Tutorial Video; Gain and Phase measurement of a Flyback - Page 1
Forum section: electronics / projects

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 04, 2013, 01:29:28 pm
content: In this video I show how to measure the gain and phase response of a flyback. I used the Venable frequency response analyzer to measure the stability of the flyback converter. The datasheet shows a crossover or bandwidth of 7KHz. But the Venable shows it to be only about 54 Hz. So always try to verify your components specification.Poor man's frequency response analyzer(Gain and Phase measurement of a flyback supply)This video shows how to use a function generator, an injection transformer, and a scope to measure the gain and phase of a flyback. By injecting a signal and measuring the amplitude and the phase delay, you can measure three critical power supply specifications such as the crossover frequency, phase margin, and gain margin.

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 04, 2013, 05:13:46 pm
content: Hi AcHmed99,I totally agree with you. That has been my complaint that there are no affordable FRA for the small business man. The scope and function generators works if you perform it on a Forward converter which is more quite than a flyback because of the output inductor. On a flyback, there is usually too much noise to see the signal.There will be an affordable FRA in the 1500 dollars range announced in about a week. I have been working with a company that make a very affordable unit. In fact this video is part of the testing that we are doing. This test of the same TI evaluation board was performed by the company. I verified their results by performing this bode plot. In short, the new 1500 dollar FRA that will be announce early next week performs as well as the 30000 dollar Venable FRA. As far as the data that I have seen, the performance the the new FRA looks very good.I will be posting some more information regarding the new FRA very soon here on the eevbog.Robert Bolanos

author: JackOfVA
author_rank: Unknown
timestamp: May 04, 2013, 06:31:07 pm
content: I believe your electronic load is set for 2.5A constant current mode. Does this have an effect upon the gain and phase measurement? The electronic load will presumably adjust itself to maintain constant current with a reasonably short time constant, so as the 50mV test signal attempts to perturb the output up, the constant current load will change its equivalent resistance with some unknown frequency response.This is an area I've never worked in, so I'm curious whether valid gain/phase data can be taken with a constant current load or if the load must be a resistive load.

author: jahonen
author_rank: Super Contributor
timestamp: May 04, 2013, 06:32:48 pm
content: I have used a soundcard and software called ARTA to measure loop gains of power converters.Works quite nicely but of course the frequency range is limited by the sound card sampling rate (with 192 kHz sampling rate you can get up to 80 kHz or so). I used ordinary mains transformer (230/6 VAC) as an injection transformer, sound card output connected to transformer secondary. As ARTA measures both the reference and response, the transformer non-ideal response won't affect the result. Also, you'll need some protection/level matching circuitry to connect power supply signals into the sound card.Here are two example measurements (plots done with Maple):Regards,Janne

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 04, 2013, 06:40:49 pm
content: Hi JackofVA,Yes this is valid using the electronic load. This test was performed with a 10 ohms resistor as well and we got the same measurement. In the many supplies that I have worked on, I have rarely have had any problems although it may be possible to have a condition where the loop of the load effects the overall response of the power supply.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 04, 2013, 07:04:00 pm
content: Hi Jahonen,That looks pretty good bode plotsI had been looking for something like that for my home labAt work we have both the Venable and the Ridley FRA which are very expensive. For most power supplies, this would be suitable since most power supplies have low bandwidth.However, some of the linear supplies have bandwidth in the 1 MHz range and therefore would need a higher frequency range to be able to measure those supplies.The new FRA that should come out this week has a frequency range up to 10 MHz. This is suitable for most applications.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: May 04, 2013, 07:36:18 pm
content: My professor used the discovery to teach analog electronics. I remember he tried using it to do a bode plots but did not have to much success. Although it was the first model that he tried it on.Just remember that a flyback is more noiser than a forward converter. You might want to sample the signal first, then do a FFT to get the fundamental frequency. By doing this, you get a natural filter which might work for you.Robert