Author Topic: Fly back SMPS, please evaluate.  (Read 977 times)

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Offline FriedMule

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Fly back SMPS, please evaluate.
« on: December 01, 2021, 07:21:25 am »
I have tried to make an fly back supply with input protection, EMI reduction for max 3.3V 1.5A
Every resistor is 1W, only missing parts is the resource electrolytes and maybe a inductor.

Is the design okay for a low loss, stable PSU?

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Offline kaindub

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Re: Fly back SMPS, please evaluate.
« Reply #1 on: December 01, 2021, 08:26:06 am »
A cursory look says the tl431 will be unstable as it lacks stabilising capacitor.
It’s a fairly basic design. I would use one of the dedicated smps chips. Why use a transistor. Mosfets are a better choice.
In the end you need to mock-up this circuit and see how it works. Perhaps run it through Spice as a preliminary check.
 

Offline Benta

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Re: Fly back SMPS, please evaluate.
« Reply #2 on: December 01, 2021, 08:32:47 pm »
I fail to see the purpose of connecting the output capacitor to the TL431 and R9.
But that's only one of the dozens of problems I see.
Use a dedicated chip, this is really 1970s design.

 

Offline FriedMule

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Re: Fly back SMPS, please evaluate.
« Reply #3 on: December 02, 2021, 07:47:12 am »
Thanks for your help, I'll find a chip instead:-)
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Offline magic

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Re: Fly back SMPS, please evaluate.
« Reply #4 on: December 02, 2021, 08:43:46 am »
Are you building this for fun or for actual use?
If the latter, what's the reason for not using an off the shelf PSU brick?
 

Offline FriedMule

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Re: Fly back SMPS, please evaluate.
« Reply #5 on: December 02, 2021, 10:46:02 am »
I do in fact plan on using it, my reason for building one is that I want to learn about electronic:-)
Can anyone maybe suggest some chips?
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Offline PKTKS

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Re: Fly back SMPS, please evaluate.
« Reply #6 on: December 02, 2021, 11:08:58 am »
I do in fact plan on using it, my reason for building one is that I want to learn about electronic:-)
Can anyone maybe suggest some chips?

Rather typical simple and fairly seen on "flyback" topologies..
( aka  offline converters )

OnSemi MC44603

Paul
 

Offline magic

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Re: Fly back SMPS, please evaluate.
« Reply #7 on: December 02, 2021, 11:30:35 am »
MC44603 is crap. Its built-in transistor can't switch mains voltage and it lacks several features to conveniently and safely drive external MOSFETs: UVLO, push-pull output, primary peak current limiting.

Popular oldschool options are:
Viper series from ST: built-in MOSFET, few external components, good enough for a few watts
UC3842~3845 series: very common, needs external FET

NCP1207, NCP1377 are another controllers I recently found in off-the-shelf PSUs which appear to be fairly available and have a nice bonus: secondary overvoltage detection and shutdown.

There is a lot of newer chips too, mostly from China, but they are more obscure, sometimes without good datasheets, often only in SOT23-6 which you may or may not like.


Whatever you do, beware of capacitors charged to 330V and remember that it's difficult/expensive to safely probe primaries of mains-powered PSUs. Such SMPS is not an ideal beginner project. Honestly, I probably wouldn't know how to design one properly, I have only ever repaired them.
« Last Edit: December 02, 2021, 11:36:19 am by magic »
 

Offline Benta

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Re: Fly back SMPS, please evaluate.
« Reply #8 on: December 02, 2021, 11:48:54 am »
The MC44603 is obsolete. Apart from that, it's optimised for TVs, not really a general purpose part.
 

Online Circlotron

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Re: Fly back SMPS, please evaluate.
« Reply #9 on: December 02, 2021, 12:02:51 pm »
LED D2 will go into orbit in microseconds. Also, the entire circuit is being fed from the voltage drop across that LED.
 

Offline PKTKS

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Re: Fly back SMPS, please evaluate.
« Reply #10 on: December 02, 2021, 12:26:16 pm »
The MC44603 is obsolete. Apart from that, it's optimised for TVs, not really a general purpose part.

May be.. I did repair man stuff for decades up to 90s..
no surprises my choices and generic stuff is biased..

Nevertheless.. it is always handy having classics which can be found today..
The rated power and specs of this IC are generic enough for that..

But sure we have newer stuff..

Paul
 

Offline PKTKS

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Re: Fly back SMPS, please evaluate.
« Reply #11 on: December 02, 2021, 12:41:17 pm »
MC44603 is crap. Its built-in transistor can't switch mains voltage and it lacks several features to conveniently and safely drive external MOSFETs: UVLO, push-pull output, primary peak current limiting.
(..)

UC38xx is more used in generic SMPS PSU units indeed.
It can still be seen inside a lot of standard SMPS.

Nevertheless even the APPNOTE schema does not use the IC driver
directly.. it is always a multi stage driver and an external driver
can handle whatever mains needed..

It should be a simple choice..
several base examples available

Paul
 


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