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Help needed for DCM SMPS using uC3842AN

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xavier60:
Onsemi have a lot for SMPS design information. This one concerns the functioning of the TL431 in the feedback loop. In particular, it points out a not so obvious  feedback path from the secondary rail directly to the opto-coupler. https://www.onsemi.com/pub/Collateral/TND381-D.PDF
There is possible trap to be aware of with the UC3842. If not enough voltage is applied to bring it out of UVLO, the output pin can go Hi-Z allowing the MOSFET's Gate to slowing charge up. As a precaution, put a 10K bleed resistor from Gate to ground.

And this is good way to connect the opto-coupler to the UC3842,
http://uzzors2k.4hv.org/projectfiles/auxsmps/UC3842%20Flyback%20Converter.GIF
The compensation would be better with a series RC.

MagicSmoker:

--- Quote from: WyverntekGameRepairs on December 11, 2019, 01:57:14 am ---I thought so, haha. After reading your explaination though, I think I understand much better, especially where I veered off. Now, I am considering the TVS or Zener diode alternative for my power supply's clamping. The lower part count should be better because that means less points of failure. Also, because it is such a simple solution, it would be pretty difficult to screw up. Would there be anything I should note about this solution?
--- End quote ---

The RCD clamp is actually much more reliable, and it is much more forgiving of careless component value selection. The TVS voltage needs to be higher than the sum of the maximum reflected voltage from the transformer plus the supply voltage by a good margin in order for the flyback to actually work (otherwise energy isn't transferred to the secondary during the switch off time, it gets burned up in the TVS diode instead). Predicting your next question, the reflected secondary voltage is the secondary voltage multiplied by the turns ratio from secondary to primary. For example, if the secondary is 8V and has 4 turns while the primary has 80 turns then the reflected secondary voltage is 160V. If the primary supply is 170V then the minimum clamping voltage needs to be >330V; the higher the better, within reason, to reset the leakage inductance as quickly as possible.

Upon reading this I realize I probably just gave you more questions to ask than answers, but, well, you're young and have lots of learning to do anyway.


--- Quote from: WyverntekGameRepairs on December 11, 2019, 01:57:14 am ---Also, in my design, should I use a cermet pot instead of a trimmer pot? Cermets look like they are more precise and reliable than trimmers.
--- End quote ---

Cermet is a type of material commonly used for small, board-mount potentiometers which are themselves usually called "trimmers" or "trimpots." Cermet is fairly stable but has a very limited rotational life because it is abrasive. Carbon film is another material commonly used and while it has a good rotational life, it has a terrible temperature coefficient, voltage coefficient (that is, resistance changes with applied voltage!) and initial tolerance. If you need the ultimate in rotational life and stability then conductive plastic is another material option, but its usually found on panel pots because trimpots aren't usually meant to be adjusted that often (indeed, usually a once-and-done thing, as is likely the case here).


--- Quote from: WyverntekGameRepairs on December 11, 2019, 01:57:14 am ---Though, I still need to know... Is it analog or digital? I'm assuming it is digital, but I could be dead wrong. The way it is being used, it sounds like it is analog.
--- End quote ---

The opto is definitely being used linearly, though you are correct in assuming that optos are more reliably devices when used digitally (mainly due to aging - the brightness of an LED at a given forward current declines over time, though with relatively low currents this time is measured in decades).


--- Quote from: WyverntekGameRepairs on December 11, 2019, 01:57:14 am ---
--- Quote from: MagicSmoker on December 11, 2019, 12:44:51 am ---The leading edge spike on the current sense signal is almost entirely the result of discharging the energy stored in the MOSFET drain-source capacitance

--- End quote ---
Wait, woah, hold up. Really? I thought that the leading-edge spike was caused by inrush current due to the transformer inductance! I almost entirely forgot about the capacitance of the MOSFET. But this does lead me to the question: Is the damper useful in dampening the inrush current spike from the transformer?
--- End quote ---

Sorry, I screwed this up by giving the wrong capacitance and mechanism to blame. It is charging up the INPUT capacitance of the MOSFET that causes the leading edge spike in the current sense waveform (plus the reverse transfer - aka "Miller" - capacitance). I honestly don't know how I screwed that up, but T3sl4co1l already caught and corrected it so hopefully no harm done.

WyverntekGameRepairs:

--- Quote from: MagicSmoker on December 11, 2019, 01:39:10 pm ---
The RCD clamp is actually much more reliable, and it is much more forgiving of careless component value selection. The TVS voltage needs to be higher than the sum of the maximum reflected voltage from the transformer plus the supply voltage by a good margin in order for the flyback to actually work (otherwise energy isn't transferred to the secondary during the switch off time, it gets burned up in the TVS diode instead). Predicting your next question, the reflected secondary voltage is the secondary voltage multiplied by the turns ratio from secondary to primary. For example, if the secondary is 8V and has 4 turns while the primary has 80 turns then the reflected secondary voltage is 160V. If the primary supply is 170V then the minimum clamping voltage needs to be >330V; the higher the better, within reason, to reset the leakage inductance as quickly as possible.

Upon reading this I realize I probably just gave you more questions to ask than answers, but, well, you're young and have lots of learning to do anyway.

--- End quote ---
Hmm, you do have a point. I think it would be best to take the less risky route then and go with the RCD clamp. While it does have more parts to it, it would indeed be more reliable than a TVS or Zener. And because it is more forgiving. I'd rather not have a tiny miscalculation cause a big *boom* on my power supply just because a diode can't handle it.

And yeah I agree, I still got quite a bit to learn. Though I do want to say that honestly, I don't find this as intimidating as most do. The only real difficulty I have is finding out the proper parts and minisicule features/details implemented to make it as efficient, safe, and cost-effective as possible. "But isn't that the whole function of a SMPS?" Yes, but I mean the really down-to-it details, like coupling capacitors and clamps and buffers. The precise art of these devices are the true difficulty. You get one thing wrong, and the entire thing either blows up or one small part blows up and causes an avalanche effect throughout the rest of the unit.

But hey, if it doesn't kill you, then I guess it is considered a learning experience! If it does kill you... Well, then not so much a learning experience, and more of a being-dead experience.

Also, thanks for defining the reflected voltage. That'll really come in handy later on while designing my power supply.

--- Quote from: MagicSmoker on December 11, 2019, 01:39:10 pm ---Cermet is a type of material commonly used for small, board-mount potentiometers which are themselves usually called "trimmers" or "trimpots." Cermet is fairly stable but has a very limited rotational life because it is abrasive. Carbon film is another material commonly used and while it has a good rotational life, it has a terrible temperature coefficient, voltage coefficient (that is, resistance changes with applied voltage!) and initial tolerance. If you need the ultimate in rotational life and stability then conductive plastic is another material option, but its usually found on panel pots because trimpots aren't usually meant to be adjusted that often (indeed, usually a once-and-done thing, as is likely the case here).

--- End quote ---
The reason I need a stable pot is actually for the prototype. Once I find the right value that I'll need, I will replace it with a regular cermet pot or trimmer pot on the manufactured boards' design. Now that you mention it, I think for the prototype boards I'll use that conductive plastic alternative you mentioned (because I'll probably be making quite a few adjustments). Do you have any suggestions on where I can find such potentiometers?


--- Quote from: MagicSmoker on December 11, 2019, 01:39:10 pm ---The opto is definitely being used linearly, though you are correct in assuming that optos are more reliably devices when used digitally (mainly due to aging - the brightness of an LED at a given forward current declines over time, though with relatively low currents this time is measured in decades).

--- End quote ---
Ah, that does clear it up a bit more. You know, this reminds me, I'm having a bit of interference in my PS1's audio. I think it could be caused by the power supply being noisy due to an aging opto. I've tried adjusting the trimpot, but it doesn't really help too much. Should I replace the opto with a new one? I'm guessing it is the opto because what I think is happening is the feedback from the opto is becoming a little too inaccurate due to the fading from age. The inaccuracy causes errors in the calculations of the PWM controller, and causes noise in the transformer which is transferred to the secondary side, and (despite smoothing capacitors) causes noisy voltage on the output.


--- Quote from: MagicSmoker on December 11, 2019, 01:39:10 pm ---Sorry, I screwed this up by giving the wrong capacitance and mechanism to blame. It is charging up the INPUT capacitance of the MOSFET that causes the leading edge spike in the current sense waveform (plus the reverse transfer - aka "Miller" - capacitance). I honestly don't know how I screwed that up, but T3sl4co1l already caught and corrected it so hopefully no harm done.

--- End quote ---

Ohhhhhhhh. Okay. Yeah, that does make much more sense. Though, I would think that capacitances between any given pins on the MOSFET could be causing something like this... But looking at Tim's diagram, I'm seeing both of yours' points now. The visual representation really helps, thanks Tim :D
And yeah, no harm done :)

station240:
To me it looks like R120 6.8M ohm between neutral and GND is to make up for the lack of earth on the mains side, and to allow a smaller value for the class Y cap.
Given it takes 20mA to get a dangerous shock, you'd need 136,000V across that 6.8M ohm.

Usually the class Y capacitor is 2.2nF, instead of 1500pF (for C103), for this style of PSU. So that 6.8M is used to reduce that.

Also need to specify safety rated parts for:
C103 - class Y 2kv or 3kv cap
C101 - class X2
R120 - 1KV or higher rated.

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