Opto isolated Flyback with opto in Common Collector, or Common emitter?

[treez]

Hello,
The attached show 5W, Opto-isolated Offline Flyback SMPS’s.
One has the feedback opto connected in common collector, one in common emitter. Which do you believe is best?

For the controller in each case, please assume it is the UCC28C43…(not the LT1243)
UCC28C43 datasheet.
https://www.ti.com/lit/ds/symlink/ucc28c43.pdf

Attached is LTspice sim showing comparison, and schem.

 

[Easy peasy]

If you have an EMC cap from pri gnd to sec gnd, then common emitter opto will work best.

It is the amount of CM noise generated by (or across) the Tx that determines how opto's should be placed for best performance.

You often see the EMC cap placed close to the opto from gnd to gnd where the opto kathode is at gnd (sec) and the emitter is at gnd (pri) this give the min CM transference to the pri side control and allows the cheapest opto...

 

[dick_freebird]

Common collector will not ever saturate the photo-BJT.

Common emitter gives you gain which you may or may
not want (highly variable and prone to aging as well).

 

[Easy peasy]

I've never heard of a 4 pin opto-coupler output xtor being saturated - how many amps to the LED for this? how does common collector prevent this? - for a 4 pin opto coupler?

how does the gain vary for C-Em over C-Col for a 4 pin opto? I look forward to the answers ...

 

[mtwieg]

I'm also a bit confused by the question... if the "base" drive is an isolated circuit, then it effectively has no bias voltage, so the bias voltage of the C/E doesn't really have any meaning with regards to its amall signal voltage/current gain. In those two circuit examples, the loop gain will definitely be different, but this is entirely because of how you're using the error amplifier in the controller, not the phototransistor itself.

 

[treez]

In those two circuit examples, the loop gain will definitely be different, but this is entirely because of how you're using the error amplifier in the controller, not the phototransistor itself.

Thanks, yes indeed, the loop gain will be different in the examples shown as they are, however, as you know, with suitable component value adjustment, both of the demonstrated examples can be made to have the same loop gain.
The point is, that the “common emitter” connection of the opto, has less components, so why would that configuration not always be used?
In the UCC28C43 datasheet, they show both of the above opto connection configurations, but make no mention as to why each one was chosen. (‘CE’ is shown in Fig 29, pg22 and ‘CC’ is shown in Fig 31, pg 24).

As Easy Peasy says,the CE connection method has the opportunity for less CM noise (though with the stated stipulation of emitter and cathode of opto being connected to their relevant ground).

Also, the ‘CE’ connection of pg 22, has an external resistor pullup to VREF… this is needless….the internal error amplifier inside the UCC28C43 will provide this pullup when the FB pin is grounded, so why have they used an external resistor when none is required?

The ‘CE’ connection of the the opto clearly requires less components, so why not just always use that one?

UCC28C43 datasheet
https://www.ti.com/lit/ds/symlink/ucc28c43.pdf