freewheel diode and output cap.  The output cap should be large enough and a low ESR type for minimizing switching noise.

It is a bit hard to tell, but it looks like diode recovers might be part of your problem.  Are you using a Shottky or at minimum a fast recovery diode ?  Are the current paths from the diode directly from the inductor to the ground point of the output cap ?  Those connections should be short and direct.  When measuring, is the scope ground short and direct to the same ground junction ?

Paul

patb:
Ok, let's start from the begining. I've made two almost exactly identical DC-DC converters - one on a breadboard and one soldered to a protoboard (a little messy in that case). Both have similar effect on the circuit. Now for the components. I do not have brand new low ESR capacitors and the ones I have are dodgy (taken from other device, I don't know their 'ESR condition'). Anyway, connecting these not-so-new Low ESR caps changes nothing. I use brand new Schottky diode model 1N5819. Output cap and the diode are very close to each other. Probe ground wire is connected to the common ground which is '-' of the main filtering cap (just after the transformer). But maybe something is in fact wrong with that 'ground rail' in the circuit as you suggested. I will investigate further.

And of course thank you for help, Paul!

Patryk.

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