Control of Current Doubler Rectifiers (CDR) - Page 1
Forum section: electronics / projects

author: sean0118
author_rank: Frequent Contributor
timestamp: June 28, 2014, 10:40:32 am
content: Hi everyone,I've been looking at current doubler rectifiers for SMPS design.From reading a few sources it seems that the use of current control over voltage control is recommended. But nothing I have read gives an exact reason why. Is it to do with each filter inductor being of slightly different inductance/resistance or is it due to the changing duty cycle of PWM control? Or is there something else I haven't considered?

author: dannyf
author_rank: Super Contributor
timestamp: June 28, 2014, 10:51:41 am
content: Posting a schematic of this CDR probably would help others help you.

author: sean0118
author_rank: Frequent Contributor
timestamp: June 28, 2014, 11:13:20 am
content: Ok good point, sorry I wasn't sure how widely used they were.Schematic at:http://www.encyclopedia-magnetica.com/wiki/Current_doublerAlso, a textbook that includes them is:Power Electronics and Energy Conversion Systems, Volume 1, Fundamentals and Hard-switching Convertersby Adrian Ioinovici

author: PChi
author_rank: Frequent Contributor
timestamp: June 28, 2014, 05:39:13 pm
content: I haven't seen the topology before. It is using two magnetic components versus one which may be an advantage or disadvantage depending on core availabilty or space or something else.Do you mean current mode control versus voltage mode control? The Current Doubler Rectifier arrangement looks to be used in a forward converter circuit. If the primary side of the transformer is using a bridge circuit any imbalance in power switch on times gives a dc component which causes unecessary power loss. Current mode control menas that every cycle (for a constant load) is reaching the same primary current so there isn't any DC component so is inherently OK. Voltage mode control relies on the two phases having exactly equal delays so could have a slight imbalance.

author: nickm
author_rank: Regular Contributor
timestamp: June 28, 2014, 08:37:02 pm
content: The current doubler uses any push-pull transformer topology and makes the output behave like a two-phase output. The reason you want current mode is it will keep your transformer from saturating. All the current must return through the inductors and any assymetry due to inductance will result in flux walking. I think.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2014, 02:47:00 am
content: Current mode means, controlling PWM based on the current flowing in the inductor (the active inductance that's switching and storing energy).In boost and buck, the inductance is explicit. In flyback, SEPIC or Cuk, the inductance is the inductance of the transformer, which could be more descriptively called a multi-winding or coupled inductor. In forward converters (SE or PP), the transformer ideally has infinite inductance and stores no energy; the primary (if current-sourcing topology) or secondary (if voltage-sourcing) inductor is doing it.The current doubler I hadn't seen before, but I expect it's just as useful as the voltage doubler is under respective conditions. Note that the two inductors serve as the energy storage inductors of the usual (choke input) filter, exactly analogous to the voltage doubler (which needs a current-sourcing topology to use with PWM).Tim

author: johansen
author_rank: Super Contributor
timestamp: June 29, 2014, 04:56:33 am
content: I believe I have seen that topology in a 300 amp 5 volt power supply.Anyhow, I read an article here long time ago:http://www.smps.com/knowledge/Idoubler/idoubler.shtmlIt looks like if you do it right, the output current ripple is far, far less than other topologies, but the efficiency will be lower, unless you spend a lot more money on it."The voltage loop of the output stage is different in center tap then in current double, and the compensation network has to be changed.For the current doubler topology to work properly, the currents in the two output inductors have to be balanced. In order to have them balanced, you can not use voltage mode control or average current mode control for the output stage. You MUST use peak current mode control or a similar control method to force equal average currents in the two output inductors. "seems to me you have the usual problems, nothing new.

author: sean0118
author_rank: Frequent Contributor
timestamp: June 29, 2014, 06:42:51 am
content: Thanks for the replies guys! You have raised a few areas that I don't know much about (I didn't know flux could walk!), so bear with me.Quote from: PChi on June 28, 2014, 05:39:13 pmIs this the same as the 'flux walking' that Nickm mentions? What causes the imbalance in switch on times? Maybe a change in duty cycle or differences between the primary side switches or output inductors?Quote from: johansen on June 29, 2014, 04:56:33 amYep, that's one of the places I have read that current mode control is required. But, I'm still not to sure what makes the currents in the output inductors unbalanced?If flux walking is the cause would voltage mode control be possible with a capacitor in series with the transformer? Also, what if the duty cycle was kept constant as with variable frequency control?

author: PChi
author_rank: Frequent Contributor
timestamp: June 29, 2014, 10:25:59 am
content: Yes any imbalance in power switch on times that gives a dc component is the same as flux walking. It's a good name because any slight imbalance causes the average flux to change slighty per cycle or walk. It's only limited by resistance so only a little imbalance can cause a big problem if the core saturates.I haven't analysed the current doubler so don't know how much of a problem imbalance is in the output inductors. I don't see the topology as inherently superior or think that the ripple is necessarily lower it just depends on the implementation.As you say a capcitor in series eliminates the problem so voltage control mode can be used but the full current is going through a capacitor which isn't small or cheap. Even if the duty cycle was kept constant if the primary circuit uses two switches there is likely to be a difference between the 2 switch on times, even a fraction of a us could be a problem.With SMPS design there are many variables so there isn't one best solution.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2014, 10:00:40 pm
content: Quote from: PChi on June 29, 2014, 10:25:59 amIf you SPICE'd it, you'd first get the error:Singular matrix: check nodes [whatever the inductors are near]Though if you have internal resistances wrapped in there somewhere, you might not.Point being, physically speaking, two inductors connected in parallel act like one inductor of the parallel value (assuming k12 = 0), but, the current in each inductor is not uniquely defined from the current through those outside terminals. Meaning, one inductor could have an initial current of +1A and the other -1A, or any other inverse combination, and the total behaves identically to an inductor of 0A initial.Likewise, no combination of outside signals can induce a difference in currents between the two inductors.Put more simply... the inductors are behind a transformer, which blocks DC.So whatever concerns you might have regarding balance, aren't new because of the topology; they are still just as necessary as usual.Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 30, 2014, 06:24:59 am
content: I've used the current doubler with voltage mode control about 15 years ago without a problem. It was a phase modulated full bridge converter at 1.5kW.However, now I would definately use current mode control. Current mode control eliminates all chance of flux staircasing and you can get rid of a series flux balancing capacitor. Current mode control will also ensure that the peak inductor current in each choke is the same regardless of actual inductance.Although I opted to use the current doubler configuration then I'm not so sure I would use it again.Dick

author: sean0118
author_rank: Frequent Contributor
timestamp: July 03, 2014, 02:12:00 am
content: Quote from: T3sl4co1l on June 29, 2014, 10:00:40 pmSo you mean the flux walking in the transformer is a separate problem to the imbalance of the output inductors? I'm still a bit confused as to why current mode control is required over voltage mode? It's just that I'm planning on building one and using voltage mode control is easier due to commercial control chips being available for the application.

author: sean0118
author_rank: Frequent Contributor
timestamp: July 03, 2014, 03:14:01 am
content: Quote from: Richard Head on June 30, 2014, 06:24:59 amSorry I don't know a lot about that type of converter, but they are variable frequency controlled, not PWM right?

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 03, 2014, 06:11:44 am
content: Sean0118The phase modulated full bridge converter has two square wave outputs that can vary their phase relationship, depending on the load. The net result is that the transformer is excited by a PWM signal.The advantage of the phase modulated full bridge over the conventional full bridge converter is that you can use the transformer primary magnetising current to slew the power switch nodes and create lossless switching. It's classed as a quasi resonant converter as apposed to a fully resonant converter.It's a great topology but has been superceded to some degree by the resonant LLC and LCC in recent years.I haven't looked recently but in its day the control IC was expensive.Dick