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uC3842AN SMPS idea for feedback - What do you think?
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WyverntekGameRepairs:
I was doing some thinking while reading over my thread I posted concerning my uC3842AN SMPS power supply questions regarding design choices of the PS1 SMPS, and I had a very interesting idea:
Because (in the way the PS1 was using it) the optocoupler is transmitting analog data, it can become dangerously unreliable as it ages and fades. It also could reduce the lifespan of the LED inside the opto due to it being in an ON state for so many hours. A great way to increase the lifespan of the LED is to allow it to turn off for a short period before turning back on again, instead of having it ON all the time. Since we are using analog in this circuit, it would make sense to use a A-D converter on the LED side and a D-A converter on the phototransistor side. Because it would become a pulse-width modulated signal at either 2.5V (or 3.5V depending on LED needs) or 0V, the brightness of the LED would not affect the output since the ON time of the LED would be measured instead of the brightness of the LED. In case you haven't caught on yet, this would be at a couple hundred Hz to even a couple KHz. To understand what I'm going on about, let's take a voltage of, say, 5V - and let's monitor it using an ADC. Let's assume this ADC can take a signal input ranging from -10V to +10V (meaning it can take AC as well as DC).Let's also assume that the ADC's median output (the output when 0V is being measured at the input) has a duty cycle of 50%. When the voltage is a stable 5V, the ADC outputs a square wave with a duty cycle of about 65%. However, as the voltage rises, the ADC measures that and increases the duty cycle depending on the voltage. But if the voltage begins to drop, the ADC measures that and decreases the duty cycle depending on the voltage. Basically, the ADC is measuring the voltage, and setting the duty cycle that coincides to the voltage measured on the input signal.
Going back to the optocoupler, if we connect the ADC output to the optocoupler, the LED will be turning on and off at a very fast rate. But as the duty cycle increases, the LED is on for a longer time, and vice versa. The LED is never purely on unless (and this is a worst-case scenario) the ADC receives a 10V input and thus outputs a square wave with a 100% duty cycle.
But what about the phototransistor side? Simple: The digital signal received would be converted to analog using a DAC that operates on the near same principle as the ADC, but in reverse. It measures the duty cycle of the input signal, and outputs an analog signal from -10V to +10V, depending on the duty cycle of the input.

This solution would greatly increase the lifespan of the LED, and would make it far more reliable in use, because instead of relying on the brightness of the LED (which changes over time as it ages and begins to fade), we are relying on the duty cycle of the high-frequency digital pulses transmitted by the LED.

I think that my own power supply will have a feature like this, because it is far more reliable and age friendly. As it grows old and begins to dim, the efficacy of the signal is not changed. The signal would not be damaged until the LED grows so dim that no signal is transmitted at all, thus causing the SMPS to stop working completely. I believe that this is also far safer - whereas in analog, the damaged analog signal due to the opto's aging and dimming LED would throw the SMPS controller IC's feedback measurements and calculations so out of whack that the SMPS would most likely suffer some form of damage or cause damage to the device it is powering.

What do you think to my solution? (I'm pretty sure a lot of people have used this solution in their projects and power supplies, but I thought of this just now and I want your opinions and feedback on it). Don't be afraid to correct me if I'm wrong about something, as I am very willing to learn something new. :D

Edit by gnif: Removed marquee, the information below isn't advertising any optos but rather providing technical data sheets on the science of optoisolators and best usage.
SuzyC:
For technical questions, contact: optocoupler.answers@vishay.com ... In general, an optocoupler's life time is a period of 100000 hours.

Vishay says, "No Worries!"

Optocouplers are in use in every powered electrical instrument you can think of , from your smartphone charger to your TV to your PC and in every case, everything else in the P/S  is likely to fail first.

The nature of the feedback using an optocoupler in a SMPS is such that the brightness of the LED in the optocoupler is held constant over years by the LM431 on the cold side of the  P/S, so aging of the LED is compensated for.

What causes them to deteriorate is high temperatures, so keep things cool, man.
SuzyC:
Lucky for you Opto's today cost only a few cents in large quantities, but you can get just one for $0.35 plus shipping from Vishay, but other cheaper Mfg's are probably just as reliable. Do a little research.

Toshiba says: https://toshiba.semicon-storage.com/info/docget.jsp?did=13438

The projected field lifetime for the LED = AF × stress hours = 184.7 × 1,000 = 184,767 hours (or 21 years). With the AF value
calculated, all data points of stress hours map to the expected field lifetime time.

Edit by gnif: de-googled the link
gnif:
@WyverntekGameRepairs SuzyC is not advertising but referring you to some real information.
gnif:

--- Quote from: WyverntekGameRepairs on December 13, 2019, 04:42:14 am ---Anyway, let's get this thread back on track

--- End quote ---

This thread was always on track, the information @SuzyC provided did not advertise any brand make or model of opto, rather the science behind them and how they operate, how best to use them in a SMPS where the LED may fade (feedback is king) and information from a very reputable (industry leading) manufacturer and researcher of high quality optos.

For the record, the link provided is an application note titled "Basic Characteristics and Application Circuit Design of Transistor Couplers", with the outline "This document outlines the basic characteristics and application design of general-purpose transistor output photocouplers". It's an excellent read on the characteristics and usage of optoisolators.
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