Simple error amplifier for SMPS feedback compensation? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: February 13, 2022, 04:08:18 pm
content: 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?

author: th_sak
author_rank: Newbie
timestamp: February 13, 2022, 08:03:12 pm
content: Hi,Search for the following two documents."Op Amps for Everyone""Switch-mode power converter compensation made easy"

author: Faringdon
author_rank: Super Contributor
timestamp: February 13, 2022, 09:02:32 pm
content: Thankshttps://www.ti.com/seclit/ml/slup340/slup340.pdf..the above, as with many others, doesnt deal with the DC feedback error amp.Is the DC feedback type often used in High voltage SMPS's...where output poles and zeros are very low and you need the low gain at low frequency?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2022, 09:51:48 pm
content: Idunno, call it type 0?If you can't compensate the loop with type 2 or 3, the error amp isn't bad, the circuit is bad.I might even say type 3 is bad, and you shouldn't need it in almost any case. Not that it's inherently bad (it's fine), but as a sign of the circuit. Example: use current mode control to separate the filter double pole.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: February 14, 2022, 05:25:44 am
content: QuoteThanks, what about battery chargers, which have an extremely low frequency load pole...because of the batteries huge capacitance.....but ...since your doing current regulation, this gets cancelled out, right?I am sure there is some use for type 2b error amps (ones with cap in pllel with R in feedback......?..is it high voltage power supplies mainly...due to their low crossover in general?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2022, 11:06:13 am
content: As in, a compensation network of "C + (R || C)" vs. "(R + C) || C"?They are equivalent, in the limit with the parallel capacitor is much smaller than the RC cap. Which is usually the case for SMPS compensation use.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: February 14, 2022, 11:20:44 am
content: QuoteThanks, i am thinking of type 2b, as in C//R in the feedback path of the opamp....on the subject...do you agree with following may i ask.......i think with batt chargers, there is a very low frequency power stage load zero, due to esr of battery...battery which has enormous capacitance...so you need the low gain at low frequency of the type 2b compensator...would you agree?There must be situations with power stage zeros at low frequency, where you need to get the gain down fast, for stability, and the type 2b does that for you?.......this must be the case, otherwise there would be no such thing as type 2b, as type 1 has only 1 capacitor in the feedback, so if type 2b was never better than type 1, then type 2b would not even exist on the "map"...?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2022, 11:34:17 am
content: Oh. That's not confusing at all...Again, that just gives poorer regulation, on account of the limited DC gain. If it's still high enough, it needn't matter for a battery; the load current at finish is part of the design. Such a design might still be susceptible to PSRR (when inverter gain depends on input voltage) so there is that consideration.I don't think it really matters, in terms of what pole/zero/load to compensate for; the battery is simply so much bigger than anything the control would care about, it swamps out everything. Design for minimum load (e.g. open circuit, or as a general purpose power supply) not maximum.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: February 14, 2022, 07:58:34 pm
content: Thanks, our customer has told us they want a type 2b instead of a type 1 because "it has more chance of being stable".Is there any truth in this?We gave them a working solution with a type 1 with Rin=10k and capacitor of 100nF. ...but they want a 180k resistor in parallel with the 100nF.They actually presented us with the circuit with the 180k already there, in pllel to the 100nF, but i took it out...then they objected.They say that in different conditions there may be instability..........they actually said a different batch of the same opamp (for the error amp) might cause it to go unstable, but obviously there's no truth in that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2022, 11:34:42 pm
content: Customer dictating control method?I mean if they stipulate what your design must be, then that's what it is. As long as it's in writing that they insist that it be done that way regardless of what outcome it (or your contract) should have...Tim

author: Faringdon
author_rank: Super Contributor
timestamp: February 15, 2022, 09:33:37 pm
content: Thanks, also, the attached are two similar Buck battery chargers, with the said R9 resistor creating DC feedback in the error amp feedback path......one (marked) is unstable, and one is stable...could you please show where we can break the loop in order to inject frequency sweep signal as part of stability testing?

author: timenutgoblin
author_rank: Regular Contributor
timestamp: February 17, 2022, 07:01:00 am
content: Just posting this here incase it's of any interest to the O.P.

author: Faringdon
author_rank: Super Contributor
timestamp: February 17, 2022, 08:32:52 pm
content: Thanks, Would you agree that at the R5 resistor is the good place to break the loop for feedback loop injection testing?(in the attached, PDF schem and LTspice sim)