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Transient response test (alone) for stability assurance of Dual Buck converter?

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Flyback

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Hello
I have recommended that my workplace use a AP300 gain/phase analyser for our Buck converter stability assurance, however they say that transient response testing alone will do. Do you believe this is erroneous?
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Firstly , My sincere apologies for the length of this email, I appreciate that you are very busy and understand entirely if you cannot respond.
This question is about the superb Dr Ridley article (“Transient response and loop gains of power supplies”) which advises against checking for power supply stability using the transient response test alone.
We are currently developing a Dual phase, Interleaved Buck converter for Vin=48V, Vout=1V5, Iout Max = 20A, Fsw = 200kHz (Interleave phased so it looks like 400kHz at output) , CCM, Current Mode, Controllers = Two LTC3892’s , Worst case load transient will be 50% to 100% load step and vice versa, Load = many processors) .
The schematic is attached in pdf should you wish to see it. (Also the LTspice simulation)
The two Buck controllers have their transconductance error amplifiers connected together, as the LTC3892 datasheet recommends.
(Incidentally, we handled the Bode plot calculation for the Dual Interleaved Buck Converter by doubling the error amplifier transconductance, also by assuming that the switching frequency was double that of the single buck case (ie 400kHz), and that the duty cycle was double what it was for the single Buck case into half-load.
I recommended that we add a loop injection resistor into the PCB layout and used the Ridley Engineering AP300 gain/phase analyser to actually measure the feedback loop..
..However, the project leader has advised against this. He says that we should simply do the stability check by transient response test alone. He says that because this is a current mode Buck Converter with a very small duty cycle, that the transient response test alone will suffice. This worries me as the worst case load transient is significant at 50% to 100% load step, and we therefore need the feedback loop to be fairly fast)
(Incidentally, the LTspice program, with an injection source as in the attached LTspice simulation, suggests a crossover frequency of 7400Hz, and phase margin of 86 degrees ………..Our own calculation gave 6kHz for crossover and 88 degrees for phase margin. We were told by authority that adding an injection source to a time domain LTspice simulation [as in the attached] was bad practice and wouldn’t give a valid result, because a frequency domain measurement is needed instead)

Anyway, Please could you possibly advise on whether transient response test alone is indeed valid for a continuous current mode Buck converter with very low max duty cycle? I doubt it is, i mean especially when we will be going for a fast response by tweaking the compensation components.

If it was a single buck then we would be confident of calculating the feedback loop, (gain and phase margin) but for various reasons, we are unsure if our feedback loop calculation is exactly correct for the dual interleaved buck case. I mean , the calculation we did corresponded almost exactly with ltspice, but we are still not 100% sure, specially when we heard that the "ltspice method" is not accurate.
 

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MikeMl

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Transient test is necessary, but not necessarily sufficient ;)
 

Flyback

Well-Known Member
thanks, specially in this case where there are significant load transients and we want a relatively fast feedback loop.....attached gives the pitfalls of transient response testing alone...but some say its different when max duty cycle is very low......then the bode plots are less necessary
 

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