How does OptoIsolator Compensate circuit Work?

[Shahriar]

Hi

If you take a look at TV or Monitor power supplies, you see a circuit Like the one I have marked red in the picture. In the SMPS guide which I downloaded from SMPS Disscussion of this forum, there was the explanation I have write at the bottom of the picture. Can you explain how that circuit compensate the Tempreture Drift. Usually there is a transistor somewhere there that the MCU sends the PS to standby Mode through it in TVs.

ThanX in advance
Shahriar

View attachment 37748

 

[Alun]

The infra-red emmiter in the opto isolator only turns on when the voltage is above a level that's set by the programmable zener, when this happens the opto transistor on the other side is turned on, this tells the switcher to lower it's output to cause the LED to switch of, the cycle repeats over and over.

 

[audioguru2]

Hi Guys,
Optoisolators change the amount of signal passed with temperature change and the LED gets a little dimmer with age.
A 2nd optoisolator can be used as a reference for the amount of signal passed and used inside another error amp. ;D

 

[Shahriar]

ThanX all for your replies

dear Alun
I don't think that the opto Isolator turns completely on or turns Completely off, but the emission of the Transmitter veries, then the Collctor-Emitter Resistance veries so the voltage at Pin(-) Of OpAmp Veries and the diffrential voltage will be amplified with a Limited Gain.
the Amplified voltage will cange the duty Cycle.
(*) If the Emitter was to switch On and Off, then there was no need of a Feed back for the OpAmp.


But two questions :
1- what is the role of the 10K Resistor and .01uF Cap.
2- When the emitter gets dim, how does the circuit Forces it to bright more?

ThanX in advance
Shahriar

 

[audioguru2]

1- what is the role of the 10K Resistor and .01uF Cap?

They provide frequency compensation for the TL431 so it doesn't oscillate at a high frequency.

2- When the emitter gets dim, how does the circuit Force it to be brighter?

A 2nd emitter also gets dim and with its detector in the negative feedback loop of the emitter or detector of the 1st emitter/detector then it causes automatic adjustment.

 

[harrychoo]

Is the feedback from the optoisolator is to let the PWM controller to properly adjust its duty cycle so it can adjust to the desired output voltage?

If that so, how about current? How we gonna know or control how much current will reach the output?

I know this related to the another thread that i post  ;D

cheers

 

[audioguru2]

I haven't seen your other post yet.
If the circuit uses an opto-isolator to sense and feedback the output current, then the circuit will also need a 2nd opto to automatically adjust it when the emitter gets dim with age or temperature.

 

[Alun]

I don't think that the opto Isolator turns completely on or turns Completely off, but the emission of the Transmitter veries, then the Collctor-Emitter Resistance veries so the voltage at Pin(-) Of OpAmp Veries and the diffrential voltage will be amplified with a Limited Gain.

This is true.

Also running the opto isolator in it's linear region will reduce the amount of ripple on the output but as audioguru has correctly stated it won't be as acurate, you could make it more procise by switching the opto isolator either on or off but this would increase the ripple.

 

[Shahriar]

ThanX all for your repeplies.

Today I saw a SMPS circuit that the OptoIsolator Works some how differently and work in ON/OFF Mode as Alun said but the Controller IC also is Different. When I get sure, I will Send the PDF to SMPS section.

Dear harrychoo
As I have seen in most circuits, Specialy TV Power section, for Over current protection (OVP), they use a a small resistor between MosFET Source Pin and Ground, then check the voltage across it. I have not seen a SMPS to control the over current by OptoIsolator.

Dear Audioguru
Can you please explain some more about Frequency Compensation. Where did the designer bring these Value? (10K Resistor and .01uF Cap)


ThanX
Shahriar

 

[audioguru2]

A similar circuit with frequency compensation is shown in TI's datasheet.