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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Just a quick...
I always wondered what is a the ''right'' way to use error amplifiers (mostly in SMPS controllers);
Since they don't have infinite gain they can be used ala comparators and still work well in linear mode.
Now in some cases I see negative feedback made with 1Mohm or higher resistor which I considered fine since it gives really high, but ''defined'' gain and doesn't affect the circuit in other ways.
But on for example TL494 I already see some kind of RC network made of 33 or 47k resistor and 10nF cap in parallel with this 1Mohm resistor. THe network is there even if the - input is always connected to Vref (5V output).
From other side, some just connect comp in pin (etc. SG3525) to ground with a few nf cap.
Sometimes resistor is in series with cap. Needless to say, it seems to work fine with the pin just left to float.
What's really a right way to do this? Any potential advantages I should consider?
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
These components exit to provide compensation for the power supply so that it is stable during operation.
The idea is to NOT make the power supply into an oscillator. The aim is to have a loop gain less than unity when the phase is 180º or more.
There are many factors that affect this including the PSU output filter characteristics.
It can be a complex subject and even seasoned power supply engineers struggle with achieving the correct phase margin to make sure power supplies are stable.
Here is a TI application notes on the subject (not the easiest of reading)
I've also attached a Fairchild PDF slideshow on the subject that makse for much easier reading
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
For many applications, the output regulation and response requirements are so relaxed that we can put low frequency poles (big RC low pass filters) in the feedback loop and it will typically be pretty stable since these poles are real (not imaginary). But, the stability is still subject to the loop gain of the feedback, so if one is willing to sacrifice steady state error for stability, then this simple/crude approach usually works fine. This is in fact how all of my SMPS design until now has been since i really had no good grasp on control theory.
Lately ive been designing a class D audio amplifier where feedback is crucial. Basically a class D amp is like a buck converter with a quickly changing reference (bandwidth is 20khz at -1dB). In order to keep the THD below .01%, the system has to respond very quickly. Since my amp is self-oscillating the feedback is even more important to get right. Ive mainly experimented with lead (for the ~350khz self-oscillating feedback) and lag (for the audio band feedback) compensators. Ive also used straight up PI control for the audio band.
Id suggest looking into PID control (often you may only need PI or PD) and lead/lag compensators. With these few tools you can really tackle a lot of SISO (single input single output) control problems.
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Error amplifier feedback
