Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

feedback compensation: transconductance vs voltage error amplifier in smps?

Status
Not open for further replies.
G

grizedale

Advanced Member level 3
Joined
Jun 13, 2011
Messages
838
Helped
17
Reputation
34
Reaction score
17
Trophy points
1,298
Activity points
8,804
Hello,

The closed loop small signal feedback transfer function of a smps with a transconductance error amplifier includes a term which depicts the output divider attenuation ratio?

However, if a normal voltage error amplifier is used then the term involving the output divider ratio is not needed becasue it is taken care of by the transfer function of the voltage error amplifier......(the transfer function of the normal voltage erorr amplifier is Z(fdbk)/Z(in) where Z(in) is the upper divider resistor.

...is the above correct?
 

This completely depends on the specific error amplifier circuit. In general there is no buffer amp between the output divider and the error amplifier network, in which case the impedance of the output divider will need to be taken into account in the error amp transfer function, so then the above statement is correct. Personally I like to put a buffer amp between the divider and the error amp network, since it allows you to easily vary the K of your loop gain without messing up your error amp. But that's just me.

This doesn't apply to a transconductance amp, since the input of the amp is always high impedance, and there is no external feedback in the error amplifier.
 

You would not likely want to use a close-in closed loop error
amp, unless you don't need the DC gain and want very high
bandwidth (but there is no practical use for BW greater than
fSW).

I like transconductance amps because compensation can be
simple (Cshunt, one pin to GND). Voltage mode amps need to
have a pretty stiff output to get high current to drive "through"
the comp network adequately and not add a zero, while the
tranconductance amp's finite & high Zout is actually a bonus.
 

The thing i am saying is that if one is NOT using a transconductance type error amplifier, there is no need to incorporate the attenuation of the output divider in the small signal transfer function.

i.e......if NOT using a transconductance type error amplifier, there is no need for the term 20.log[(VREF/VOUT) ] in the transfer function.?
 
Last edited:

Again, you'd have to post the circuit in order to give an answer for certain, but unless you have a buffer amp between the divider and the error amp, then you likely do have to merge the two into one transfer function.
 

Page 1 of this shows a boost converter with a voltage error amplifier..........

http://www.ti.com/lit/ds/symlink/tps40210.pdf




Page 1 of this shows a boost converter with a transconductance error amplifier...........
http://cds.linear.com/docs/Datasheet/1871fe.pdf




There is a difference in how the small signal feedback loops are made out.

I am trying to find out what that difference is.

I think the main difference is that for the transconductance type, the attenuation factor of the feedback divider has to be accounted for.....whereas it does not for a voltage type error amplifier. ?
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top