UC3842 basics

[budm]

Re: UC3842 basics

The size of the transformer is rated in VA hwich is how much power it can supply, so let says that the old transformer is made to produce 5Volt at 1A, if you want it to put out 10V, then you max curent will be only 0.5A. Since the turn ratio does noe not change, the you primary voltage has to go up for the secondary to put out higher voltage. The 3842 is just the controller with its output is used for driving the transistor or MOSFET that actually drive the transformer, the feedback circuits are used to regulate the desired output voltage.

[rogfanther]

Re: UC3842 basics

Then , theoretically, if a transformer is already for 5V 1A, it could be used in another circuit producing the same 5V 1A, even if different MOSFETs and controllers are used ?

[ben7]

Re: UC3842 basics

Then , theoretically, if a transformer is already for 5V 1A, it could be used in another circuit producing the same 5V 1A, even if different MOSFETs and controllers are used ?

Yes, I think so. As long as the mosfet and controller are properly rated for the situation (correct frequency, feedback, operating mode, etc...).

[budm]

Re: UC3842 basics

Yes, same as ben7 statement. You will also need to know more about the spec of the transformer since it will have stary capacitance and resonance frequency of the tarnsformer, etc. Best to learn by searching "how to design switching power supply" it has many mode of operation as well.

[rogfanther]

Re: UC3842 basics

I understand the concept. My original idea was to have ready some skeleton boards, with most of the components in place, then just complete the circuit with the control part to configure it for the desired voltage. But that was in the case when I could re-use the transformers.

Barring that , I think what would work better is to get a bunch of cores that satisfy the design parameters for most of the voltages I need, then winding them accordingly for said voltages.

Of course, it would be easier to buy some power supplies from China / AliBaba, but where is the fun in that ?

[ben7]

Re: UC3842 basics

Of course, it would be easier to buy some power supplies from China / AliBaba, but where is the fun in that ?

Load testing them and blowing them up

I like the idea of a skeleton board. I have even thought about doing that for using the DM311 5vsb mod to replace the crappy 2-transistor 5vsb.

[rogfanther]

Re: UC3842 basics

Yes, having it ready would make the decision easier when the need arises, without having to source parts, building another board, etc.

[ben7]

Re: UC3842 basics

Yes, having it ready would make the decision easier when the need arises, without having to source parts, building another board, etc.

Maybe we should design a board

[ben7]

Re: UC3842 basics

Ok I had a go at this again, and I did get it to work!

Im using a UC3844, and a 5vsb transformer. (again xD)

Its regulating the output to just about 5v like it should, BUT, the transformer whines! Does this mean the feedback loop is unstable??? I have the filter capacitor (an NCC RX(discontinued!), 330uF, 35v, low esr) right after the diode, but before the TL431. Could that be why it is whining?

The opto is a PC817.

I can post pics tm

[budm]

Re: UC3842 basics

Did you check the frequency it is running at? May be running at the wrong frequency or it has loose core assembly.

[ben7]

Re: UC3842 basics

Did you check the frequency it is running at? May be running at the wrong frequency or it has loose core assembly.

My fluke 87 says its running at 21khz.

Strange, b/c I built the circuit just like the one Th3_uN1Qu3 posted. (used the same cap values for the UC384x)

[ben7]

Re: UC3842 basics

Update. When I put the probe to measure the voltage on the output (it was 4.8v), the pitch of the whine changed! So, there IS a problem with the feedback being unstable. Nothing is getting hot, the only things that are warm are the load resistor and the snubber resistor.

[ben7]

Re: UC3842 basics

ANOTHER update

I found another circuit which is MUCH nicer.

I modified the psu on my breadboards, and now it is working better. That is, with a light load. A heavy load makes it hiss. So it still has some tweaking to be done. But I tested it with a 1A load, and the supply happily kept up a constant 4.95v output, and 4.99v with only 10mA load (an led).

[budm]

Re: UC3842 basics

When you build this kind of circuits on bread board like that, you will have to really watch out how you route the ground current return path for each section of the the circuits. You may want to search "how to do PCB LAYOUT for switching power supply" to get more understanding. You need to keep high current path as short as possible for the return current to flow back to the source without modulating other sensitive circuits such as feedback, ref voltage source, etc.

[ben7]

Re: UC3842 basics

When you build this kind of circuits on bread board like that, you will have to really watch out how you route the ground current return path for each section of the the circuits. You may want to search "how to do PCB LAYOUT for switching power supply" to get more understanding. You need to keep high current path as short as possible for the return current to flow back to the source without modulating other sensitive circuits such as feedback, ref voltage source, etc.

Ok, I had a feeling that was the problem, will do some research now...

[budm]

Re: UC3842 basics

Please read the good app notes here:
https://cdn.badcaps-static.com/pdfs/...c8af2b1f18.pdf
i.e. page 12. That is how you learn. I and my co-worker go through this exercise every time we lay out new PCB's, it takes more than one time to get them right.

[ben7]

Re: UC3842 basics

Please read the good app notes here:
https://cdn.badcaps-static.com/pdfs/...c8af2b1f18.pdf
i.e. page 12. That is how you learn. I and my co-worker go through this exercise every time we lay out new PCB's, it takes more than one time to get them right.

You design PSU's? Awesome! I wanna do that

Ah, gonna go re-build it now, the ground rail is carrying current from one end to the other end of the breadboard

[ben7]

Re: UC3842 basics

Update.

So, I used wires to connect the mosfet and transformer directly to the bridge rectifier. Now there is no hiss or whine, but the output voltage is low

I used some 20 gauge single conductor copper wire, but it didn't do anything better.

[budm]

Re: UC3842 basics

Check your feedback section. I work with an expert that used to design power supplies for ALCATEL, very lucky to have good him to guide me, still learning every day.

[ben7]

Re: UC3842 basics

Check your feedback section. I work with an expert that used to design power supplies for ALCATEL, very lucky to have good him to guide me, still learning every day.

It turns out that I was missing a 470pF capacitor on the Isense pin, so it was triggering the overcurrent protection, causing the whole circuit to turn on and off, making the whine/squeal sound.

With a 1A load, it is dead silent now!