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SMPS feedback compensator: Simple inverting error amp?...not integrating type.

Flyback

Well-Known Member
Hi,
You have type 1, 2 & 3 error amps for SMPS feedback compensation...
But what is the error amp called that is just a high gain inverting amp?...ie, not an integrating error amp.

Also, can you confirm, that using this (unknown name) error amplifer, there is no gaurantee in eg a Buck battery charger that you would reduce the chances of instability, by using it, instead of a type 1, 2 or 3 error amp?
 

Flyback

Well-Known Member
The subject is such a rarity, that i dont expect any answers to be honest.
I wish i hadnt lost my Marty Brown book "power supply cookbook"
 

kubeek

Well-Known Member
Most Helpful Member
Error amp is an error amp is an error amp. It your feedback loop is compensated or not is a different thing and not necessarily related to the error amp.
The compensation can happen at the primary side inside the pwm IC using what you would normally call THE error amp, but it could also be done at the secondary side like how the circuits with TL431 and optocoupler typically do it.

And a fedback loop without any compensation is called feedback loop with no compensation.

As to the second question, I have hard time grasping what yet another twisted rule of thumb are you actually trying to get confirmed to suit your biases and misconecptions.
 

Flyback

Well-Known Member
Thanks, and as you know, there is a type 1 error amplifier, and a type 2b....type 2b is a type 1 but with a resistor connected in pllel to the capacitor.
As you know, the resistor in this place reduces gain at DC......and usually, the power stage poles and zeros in an SMPS mean that the very last thing that you ever want to do is sacrifice gain at DC.......so why have a type 2b error amplifier which cuts down your gain at DC?....there must be very niche, specific applications for it.......it would need to be an SMPS application which involves a power stage with a very low frequency zero....but what SMPS topology has such a thing?, that a type 2b error amp is needed?

Is it a battery charger?...because the battery is basically a very big capacitor, and the ESR zero is therefore at a very low frequency?...but then again, i thought that the current regulation of battery chargers, basically cancels out the big battery capacitance?

There must be a good reason why people take a type 1 error amplifier, and then add a resistor in parallel to the capacitor?...i cant think of any SMPS power stage that would need such compensation?
...***...
In relation to this, please see bottom pf page 8 of this..
https://www.microsemi.com/document-...tage-mode-current-mode-and-hysteretic-control

"Unfortunately, DC gain does tend to fall at high load currents"

..this epitomises what i mean...high DC gain is desirable in most SMPS's....so why ever drop DC gain by using a type 2b error amplifier?
 
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kubeek

Well-Known Member
Most Helpful Member
Let me say first that I am not very well versed in feedback stability and compensation at least at the analytic and algebraic side of things. Also I have no idea what is the difference between type 2A and 2B feedback compensation as I have never seen that distinction and cant find anything online.
Why is your focus at DC gain? What is the difference in DC gain between no compensation and the type 2? Does it have a real impact on the output impedance of the whole PSU?

More importantly, do you realize that without any compensation or maybe as you would like to call it "type 0 error amplifier" the supply will simply never be stable, and in order to achieve stability you need to compromise and use feedback compensation even if it brings some negative side effects?
 

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