Power supply frequency response measurement - am I doing it wrong? - Page 1
Forum section: electronics / projects

author: krokodyl1220
author_rank: Contributor
timestamp: March 11, 2021, 12:32:56 pm
content: Hi all,I would like some help with making the power supply response measurements.I have made an injection transformer inspired by this thread:https://www.eevblog.com/forum/testgear/injection-transformers-bode-plots-application/and Maxim appnotehttps://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3245.htmlSimplified schematic of the injection transformer:470R is there to limit the current from the frequency generator. I have measured the transformer response by shorting one side of the secondary-side resistor and measuring the voltage on the other side:Now, I would like to verify the design of a power supply, here is a simulation from the ST's workbench (note that the input voltage is set to 48V, but during the measurement I used 31V, for which the fc=92kHz and the phase margin=79.3deg):To measure the response I used a circuit and loaded the power supply with 1.2A:Here is where the problems start. Based on the simulation, for the input voltage of 31V I should get a crossover frequency, depending of load of somewhere of 92kHz. But when I plot the response of the power supply I get the crossover frequency of about 40kHz.When reducing load, the crossover frequency goes down (to about 5kHz for 50mA) and it goes up when increasing the load. I initially thought that this single circuit may be faulty, but I tested 3 different designs and have observed the same behaviour.I thought that perhaps the measurement technique is wrong, so in no particular order I tried:- disconnecting the 49R resistor in the injection transformer- measurement using a line transformerand got the same results. Am I wrong for assuming that the PM I measure is the same PM from what is calculated by ST's utility? The gains are all different and the phase also looks different. Should I just assume that since the phase margin is greated than 45 degrees and the gain margin is positive the supply is stable and leave it at it? I am quite sure that something is wrong, but even when verifying the output voltage signals according to TI's SNVA364 I get the similar frequency where the two voltages have the same Vpp values.Also, when measuring other rail I got the results more in line with what the simulation is telling me:but this was at the 3V3 rail that loaded by the damaged MCU at about 1.5A.

author: bob91343
author_rank: Super Contributor
timestamp: March 11, 2021, 06:01:12 pm
content: I think too much emphasis is put on making simulations. There are subtle parameters that can't be modeled easily which affect results.I would believe the experimental data.

author: krokodyl1220
author_rank: Contributor
timestamp: March 11, 2021, 08:11:15 pm
content: Yeah, normally I would also believe my measurements, but less than half the expected bandwidth?It seems to me that either I am measuring something incorrectly, expecting the supply in a different way than it will behave in reality (FC changes more dramatically with the load than I think it would) or I am simply mixing up two bandwidths (the gains, margins and phases between real circuits and the design SIM are completely different).

author: Tomorokoshi
author_rank: Super Contributor
timestamp: March 11, 2021, 08:49:53 pm
content: For instance, the ceramic capacitors might have only half their value at the operating voltage. Electrolytic capacitors might be 10% at that frequency. Inductors might start to become shunted by parasitic capacitance at that frequency. There could be a layout issue. The power supply also has some frequency response. I'd be happy with the simulation being around 50% of measured value the first time around.

author: krokodyl1220
author_rank: Contributor
timestamp: March 12, 2021, 11:24:35 am
content: Today I did more work, and after some testing I was able to achieve the power supply bandwith very similar to what was simulated.Measured 105 kHz / 72deg at 29V input vs 100khz / 85deg simulated:and measured 125kHz / 68deg vs 138 / 88deg simulated:To achieve that I had to adjust the feedback capacitor from simulated 39pF to about 420pF. Seems higher than I expected, but if that must be it must be.However the one thing that I would like to ask is a verification of the measurement setup, even though I followed what is generally described as a procedure (even in TI app note) and I believe that there is not much to be done wrong, does it seem reasonable?Next step for me will be to verify if the load step response changes, but that is for the next revision.