LED driver with or without cascoded optocoupler feedback?

[treez]

Hello,

The follwing two PDF Schematics (also attached as .txt LTspice simulations) are both LED drivers, with this spec…

Spec:
Buckboost
Vin = 12-100V
Vout = 40 to 64V
Switching frequency = 200KHz
LED current = 100mA
Discontinuous Mode.

One of the examples has cascaded optocoupler feedback, and the other has plain optocoupler feedback.

When trying to stabilise them, it is far far far easier to stabilise the one with cascaded optocoupler feedback…
Stabilising the one without the cascode is like trying to balance on the point of a pin.
Stabilising the one with the cascode is so simple its like falling out of bed.

Given the ease of stabilising the one with cascaded optocoupler feedback, why would anyone not use the cascaded optocoupler feedback? The extra components are very cheap, and give extra robustness in terms of improved stability.

Also, the one without the cascode has a longer start up time, and overshoots considerably more.

Whats the disadvantge of the cascoded opto feedback?

 

[treez]

I see the above drew no reply.
But the massive advantage of cascoded optocoupler feedback goes on.
In offline flybacks, with optocoupler feedback, why doesn't everyone cascode the opto coupler?
-It makes the feedback loop so much easier to stabilise, and more easily provides excellent transient response, as well as better gain and phase margins being easier to get with the cascode.

Note these two equivalent offline flyback simulations.

one has cascoded opto feedback , and the other does not.

Note how the non-cascode version has 2V of overshoot on vout at start-up. Due to the lack of a cascode, this overshoot is virtually impossible to get rid of whilst keeping sufficient gain and phase margin.

Cascodeing is extremely cheap to do...why does everyone not do it?

Schematics and LTspice simulations of offline flybacks given. (one cascode and one no-cascode, but they are the same power level, vin, vout, np/ns, Fsw.

 

[schmitt trigger]

I have not used myself the cascode in a power supply feedback configuration, but have used it to transmit pulses thru an optocoupler faster, much faster.

Therefore, I believe you that without the optocoupler's roll off, the loop would be simpler to stabilize.

 

[treez]

There must be some disadvantage, as few people do it, but I can't think what, the cascode BJT stage costs a cent or two so cost isn't the reason.
I wonder why its so seldom used?

The problem with opto's isn't just the low pole frequency, its the terrible poor tolerance on CTR. Due to the poor tolerance on CTR, the pull up resistors that get used tend to be of too low a value....and this must be so because if a low CTR optocoupler is in situ, then the current through the pull up resistor will be more, and if it was not a low value resistor then it may drop too much voltage.
The problem with using too_low_value pull ups is that it worsens gain and phase margin.
-but there's nothing that can be done about this -unless of course you use a cascode stage and push the pole frequency well up, and then also the ctr variance effect is less damaging.

Please see the following comparison simulation between a cascaded opto feedback smps and a non cascaded feedback smps.
The difference is dramatic...the cascaded one is stable, the non cascaded one goes wildly unstable.

With such dramatic improvement of feedback loop robustness, why do more people not use the casc0ded connection for the opto? (lt spice sim amd schem attached)