Do we really need a multi function SMPS control chip when we have a micro too?

[Flyback]

We are doing a 60W offline flyback.
We realise that all offline SMPS’s need to have overload protection and short circuit protection.
The UCC28C45 PWM controller IC does not have these functions built in, but the L6566B PWM controller does have these functions built in. However, the L6566B is a far more complex chip to get through prototype test..
However, we have a microcontroller on the board, which is supplied by a separate 5V output “housekeeping” flyback, and surely that microcontroller can do all the short circuit and overload protection?, meaning we don’t need a pwm controller IC with in-built overload and short circuit protection? -Because this microcontroller can simply sense the output voltage of the flyback and shut the flyback down if there is a drop in output voltage indicating output short circuit or overload.
The UCC28C45 is a cheap IC which, having only 8 pins, is far easier to get through prototype test than some of the “all singing all dancing” pwm controller IC’s.
So, since we have a microcontroller which can switch off the pwm controller ic on the board, do we need to use a pwm controller that has in-built short circuit and overload protection.?…it seems like overkill?
L6566B datasheet:
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf
UCC28C45 datasheet:
https://www.ti.com/lit/ds/symlink/ucc28c45.pdf

 

[crutschow]

It's possible to use a micro for those functions if it can respond fast enough to protect the power transistor from a short circuit or overload.
How will an overload cause a drop in the SMPS output voltage?
You may have to add a small shunt resistor and amplifier to detect such a current overload.

 

[tvtech]

Aaghh

Flyback you need to get involved with people and/or Companies currently practicing this kind of stuff to learn.

I had more to say but deleted it.

Regards,
tv

 

[Flyback]

if a flyback is short circuited, its output voltage simply drops, a flyback, being transformer isolated, and delivering power on the off-stroke of the fet, does not deliver enormous current into a short......the output voltage just drops, and the micro could detect this and simply turn off the pwm controller.
So the micro would be able to perform the shutdown no problem, but its just a case of whether or not the safety and regulatory agencies would agree to having a micro do this function?

I am afraid that in UK, the SMPS industry is pretty much all gone, -gone to China, so there isn't too much opportunity to go to places where smps's are being made, as this doent happen very much on the green pastures of the UK.

 

[ronsimpson]

The UCC28C45 is current mode and should be good under a short.
You can measure the voltage on COMP or the average voltage on OUT to get a idea of duty cycle. average=RC filter
Pull down on COMP or up on CS will stop the part. You can also stop the OSC.

 

[Flyback]

I must admit its amazing that ucc28c45 still sells when it has no inbuilt protection functions...it just shows that circuits like you suggest in combo with the ucc28c45 are often better than these "all singing and all dancing" ic's, which have every protection function under the sun integrated into them.

 

[ronsimpson]

What protection are you after?

 

[Flyback]

We basically don't want the smps, if it gets a short on its output, to keep dumping power into the short, and if it gets an overload on its output, then
we don't want the smps to keep dumping power into the overload. We want the smps to be shut down in these cases.

 

[ronsimpson]

If this is a off the line switcher the IC will not be able to support its self under a short. So it will reset and try again. Depending on the RC that powers up the IC it will retry maybe two times/second. The duty cycle will be about 2% of the time.

Your micro can watch for that and just hang up the supply until there is a hard reset.
I do think it is a good idea to watch the heat and shutdown if the temp gets too high.

 

[Flyback]

I know what you mean, but sometimes the bias winding doesn't collapse under short output due to leakage ring I n the auxiliary coil.
Also, we actually have a separate low power, housekeeping flyback to supply the controller, so the "bias winding collapse" method cant be done, if that's what you meant.
We cannot supply the chip with the bias winding as the output is variable from 5-24v and when its 5v the bias winding would effectively drop out.

 

[Flyback]

This is a new thread, but is relevant to this controller, so I post it here...

*****************************************************************************************
We are designing a 60W offline flyback.
We notice that the L6566B pwm controller, in fixed frequency mode, can vary the switching frequency from 82KHz to 88Khz, if we wish it to. (ie the switching frequency is constantly going up and down between these two frequencies, at about 1Khz)
It is said that this offers ease in passing EMC.
But how can this be so?
The main problem for EMC in SMPS is the harmonics of the switching frequency, not the switching frequency itself. It is the high di/dt and high dv/dt currents and voltages that create the problem, and these are not eased by varying the switching frequency as described. So what is the point of varying the switching frequency as described?
L6566B datasheet:
https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00167474.pdf

 

[ronsimpson]

The main problem for EMC in SMPS is the harmonics of the switching frequency, not the switching frequency itself

And where do the harmonics come from?
.....Grandad said; "All my grand children have big ears. Is that because I have big ears?"

 

[JimB]

It is said that this offers ease in passing EMC.

It is due to the response* of the detector used in the spectrum analyser used in EMC evaluation and certification.
This frequency shifting trick is just a frig to get away with cheap inadequate screening and filtering.

* A Quasi-Peak detector if I remember correctly.

JimB

 

[Flyback]

the harmonics come from the switching edges, and these are the same and are not changed by slightly varying the switching frequency.

 

[ronsimpson]

Harmonics come from more than one source.
A square wave has harmonics. These harmonics are integer multiple of the fundamental.
There are also high frequency noise from other sources.

 

[Flyback]

I agree, and I am sure you would agree that varying the switching frequency slightly, is pointless, because it does nothing about the harmonics....ie , it does nothing about the di/dt of the switching edges., and nothing about the dv/dt of eg the drain node.
"Dithering" the switching frequency, is , I am sure you will agree, just a hoax to flog more pwm controllers?

 

[Mosaic]

Flyback I use the TL494 or equivalents for SMPS with protections. The TL494 is cheap and well documented, there is an app note for it.
https://www.ti.com/lit/an/slva001e/slva001e.pdf
It has built in err amps for current and voltage regulation with a separate DTC for use with short circuit protections.
Found in most PC PSU's.

 

[ronsimpson]

I have used millions of TL494s. Literally.
The error amp can not have a gain below one. Can be a problem.
The goofy thing is backwards compared to most PWMs.
The current limit is slow. (Not cycle by cycle)

 

[Mosaic]

Nevertheless you use them, is the price the reason?

 

[ronsimpson]

1)When I started with the TL494 the part was new.
2)No one did cycle by cycle current limit. I did not need it.
3)I needed a part that could run at the same frequency as another PWM. Most importantly run in phase.
Most PWM turn on current with the clock and turn off current (x) amount of time later.
The 494 turns off current with a clock and turns on current some amount before that. (back wards)
I needed the PWM to turn off with a master clock.
4)Before the 494 I had a desecrate PWM made with op-amp, comp., osc, ect.