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SMPS's with cascoded opto feedback

(*steve*)

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ok thanks, ill read these but we are pitting linear.com of my #7 post against your Vishay and Renesas....we'll see which of these has it wrong soon

Make sure you compare apples with apples.

Your two circuits use totally different ways of controlling the regulator.

I have no problem with one being better than the other, but without a base connection, it being common emitter or common collector isn't going to be one of these reasons.
 

Arouse1973

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"VREF" and "VREF1" are the supply rail provided by the LT1243.... this is what they are commoned with.
Its the opto transistor, inside the CNY17 optocoupler that is connected either CC or CE.

Pages 16, 17 18 of the following state that common collector is better...

..it states how "The common-collector configuration eliminates the miller
effect of the output transistor’s collector-to-base capacitance"
and generally increases achievable loop bandwidth.
http://cds.linear.com/docs/en/datasheet/4430fc.pdf

Hey eem2am

I have done a test using a 3 terminal NPN photo transistor BFX43. Connected up as what we will call CE with 100K as load, no base resistor the rise time was 12.8ms. Connected up as what we will call CC the fall time was 8.4ms. So if we forget about what we call the configuration for a minute then it shows as I first thought before people tried to confuse me is that if the output is taken from the emitter of the transistor there will be a reduction in the timing in this configuration.

So if we forget configurations and component values for a minute you are correct in your initial findings.

Picking out bits of this and that from the internet is fine but we have to take a back seat sometimes and go naahh that's not right.

Take the guy from HP that said right angled tracks are no good for high frequencies, this changed industry for the next 20 years. But actually he was wrong and he said so 20 years later, people have done tests over many Ghz and the difference is extremely small.

Thanks
Adam
 

Arouse1973

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Yes

Phase shift can be due to a time shift, but 180 degrees of phase shift is a simple inversion and applies to this signal.

I'm not sure what you are suggesting here. Are you saying that in a phototransistor the emitter current is different from the collector current?

edit: Let's be quite pedantic. Without a base connection a phototransistor is essentially operating as a field effect device. The photons are causing a charge to accumulate and recombination is discharging it.



Ah, so you *do* think that a 2 terminal device can have a different current in each of its 2 leads.

I will now sit back and watch you explain how the voltage drop across a phototransistor is different depending on whether a resistor is connected in series with the collector or the emitter.

Or even better how, with the base open, that the collector current can differ from the emitter current.

Perhaps you could also venture to tell us whether we should place resistors in series with the anode or the cathode of LEDs and which provides the better performance.

I agree. And I suspect that when you've shown me how it works, I'll be more educated on the subject.

The photo-transistor is a three junction device BCE an LED is a two junction device so they are different. If you were talking about a photodiode then I would agree with almost everything you have said It does not use a field effect of anything it uses light which is an EM wave not an E wave. A small base current is generated in the base collector junction of the transistor which adds to the collector current, but this is generally swamped by the collector current.
Thanks
Adam
 

Arouse1973

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How can a transistor whose base is not connected to anything be said to be operated in common emitter or emitter follower configuration?

Hi Kris

So what are you saying, physically connected to the base. What about the input of an audio amplifier which has a coupling cap between the input and the base. This then has no direct connection to the base.

No signal current will flow through the capacitor, only displacement currents because of the insulator. So what do we call an amplifier with a input capacitor connected between the base and the input?

Thanks
Adam
 

KrisBlueNZ

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1. A transistor audio amplifier with a capacitor connected to the base does have something connected to the base. You said it has no DIRECT connection to the base. If you had said that it has no DC connection to the base, I would agree, but it's irrelevant because I didn't say "how can a transistor whose base is not DIRECTLY connected to anything ..."
2. A transistor audio amplifier with only a capacitor connected to the base will not work.
3. LOL!
 

Arouse1973

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Ok this will probably be my last post on this subject, thank Christ I here you cry, as I would like to answer some of the others in more detail. The original post was to ask the question of what we thought was the better switching arrangement for an opto coupler used as a feedback element in a SMPS.
The OP rightly or wrongly made the statement common collector or common emitter. But that was not the original question and we all went of the rails a bit but it was interesting to get every bodies understanding on this matter.
This is my basic take on this and my outcome from actual tests that I have conducted. I would love to have the time to supply all the formula to back this up. But all this is in one of my books, I can’t give you them all of the top of my head. And I am not saying I would understand all of them fully anyway lol.

CE, CB, CC.
Anyway it is my understanding that a transistor switch can only have 3 main connection configurations, common base, common emitter and common collector. The terminology for each states that the input and output nodes have a common point.
Well if one terminal is used as the input and one used for the output then the other one left bust be the common terminal otherwise the input and output would be connected together. But this is not the case. Using this method you don’t even have to think of the direction of current or the polarity of the voltage.
Rise time and fall time CE,CC
It has been debated that there is no difference in which configuration CC or CE is used. Well there is and I have done some tests to prove it. In the tests I carried out I took a BFX 43 NPN THREEterminal photo transistor with base lead and a 100K load. In both configurations the transistor was in saturation. The rise time for the CE was 12.8ms and the fall time for the CC was 8.4ms, make your own mind up about this.
Current and voltage differences
The photo transistor is a THREE junction device it has a base, collector and emitter. Some people struggle with this because in most cases the base lead is missing. But it still has a base junction. When light (Photons) falls onto the active base junction this produces electron hole pairs. The movement of these electrons in the base junction is a real current a real base current. So just as in normal BJT the input caused a base current to occur.
It has been mentioned that the Ice will be the same, unlike conventional BJTs. This is incorrect. I have done another test to prove this. An OP293 driven with 50mA in a tube pointing toward the transistor with the same 100K load. Collector current was 88.864uA but the emitter current was 99.463uA. Indicating a base current.

Voltages
And yes you do get the same voltage as you do in a BJT, funny that considering it’s a transistor.
Vbe 0.557V measured from base to emitter. And with a 100K load in CC mode I get 9 Volts on the emitter with respect to 0V. Guess what voltage I get on the base then, yep 9.55 Volts. Am I surprised? No not really because it’s a transistor and guess what this indicates yep it has a voltage gain of less than 1.
Regards
Adam
 

KrisBlueNZ

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The photo transistor is a THREE junction device it has a base, collector and emitter. Some people struggle with this because in most cases the base lead is missing.
In my experience, the optocouplers that I've used have all had the base connection brought out. Do you mean to imply that Steve or I "struggle with this"?
When light (Photons) falls onto the active base junction this produces electron hole pairs. The movement of these electrons in the base junction is a real current a real base current. So just as in normal BJT the input caused a base current to occur.
That's right, and that base current comes from the collector.

From http://www.johnloomis.org/ece445/topics/egginc/pt_char.html ("Characteristics of Phototransistors" from EG&G Reticon):

"The collector-base junction of the phototransistor functions as a photodiode generating a photocurrent which is fed into the base of the transistor section."
It has been mentioned that the Ice will be the same, unlike conventional BJTs. This is incorrect. I have done another test to prove this. An OP293 driven with 50mA in a tube pointing toward the transistor with the same 100K load. Collector current was 88.864uA but the emitter current was 99.463uA. Indicating a base current.
Pics or it didn't happen :) And ditto for your risetime experiment. (You do realise that risetime and falltime are reversed when the transistor and resistor are exchanged, right?)
And yes you do get the same voltage as you do in a BJT, funny that considering it’s a transistor.
Nice snarkiness, but I think we already knew that a phototransistor behaves like a normal transistor when it's dark!
 

(*steve*)

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Ok this will probably be my last post on this subject, thank Christ I here you cry,

But perhaps not for the reason you think.

I m sure you have a fundamental misunderstanding of some basic concepts and/or some issues with the way you've done the measurement.

This stuff IS relevant to eem2am's question lecture, because I believe he suffers from the same (or similar) misunderstanding.

It stems from the belief that the Miller effect can somehow make a difference to the transistor's characteristics if there is a resistor connected to the emitter vs connected to the collector, with no base connection. If you were to think logically about that for even a second, you'd realise it simply can't be so.

I'm obviously going to do what I asked you to do and show that the voltage change across the transistor is exactly the same independent on whether the same value resistor is connected to either the emitter or collector at a given supply voltage.

If it were to make a difference, then it would also make a difference if in a LED/Resistor pair whether you placed the resistor before or after the LED.

You also seem unfamiliar with the role of electric fields inside a bjt.
 

Arouse1973

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But perhaps not for the reason you think.

I m sure you have a fundamental misunderstanding of some basic concepts and/or some issues with the way you've done the measurement.

This stuff IS relevant to eem2am's question lecture, because I believe he suffers from the same (or similar) misunderstanding.

It stems from the belief that the Miller effect can somehow make a difference to the transistor's characteristics if there is a resistor connected to the emitter vs connected to the collector, with no base connection. If you were to think logically about that for even a second, you'd realise it simply can't be so.

I'm obviously going to do what I asked you to do and show that the voltage change across the transistor is exactly the same independent on whether the same value resistor is connected to either the emitter or collector at a given supply voltage.

If it were to make a difference, then it would also make a difference if in a LED/Resistor pair whether you placed the resistor before or after the LED.

You also seem unfamiliar with the role of electric fields inside a bjt.

Steve and Kris
Apologies if I have upset you both. that was never the intention. I did do the experiment and yes I understand about the rise and fall timings, it's the slowest of either that would be the issue. Please if you have time do the experiment and you will see.
I need to be less confrontational in the future.
Thanks
Adam
 

(*steve*)

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You also need to learn to quote. Please go back and cut down the quotes to the required stuff. I've dome it a few times today for you already. Quoting everything makes it hard to know what you're responding to exactly.

This is just a hint to make your posts more readable.

And yes, I will be doing the tests.
 

(*steve*)

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Steve and Kris
Apologies if I have upset you both.

I'm not upset, you'd know if I was :D

yes I understand about the rise and fall timings, it's the slowest of either that would be the issue.

In the cases we are discussing it isn't.

Both circuits are designed so that it is the turn on time which limits the overshoot.

There are several reasons why the "CC" version is faster than the "CE" version, and they're all adequately explained by reasons other than the Miller effect which can't change the performance of the opto if other parameters stay the same regardless of where you place the series resistance.

If you look at both cases you will see differences in (from memory) Vce and Ic. These have a significant effect on the turn on time of the device.

If you read the Linear datasheet carefully, I believe you can see this. Whilst the rise time can be described using the Miller effect, the problem with the datasheet is that it describes the configuration and the Miller effect as if one affected the other in this instance. The fact is, different operating conditions are responsible for the observed effect.

Please if you have time do the experiment and you will see.

I will, and I will provide sufficient documentation to allow people to interpret the results for themselves as well as reproducing the experiment.

If I'm proven wrong, I will gladly eat humble pie.
I need to be less confrontational in the future.

We all do.

I was on my way out when I left the last message. I'm travelling now so I have more time to be polite :-o
 

KrisBlueNZ

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Steve and Kris
Apologies if I have upset you both. that was never the intention.
No, I'm not upset.
I need to be less confrontational in the future.
Likewise. This thread has gotten a bit out of hand.

I'm looking forward to the report from Steve's testing!
 

(*steve*)

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OK, here is the way I plan to test for this effect:

attachment.php


V1 and V2 are a pair of power supplies which can be configured as floating.

Q1 and Q2 are the output side of a pair of CNY17 optocouplers.

Channel 1 and 2 are the inputs to my scope.

The two power supplies are required because both scope channels share a common ground, and because I want to see if the transistor turns on (or off) any differently if the resistor is in the emitter or collector lead.

I am measuring across the emitter/collector because I wish to ensure that I don't have a different load on one transistor as compared to the other.

I do not show the input side of the optocoupler or the signal source. I will connect both LEDs in series to ensure they have the same current and drive it from a signal generator.

R1 and R2 will be the same value -- probably 4k7 to start with.

My prediction is that aside from variables inherent in the optocouplers and resistors (tolerance issues), the results shown on both channels will be identical.

If there are any differences, I will swap devices, resistors, channels, power supplies, etc to try to isolate the cause.

If a consistent difference remains, I will be convinced.

Don't expect results for a couple of days :)
 

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Arouse1973

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Hi Steve

I think it's just dawned on me what you are on about. I think I have confused matters, I was comparing the switch off time of both which would cause issues also. Which one will be a rise time and one will be a fall time. I'll do some more work also. The circuit you plan is the same as what I did.
This may have come from the confusion about what we see as rise and fall. My take is fall time is high to low transition 90-10% time and vise versa for risetime. Now switch on time is different because it's the fastest time for both modes. Is this what you mean.

I would still like to explore the correct operation of the photo transistor current and voltages because we still have a difference of opinion on this matter and I would like to know what is right. An experiment is the only real way.
Thanks
Adam
 

(*steve*)

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I think it's just dawned on me what you are on about. I think I have confused matters, I was comparing the switch off time of both which would cause issues also. Which one will be a rise time and one will be a fall time. I'll do some more work also. The circuit you plan is the same as what I did.

Yeah, I think it's important we're talking about the same thing :D

This may have come from the confusion about what we see as rise and fall. My take is fall time is high to low transition 90-10% time and vise versa for risetime. Now switch on time is different because it's the fastest time for both modes. Is this what you mean.

I think so.

Rise and fall time for the output of these two configurations will be different because the rise time is controlled by the turn-on time in one configuration and the turn-off time in the other (and conversely for the fall time).

I would still like to explore the correct operation of the photo transistor current and voltages because we still have a difference of opinion on this matter and I would like to know what is right. An experiment is the only real way.

Agreed.
 

Arouse1973

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Yes I think we were talking about different things which is what led to confusion, we were both right by the look of it but on different characteristics.

How do you split quote, I wasn't being Lazy just could work it out. lol

Thanks
Adam
 

eem2am

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There are several reasons why the "CC" version is faster than the "CE" version

Can you say what any of these are?

Basically, in this thread , I have presented three ways of connecting up an opto in an SMPS feedback circuit.

1. "common emitter"
2. "common collector"
3. "casc0ded"

When running each of these on the simulator, (and I have done this time and again over several years), it is *always* noticeable that getting a fast Vout response, as well as maintaining stability, is massively more difficult with the "common emitter" arrangement than the other two.
I can almost guarantee that nobody here (or anywhere else ) will be able to adjust the "common emitter" feedback SMPS simulation (the one attached earlier) to make it stop its vout from overshooting at start-up.

Of the "common collector" and "casc0ded" connection methods, the casc0ded method *always* offers more ease in getting fast transient response in vout whilst maintaining stability.

The Renesas and Vishay say that the Miller effect is not relevant when we don't have an actual base connection......the Linear.com app note of post #7 definitely does point to the Miller Effect.

Christophe Basso, one of the greatest SMPS engineers that ever lived, refers to the connection methods as "common emitter" and "common collector" in pages 288 -308 of his book "switch" mode power supplys".
The really strange thing is, that Basso makes no mention of the fact that the "common collector" connection method offers greater ease in achieving fast transient response in vout, whilst simultaneously maintaining stability .
Basso also does not speak of the Miller Effect being prevalent at all. (He doesn't even mention the Miller Effect.)

Adding a physical base resistor has been put forward, but this reduces CTR and CTR is low enough at the low photo diode currents used in modern SMPS, and also CTR degrades over time and temperature, so anything that reduces CTR is not welcome really..........

...so given that we can't use a base resistor, in the field of opto-coupled SMPS feedback, can we correctly make the sweeping statement that....

--------------------------------------- - -----------------
*'common collector' connection of the opto facilitates faster transient response and improved phase and gain margins over 'common emitter' connection of the optocoupler.* ?
--------------------------------------- - -----------------

Also, I am looking for an email address for linear.com, as they appear to believe that the miller effect is at work here.
 
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(*steve*)

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Can you say what any of these are?

Basically, in this thread , I have presented three ways of connecting up an opto in an SMPS feedback circuit.

1. "common emitter"
2. "common collector"
3. "casc0ded"

If you've been following the thread, you would note that there is real no CC or CE if ther is no base connection.

I provided several links to support that and also which provide a multiplicity of reasons why the so-called CE arrangement is slower than the so-called CC arrangement in the circuits you've shown.

the cascoded solution is really just a way of allowing the opto more favorable conditions for fast switching. Your other two methods are doing totally different things and it's pretty clear why one is faster than the other from the links I've given.

When running each of these on the simulator, (and I have done this time and again over several years), it is *always* noticeable that getting a fast Vout response, as well as maintaining stability, is massively more difficult with the "common emitter" arrangement than the other two.

Yes, but this is simply due to the different operating parameters which allows one optocoupler to switch faster than the other.

The Renesas and Vishay say that the Miller effect is not relevant when we don't have an actual base connection......the Linear.com app note of post #7 definitely does point to the Miller Effect.

Yes, but you're comparing datasheets describing the actual functioning of an optocoupler with a datasheet that uses an optocoupler in two different ways. Find a Linear datasheet on optocouplers and see what it says.

Christophe Basso, one of the greatest SMPS engineers that ever lived, refers to the connection methods as "common emitter" and "common collector" in pages 288 -308 of his book "switch" mode power supplys".

And that's probably because it is shorthand to describe whether the device is used to pull up or pull down. Just don't think that because it say's "common collector" that the configuration magically takes on the characteristics of a common collector transistor circuit.

The really strange thing is, that Basso makes no mention of the fact that the "common collector" connection method offers greater ease in achieving fast transient response in vout, whilst simultaneously maintaining stability.

Not really surprising at all. Since it is not the method of connection that makes the difference.

Basso also does not speak of the Miller Effect being prevalent at all. (He doesn't even mention the Miller Effect.)

Also not surprising since the miller effect is not going to result in a different behavior of the optocoupler when the only change in operating conditions is that the resistor is connected to the collector instead of the emitter.

Adding a physical base resistor has been put forward, but this reduces CTR and CTR is low enough at the low photo diode currents used in modern SMPS, and also CTR degrades over time and temperature, so anything that reduces CTR is not welcome really..........

And this comes down to a cost issue anyway, and that was the answer I gave to you in the second post in this thread. In addition, I suspect that SMPS's benefit rather than suffer from the longer switch-off period of the optocoupler transistor, but that is pure conjecture.

...so given that we can't use a base resistor, in the field of opto-coupled SMPS feedback, can we correctly make the sweeping statement that....

--------------------------------------- - -----------------
*'common collector' connection of the opto facilitates faster transient response and improved phase and gain margins over 'common emitter' connection of the optocoupler.* ?
--------------------------------------- - -----------------

I say NO (with the caveat that we're talking about the switch on delay, which is a pull-up in one case and a pull-down in he other).

I am about to conduct the experiment I mentioned above. I'm not sure if I'll get it done right now, but I may have the results in a couple of hours.

At the moment I'm assuming that you will say the outputs will differ, with channel 1 showing a consistently faster response than channel 2.

If I find they are the same, I'm going to say you are wrong.

You may have a couple of hours to modify your prediction. Or are you sticking with CC is faster than CE?

Also, I am looking for an email address for linear.com, as they appear to believe that the miller effect is at work here.

I think that if you read the datasheet carefully, and with a full understanding of how optocouplers work, you'll realise they're not actually saying that, although I think they could have been clearer.
 

KrisBlueNZ

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Basically, in this thread , I have presented three ways of connecting up an opto in an SMPS feedback circuit.

1. "common emitter"
2. "common collector"
3. "casc0ded"

When running each of these on the simulator, (and I have done this time and again over several years), it is *always* noticeable that getting a fast Vout response, as well as maintaining stability, is massively more difficult with the "common emitter" arrangement than the other two.

I can almost guarantee that nobody here (or anywhere else ) will be able to adjust the "common emitter" feedback SMPS simulation (the one attached earlier) to make it stop its vout from overshooting at start-up.
I think you're probably wrong about that. The emitter output and collector output configurations are fundamentally different in how the feedback is applied to the control IC, and I believe this is the reason for the different feedback loop response (i.e. the overshoot).

Have a look at the connections to pins 2 (voltage feedback) and 1 (compensation) in the two designs in your "Flyback_opto_CE_CC_compare" files. I'll use "VFB" and "COMP" to refer to these pins here.

In both versions, feedback through the optocoupler acts to reduce the output voltage. That is, the current in the optocoupler's LED is increased in response to increased output voltage, and the corresponding conduction in the optocoupler's transistor acts to reduce the output voltage. But the way the feedback is applied to the control IC is quite different.

In the emitter output ("CC") configuration, increasing conduction in the opto's transistor causes an increasing voltage at its emitter, which is coupled through a resistor into VFB, the inverting input of the error amplifier. Negative feedback is applied to the error amplifier by the paralleled resistor and capacitor from COMP to VFB. The main effect of this is to reduce the speed of the error amplifier's response, i.e. it slows down the control IC's response. Given that there are delays in the rest of the loop (from the control IC to the output voltage, and from the optocoupler's LED to its phototransistor output), I think this delay is enough of an explanation for the absence of overshoot.

You should also see that at startup, the output voltage rises more slowly than with the other design; this is an important clue. It will also respond more slowly to load current changes.

In the collector output ("CE") configuration, VFB is simply grounded, and the error amplifier has no effect. Feedback is applied to COMP, which is actually the output of the error amp, but this pin is not a standard op-amp output, as you can see in the diagram at the bottom of page 5 in the TI data sheet. The error amplifier effectively has an open collector output which can only pull down; with VFB grounded, this transistor is turned OFF and has no effect. A 500 µA current source pulls the compensation pin positive.

There are two diode drops between COMP and the following circuitry, so anything connected to COMP to provide feedback doesn't need to pull all the way down to 0V. This is quite a clever design, because it allows the feedback to be applied to either pin, according to its polarity. But when feedback is applied to COMP, instead of VFB, there is no way to slow down the control IC's response, because the error amp isn't used. The delays in the loop that I mentioned earlier are the reason for the overshoot; the control IC responds too quickly to the feedback when it is applied to COMP.

Of the "common collector" and "casc0ded" connection methods, the casc0ded method *always* offers more ease in getting fast transient response in vout whilst maintaining stability.
I'm sure that's true. The reason is the reduced change in the voltage across the transistor in the optocoupler, which reduces the Miller effect greatly and causes much faster response in the optocoupler. This is the reason for the cascode design in the first place.

BTW why are you putting a "0" in "cascode"? It's just a normal word.
The Renesas and Vishay say that the Miller effect is not relevant when we don't have an actual base connection......the Linear.com app note of post #7 definitely does point to the Miller Effect.
The Miller effect will be present whether or not the base is connected, and is the reason for the improved response with the cascode configuration. I can't see any reason why anyone would say otherwise. Can you link to the documents where they make this claim, and give me section or page references?
Christophe Basso, one of the greatest SMPS engineers that ever lived, refers to the connection methods as "common emitter" and "common collector" in pages 288 -308 of his book "switch" mode power supplys".
Back in post #17 on this thread I said:
If a document refers to the collector output topology as "common emitter" and the emitter output topology as "common collector" or "emitter follower", it's because the author hasn't realised that those names they aren't appropriate, or the author is using them as (misleading) shorthand for collector output and emitter output, because they are easily recognised.
I meant that the names "common emitter" and "emitter follower" (or "common collector") are well-known. But they are misleading, which is why I'm using "collector output" and "emitter output" here.
The really strange thing is, that Basso makes no mention of the fact that the "common collector" connection method offers greater ease in achieving fast transient response in vout, whilst simultaneously maintaining stability.
That's because it doesn't, as I explained earlier. The difference is because the feedback is applied to the control IC at a different point, and in the collector output configuration, the error amplifier is bypassed so the control IC's response cannot be slowed down.
Basso also does not speak of the Miller Effect being prevalent at all. (He doesn't even mention the Miller Effect.)
*shrug*
Adding a physical base resistor has been put forward, but this reduces CTR and CTR is low enough at the low photo diode currents used in modern SMPS, and also CTR degrades over time and temperature, so anything that reduces CTR is not welcome really..........
Optocouplers really aren't a very good way to pass feedback. The variations in CTR due to temperature, age, phase of the moon etc will also affect the response of the control loop significantly, hence the overcompensation used in the emitter output design. I'm sure someone who knows something about control loops could elaborate here :)
...so given that we can't use a base resistor, in the field of opto-coupled SMPS feedback, can we correctly make the sweeping statement that....
*'common collector' connection of the opto facilitates faster transient response and improved phase and gain margins over 'common emitter' connection of the optocoupler.* ?
No, I don't believe that's the reason for the difference.
 
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