need help getting the 555 smps to work

Marcos - Munky · June 20, 2018, 09:32:26 PM

Some time ago, I asked a few things on the 555 smps. So I got a 100uH inductor and finally build it. The thing is it doesn't work...

Input voltage is 11.99V, and the output voltage is about 11.3V in every trimpot setting. Since I don't have any experience with this kind of circuit, I need some help to get it working. What should I check first?

Here are the files I've used. Original schematic, eagle schematic and layout. On the schematic I've used pads instead of the trimpot and inductor because I don't have the correct library. Also, I made the layout using a 2N5089 but built it using a BC547 like the schematic (yes, I've rotated the transistor).

The Red Rooster · June 21, 2018, 06:56:15 AM

Hi :-)

First: SMPS'er are difficult to get running as parameters related to switching transistor and inductor are critical. On top of that voltage and current feedback control adds to complexity.

Original schematic states: R5 220k, your sch R12 2k2.

This difference will influence whole system.

BR

Kurt

Marcos - Munky · June 21, 2018, 09:34:47 AM

Thanks a lot, Kurt. It was really the resistor. My bad when I drew the schematic on eagle. I swapped it for the correct value, and the smps is now working. With the trimpot at middle, I got 137V (didn't tested the max voltage, just checked if the smps is working):

Transmogrifox · June 21, 2018, 11:32:40 PM

Quote from: Marcos - Munky on June 21, 2018, 09:34:47 AM
Thanks a lot, Kurt. It was really the resistor. My bad when I drew the schematic on eagle. I swapped it for the correct value, and the smps is now working. With the trimpot at middle, I got 137V (didn't tested the max voltage, just checked if the smps is working):

Pretty awesome feeling, isn't it?

Marcos - Munky · June 22, 2018, 05:45:52 PM

Quote from: Transmogrifox on June 21, 2018, 11:32:40 PM
Pretty awesome feeling, isn't it?

Yeah, it's pretty awesome. Now that I got my first smps working, there's a lot of possibilities of circuits to build that doesn't require a expensive power transformer. Now, need to get my submini tube amp using this smps working

.

Marcos - Munky · June 24, 2018, 01:03:38 PM

A little bump with a question. Today I did a quick test using my submini tube amp. It worked, and the mosfet got hot (it doesn't have a heatsink yet). Then I turned off everything. After a few minutes, I decided to try it again to set the best voltage by ear, and things got strange.

I got no difference in the sound in every setting of the trimpot. Then I measured the voltage: about 60V and dropping. I know I should expect a voltage drop, but that was too much.

Then I disconnected everything and tested just the smps. Around 140V at the middle of the trimpot, a little above 180V full clockwise but the voltage started to drop as the trimpot started to get hot. Is this normal, right?

I'll test it again with a heatsink to see how it goes.

Additional info: I used a 12V 5A power supply to power the smps.

Marcos - Munky · July 07, 2018, 03:54:46 PM

Got it working! Made a new layout, the problem was still there, then I found out it was a fault mosfet.

Just one more question, I saw 2 variants of this circuit, one is missing that 56K resistor (R1). What that resistor do, and should I use it or not? The one I got working is missing the resistor, I removed it to see if it changes something.

The Red Rooster · July 09, 2018, 05:59:51 AM

I am so glad that you got your SMPS working.

There is so much good energy going around in here :-)

Greetings from Denmark to all.

/Kurt

Marcos - Munky · July 09, 2018, 04:50:17 PM

Thanks Kurt. Everything is working fine.

As for the 56K resistor, I put it back in the circuit and tested everything again, can't see any difference in the circuit with or without the resistor. I ended up keeping the resistor, since it was already soldered

thomasha · March 09, 2019, 05:52:50 AM

After one of my last builds my SMPS had much more noise than previous version, checking the internet I found this:
https://www.powerelectronicsnews.com/problems-solutions/pcb-layout-for-emc-power-supply-design-tutorial-section-3-1

Very interesting tips to avoid a noisy SMPS layout. It could be of some help for the next guys looking for a solution to ringing or squealing SMPS.

rankot · April 07, 2019, 02:21:39 PM

I've designed a nice 555-based SMPS PCB, just need to check it more thoroughly with HV/HF oscilloscope, but it seems to be working very stable right now. I'm also doing some calculations using theoretical articles, but they differ from LTspice simulation a lot. Must figure out what's going on.

temol · April 07, 2019, 03:46:25 PM

Ranko - what's the maximum voltage and current you can get without IRF and inductor getting hot?

T.

Marcos - Munky · April 07, 2019, 04:16:18 PM

Could you share the layout, Rankot?

rankot · April 08, 2019, 12:26:12 AM

I certainly will, once I check it with oscilloscope.

rankot · May 04, 2019, 03:29:50 PM

This is my old (and tested) layout. Still didn't have time to test it with an oscilloscope, but it can produce 250V with 33k load and with sufficient power supply. I will try it with bigger loads, too. I'm still reconsidering this layout and trying to make it better, but I'm too busy with my regular jobs these days, so I can't find some spare time for the purpose. I'm trying to make single layer PCB as good as possible, cause it's too complicated for me to produce or order dual layer.

This is the schematic - actually using voltage doubler. Usable range is 60-250V on this one I built.

I'm not sure which inductor I used, but I will desolder this one and measure it. It is some kind of small toroidal inductor, and I've read somewhere that it shall have broken core to avoid DC saturation, so I will try to cut the core with a tiny saw.

I didn't run it for a long time, but after few minutes no components get hot, and IRF740 doesn't have a heat sink.

This is my most recent (untested) layout.

I'll post a final schematic and final layout once I check it completely. I'll probably get rid of this (probably unnecessary) 0.33 Ohm resistor, and I will try to use better MOSFET, because IRF740 is not so good for this purpose (losses are big). And I will probably have only one LC filter at the end.

And since I'm an architect and not an EE, I ask you guys who understand those inductance/impedance related stuff better to take a look at those PCBs and give your precious comments.

Stay tuned.

rankot · May 05, 2019, 05:35:37 PM

I have measured inductor used in my first layout, it is 33uH. Also tried with 150uH and I get the same max voltage, however, it seems that circuit is more stable with 150uH inductor, allowing lower voltages to be produced. But this "stability" is not confirmed with an oscilloscope!

R9 should be smaller, 100 Ohm is quite sufficient. Maybe I don't need it at all???

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need help getting the 555 smps to work