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SMPS optocoupler feedback with massive improvement...?

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Fig 13, page 9 of this app note shows an improvement on the "normal" optocoupler feedback connection for transformer isolated SMPS's.....


....On page 9 it says that the opto diode is driven by a current source.
Does this configuration, with the extra BJT, Q1, absoluteley need have to have a current source to drive the opto diode?

What would happen if i drove the opto diode with a typical TL431 circuit as in the following......

(please see schematic on bottom right of page 5 of the following)

Also, since using the BJT, Q1, in fig 13 of the first app note gets rid of the opto-diode pole problem, why is this method not more frequently used?


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The app. note shows a way of reducing the response time of the opto isolator, by making sure that the phototransistor is not saturated. The cascode arrangement with the BJT does that. All transistors take time to come out of saturation.

The idea of the current source is that the error amplifier stays in its linear region, even if there is a large error. That will also speed up the response when the error comes back towards zero. The data sheet points out the improvement during start-up when the error is huge and I guess that overshoot is reduced.

TL431 circuits are by far the most common for SMPS. They work fine for most applications. However, if there is a large error, the op-amp within the TL431 will be in a non-linear part of its operating range, and the output transistor within the TL431 will be saturated. Either or both of these could introduce a significant delay and result in poor transient response, because the LED will take time to turn off after the error reduces to zero. All sorts of factors will affect how bad the problem is and how much it affects the load.


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making sure that the phototransistor is not saturated
though surely its the NPN BJT that is kept out of saturation?
The optocoupelr is able to saturate?

Here is the schematic:

From what you are saying, i suspect that the Opamp U4, in the schematic plays a bigger part in increasing transient response than the cascode stage?

Also, optocouplers, with less than 5mA of current in them , have horrendously wide ranging CTR values.
I dont see how an smps that uses an opto without a cascode stage can be workable at anything above 200Hz loop bandwidths?

Also, is the following PNP cascode going to have the same effect as the NPN one?...ie it pushes the optocoupler pole well up higher in frequency?


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