Hello,
I am working on PSFB DC-DC converter and I prefer analog control so I need to pick a op-amp for type 2 compansation . I also add buffer op-amps in current or voltage feedback paths for stability. Which specifications should I concern for fast response ? and Can you give an example op-amp model ?

 

You need to quantify what "fast response" is and then pick an op amp from a manufacturer with a gain-bandwidth that will meet that requirement.
Other considerations, are input voltage offset, common-mode range for single-supply operation, and cost.
Op amp manufacturers include Analog Devices, Texas Instruments, and Microsemi among many others.
A Google search will list more.

 

There are several performance requirements, but amongst them:
-single supply
-the input common mode range should include ground.
-The output should also swing very close to ground.
-Wide supply voltage range.

As Cruts has mentioned, the gain-bandwidth product depends on your loop bandwidth, which in most cases it isn’t very demanding.

EDIT: the ubiquitous LM358 has been successfully used in SMPS error amp.

 

slew rate and overshoot/settling may be important to consider (sometime the slower amp may perform better due it's slower/ more relaxed response HF cut off)
your schematic shows a differential (AC) feedback chains only ?
using buffer op amps adds latency (more accurate design requirements to gain a predictable control loop) ~ in case implementable using transistors is occasionally faster and cleaner (in a sense of ringing noise settling stability)

 

PWM Frequency?

Desired / required transient response?

PWM controller manufacturer and part number?

ak

 

PWM Frequency?

That's only indirectly related to the response time of the feedback loop.

 

No op-amp will be too fast! The frequency response is set by C1, C2 and R2.
The faster the op amp you choose the less it will be influenced by its own slew rate and internal dominant pole capacitor, and the more it can be predicted by the time constants of C1, C2 and R2.
For instance, if you choose the LM358 as @schmitt trigger said, its slow speed might be doing the job of C1.

[EDIT] The TL431 is a popular choice, and its open-loop response looks like this. So presumably, it must be fast enough.
Its only problem here would be its lowest output voltage being 2.5V.

 

The TL431 is a popular choice,

That is a programmable voltage reference, not an op amp.
It is often used to control an opto isolator in a feedback loop, but it doesn't necessarily replace an op amp or perform its feedback compensation.

 

I often used the TL431 as a reference and op-amp in one.

The TL431 can be used an amplifier.

Audio amplifier.

Voltage comparator

 

That is a programmable voltage reference, not an op amp.
It is often used to control an opto isolator in a feedback loop, but it doesn't necessarily replace an op amp or perform its feedback compensation.

On-semi has an entire application note concerning how to use the TL431 as an error amplifier.
https://www.onsemi.com/pub/Collateral/TND381-D.PDF

 

i have perhaps more than couple of times tested various DiY feedback nets with the best stability/latency using 1 to 3 direct coupled transistor stages - ! however ! - these were the feedback/control-loop tests which had each a varying operating window - e.g. - startup to different loads / various fixed loads / near max load - suppressed starting or killed the oscillations at certain circuits = none of them was a clocked SMPS -- problem with op amps are deriving from

• required** supply (for many it's ±15V to get its **full potential)
• I/O ranges
• sensitivity e.g. some higher speed op amps require heavy driving
• over-gain too hard feedback forces output becoming unstable or unstable at certain loads
  -vs- low gain may result at low precision or occasionally poorly regulated output . . .
• as i mostly targeted low supply I/O voltage levels - it fast reduced available/affordable op amp options**

i guess - today there's sufficient variety of controllers available for common power@voltage output ranges -- many of which can be paralleled

 

On-semi has an entire application note concerning how to use the TL431 as an error amplifier.

Good to know.