UCC38C52: How optocoupler is selected

Monica bagade

Part Number: UCC38C52
Other Parts Discussed in Thread: TL431

I am not able to understand design criteria for opto coupler selection.

1. How resistor connected to emiiter is calculated.

2. What is the selection criteria for selection of CTR of opto coupler?

3. How voltage at FB pin varies with optocoupler connection?

Is there any detailed calculation?

Thanks and regards,

Monica

0 Steven Mappus1 over 1 year ago

Monica,

Here are some recommended resources to provide a better understanding of optocoupler feedback

Regards,

Steve

0 Monica bagade over 1 year ago in reply to Steven Mappus1

Dear Steve,

Thank you for sharing. I have read that but I am still confused with selection of Ropto=1k ohm?

How is that value calculated? How to select CTR of opto coupler?.

As per this calculation, Iled~ 1mA. Based on this, How collector current is varying?

0 Monica bagade over 1 year ago in reply to Monica bagade

I have taken this as reference for Ropto

0 Steven Mappus1 over 1 year ago in reply to Monica bagade

Monica,

ROPTO is arbitrarily selected as 1kΩ
CTR is selected based on worst case rated over temperature and frequency range of interest. Assume CTR is 100% but this can vary in practice.
You mention ILED~1mA. 1mA is the minimum current needed to allow the TL431 VREF to remain at 2.5V. RTLbias=1.3kΩ is used to assure the TL431 remains biased even if the opto diode is OFF. ILED=(VOUT-VLED-VKA)/RLED, where Vout=12V, VLED (opto diode forward voltage drop)~1V and VKA (min cathode to anode TL431 voltage)=2.5V. ILED=(12V-1V-2.5V)/1.3kΩ=6.5mA. The TL431 cathode voltage is varying according to the TL431 gain multiplied by the error voltage. As the cathode voltage varies, the ILED current varies causing a variation in IOPTO, causing a variation in IOPTOxROPTO, which ultimately varies FB.
The opto Gain is given by Gopto=(CTRxROPTO)/RLED. Gopto=(100%x1kΩ)/1.3kΩ=77. Now you see that you can adjust the opto gain by varying the ratio of ROPTO/RLED and you can make this adjustment to compensate for the CTR not being exactly what you might expect. Once the circuit is build, you can use trim pots to adjust ROPTO and RLED to see exactly what gain is required to maintain output voltage regulation over all line/load conditions. In this way, you can verify your calculations and make any necessary adjustments.

There are many good resources/app notes and YouTube videos available for biasing the TL431 opto and compensating the control loops.

Regards,

Steve

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