Control strategy for a 3 phase asynchronous buck converter - Page 1
Forum section: electronics / projects

author: MarkoAnte
author_rank: Regular Contributor
timestamp: July 27, 2017, 05:35:47 pm
content: Hi,I'm looking to make a prototype DC-DC step down converter for power regulation in a induction heater. I chose a 3 phase buck topology so I can reduce the choke size and get better efficiency. I will use a delfino MCU and the sigma delta iso adc to get the current in every phase. My specification are:Input: 800V form a 3 phase Boost PFCOutput : ideally (0-800) really (100 - 750), while keeping the output power constant at 10KW. So that makes the current form 100A (@100V)- 12.5A (@750V) for full power. The voltage and current ripple do not have to meet any stringent requirements.My question is, how do you decide when will you shut down the phases to get the best performance - especially efficiency. I read on a couple of online sites different approaches and I'm a bit confused. Would it be correct to say that the goal is to keep the phase currents as low as possible without going to DCM mode?E.G. If we have inductor ripple current of 5A. If I would need 12 A peak together I would use 2 phases (180 deg phase shifted) to get 6A pek per phase and so I would still stay in CCM. Because if I used all 3 phases, that would mean 4A per phase, and the converter would go to DCM. But if I needed 50A then I would use all 3 phases.Also why is CCM preferred to DCM (if we ignore the more complex regulation requirement as my system does not need a good dynamic response)? In SLVA057 it states "It should be noted that the buck power stage is rarely operated in discontinuous conduction mode in normal situations, but discontinuous conduction mode will occur anytime the load current is below the critical level." why is this?Is CCM more efficient then DCM or is it roughly the same?What are more in depth differences of CCM vs DCM (radiate noise? over voltage peeks? The stuff that they don't say in textbooks.)Best regards,Marko

author: jbb
author_rank: Super Contributor
timestamp: July 28, 2017, 09:29:14 am
content: That's quite a chunky power supply. Have you worked on mains power electronics before?Two good questions.Phase dropping can indeed be done to improve efficiency. You can best determine when to do so by analysing the converter efficiency over a range of conditions. You'll also need to look at thermal and current limits of individual parts - for example, you don't want to saturate an inductor.CCM vs DCM is an important question. In general, all things being equal, for similar sized converters, IMHO, DCM is less efficient for large converters. This is for two reasons: 1) the more spiky currents have a higher ratio of RMS current (generates heat) to average current (DC output) and 2) the inductor cores experience higher flux swings and therefore higher core losses.Re control loops: DCM is in some ways easier to control because the inductor resets every cycle and it's easy to change the current fast. CCM isn't too bad for dynamics - as long as you use a current control loop. Good CCM dynamics all the way down to 0A output can be provided by using synchronous rectification but you would need some control logic to prevent energy being sucked back in from the load.For a converter this size, nanocrystalline / amorphous iron is an interesting candidate material for CCM inductors. They can take a lot of DC flux - almost 2T, but only a modest amount of AC flux - say 0.1T. But they do make a lot of accoustic noise at the switching frequency.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: July 31, 2017, 09:22:00 am
content: Hi jbb,yes I have worked with mains power before. I just finished optimising a 3 phase boost PFC that I made from scratch and am now moving onto the next power stage in my project. I need a step down converter and I chose a multi phase buck topology.Now I'm stuck at choosing a inductor and capacitor value. My sistem is ripple non-sensitive, so my requirements are lax. But I don't know how should I start with the calculations. Normally you start from some current/voltage ripple limits and then chose the L&C to get them - at a constant voltage eg. 3.3V. But I have changing output voltage and current and a differing number of phases, so it is difficult to determine the L&C that way, also there is phase dropping to take into consideration.First I thought that I would chose the L so that I would stay in CCM for the majority of operation area, but it seems that this my not be as strict requirement as I thought. I posted the same question on TI forums and there I got the tip to not worry if the converter is in CCM or DCM as the losses are a trade off between AC and DC loses, and so it makes it hard to say: CCM is always better.But for my design I will probably want to stay in CCM as I intend to use iron core inductors with thin laminations because of the price. For the PFC I got 3 iron core inductors (locally) for the price of 1 powder iron core from abroad. For the final design I will look into buying a pack of powder iron cores and then give it to the local guy to wind it for me, but for the prototipe its just too expansive.I will probably ask the local manufacturer if it would be possible to make a 1/3 tap, so I can experiment with the efficiency graphs when my project is running and then determine the optimum value.

author: oldway
author_rank: Super Contributor
timestamp: July 31, 2017, 10:17:42 am
content: There are a lot of 3 phase boost PFC configurations....What has been your choice ?Some interesting papers about it:https://vtechworks.lib.vt.edu/bitstream/handle/10919/28651/Barbosa_ETD.pdf?sequence=1

author: MarkoAnte
author_rank: Regular Contributor
timestamp: July 31, 2017, 01:55:49 pm
content: Hi oldway,I'm using a topology like thishttp://patentimages.storage.googleapis.com/WO2001011763A1/imgf000051_0002.png.I was debating on if I would control it with a mathematical approach where you go form 3 phases to 2 phase a/b vectors and then into d/q space and then use space vector modulation to control the phase currents.But I ended up using a discrete time interval sheme, because of the complexity of the above mentioned sheme. I ended up adding a whole lot of little improvements like: PWM phase shifting, trend calculation for each phase, PWM lock when the sin current is at max and so on.It turned out it was a good thing, because simultaneously a collage in the university was working on the other approach and after 1 year he only got it to work to 1kW. Then he had problems with noise and he didn't calculate things correctly any more. Mine approach, tho lower tech and with the problem of a non constant switchng frequency, is working at full power of 16A per phase - limited by the power connection to my lab. I had no problems with noise or something like that and I'm quite happy with my first go at a PFC. Some times a K(eep)I(t)S(imple)S(tupid) is the best way to do it for the first time.Maybe I will make a forum post about it someday.But there are so many different typologies and control schemes that you can spend years just looking at them. Especially with the SiC and GaN fetts coming on there own.Best regards,Marko