How could 4 diodes together reduce switching losses of half Bridge? - Page 1
Forum section: electronics / general

author: WuhanBear
author_rank: Newbie
timestamp: December 07, 2020, 11:19:02 am
content: 2020-12-08The document provided by David Hess has an explanation. I have attatced it as"Inverter_diodes.pdf" See Figure 6/Page259.It uses the series diode to deactivate the body diode of Mosfet. Thus it could reduce the switching loss. I am not sure, whether this design is sill neccessary for modern Mosfet.I have also seen this design in Voltage Source Inverter and LLC resonant converters. Series diode and parallel diode are added to the Mosfets of the H bridge. I am wondering, besides the Loss reduction, could this design bring other improvement, for example, zero current clamping distortion of the VSI when unipolar modulation is applied?----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2020-12-07Hi,I have seen this design in the attachment for several times.Besides the anti parallel diodes, serial diode is also added to the Mosfet. One paper says that this design could reduce the switching loss, but it does not provide any explaination.Could any one give me a hint?Thanks in advance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2020, 12:02:41 pm
content: Look up the typical recovery time for MOSFETs of various voltage ratings. Compare with diodes of similar ratings.This is a rather old-fashioned solution, as MOSFETs capable of synchronous rectification are economical these days, even at fairly high voltages. This type of operation, timed correctly, does not incur any recovery loss.The series diode is still required for certain applications, in particular, current-sourcing inverters where reverse bias (and almost no current flow, as opposed to the body diode's reverse bias with almost no voltage drop!) is required. These also tend to be old-fashioned applications -- formerly served by SCRs, which don't conduct in reverse, but are a pain to use in continuous switching duty and at high frequency. So except for very high power applications, SCRs and current-sourcing inverters have largely been replaced by MOSFET or IGBT voltage-sourcing inverters.Tim

author: langwadt
author_rank: Super Contributor
timestamp: December 07, 2020, 03:58:41 pm
content: a bit about it here,http://www.moodle2.tfe.umu.se/pluginfile.php/60819/mod_label/intro/nxp/APPCHP3.pdf

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2020, 10:09:51 pm
content: The two extra diodes prevent forward biasing the body diode of the power MOSFET which can contribute considerable switching loss. More modern power MOSFETs have body diodes which are faster and store less charge so this is less necessary.Note that while a schottky diode with its low forward voltage drop could be used in parallel with the body diode, schottky diodes are not available at higher voltages.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2020, 10:51:27 pm
content: Beefy enough SiC diodes are actually tantalizingly close. One obstacle is, body diodes have strangely almost no forward recovery, at least that I've measured -- i.e. they turn on quickly, having low Vf right away. (I'm not entirely sure whether it merely looks this way due to synchronous rectification -- i.e., as Vds goes below -Vgs(th), the channel turns on and sinks (majority carrier) current, shunting (minority carrier) current away from the body diode.) So, whereas you might have some advantage if there was significant forward recovery -- the SiC shunts it, preventing it from turning on very much, at least initially -- that doesn't seem to be the case here.I suppose GaAs power schottky would be ideal, but they were only ever a boutique item at best, AFAIK. As I recall, IXYS (IXYS RF?) had a line of 'em, up to 200V or a bit beyond I think. A segment now satisfied by advances in Si schottky, which go up to 250-300V now (super junction, field effect types; some of which actually have reverse recovery losses too, but much less than PN types).There are also schottky FETs (not sure if they're co-pack or actually monolithic?), which are effective at the low voltages they're available in (Si schottky having Vf < 0.5V or so, vs. ~0.7V body diode).(And yeah, speaking of: those high voltage Si schottky have higher Vf, too. So not much luck there either.)Tim

author: xavier60
author_rank: Super Contributor
timestamp: December 08, 2020, 12:44:45 am
content: The lowside MOSFET would be getting turned on soon after the body diode conducts. Does anyone know how much this would affect reverse recovery?

author: WuhanBear
author_rank: Newbie
timestamp: December 08, 2020, 01:19:34 am
content: Thanks a lot, I have downloaded the file. The explaination is great.

author: WuhanBear
author_rank: Newbie
timestamp: December 08, 2020, 02:09:01 am
content: Thanks a lot for your answer.Yes, the papers and devices with this design were made before 10 years. So it is old fashioned solution.I read this design in a paper about LLC converter publised in 2005. The download linkage is as belowhttps://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/01518368.pdfFurther more, I have seen this design in an old programmable AC voltage source, which is almost 20 years old. Series and anti parallel diodes are also added to the Mosfets of H bridge.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: December 08, 2020, 02:58:53 am
content: SiC MOSFET's don't have this problem and you can get them to 1700 V and above. They are fast and high current.https://www.digikey.com/en/products/filter/transistors-fets-mosfets-single/278?s=N4IgjCBcoKwOwAYqgMZQGYEMA2BnApgDQgD2UA2iACwIBMMAnAMwgC6xADgC5QgDKXAE4BLAHYBzEAF8pxWhRABbErnT4erKUA

author: Weston
author_rank: Regular Contributor
timestamp: December 08, 2020, 03:51:00 am
content: Quote from: T3sl4co1l on December 07, 2020, 12:02:41 pmI am curious, what specifically has made the MOSFET "capable" of synchronous rectification through recent process improvements ? If the timing is correct and there is no recovery loss, its not due to improvements in the reverse recovery charge.

author: David Hess
author_rank: Super Contributor
timestamp: December 08, 2020, 08:42:19 am
content: Quote from: Weston on December 08, 2020, 03:51:00 amThere was more demand with lower output voltages because the forward voltage drop of a diode represents a higher fraction of the output voltage limiting efficiency. MOSFETs also became relatively less expensive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2020, 09:32:52 am
content: Quote from: xavier60 on December 08, 2020, 12:44:45 amAgain, it acts in parallel. If the channel voltage drop is higher than a diode drop, then the diode part will carry some current. If the voltage drop is less, it will be shorted out and little diode current will flow.In an inductive load condition, the diode may be reverse-biased at first, then load current reverses (and the transistor switches on anywhere during this reverse period), recovery occurs seamlessly and the channel carries load current. With Vds > 0 by the time it's switching off again, there's no recovery loss.I've done this before with 1200V FETs, rated for... max 300ns recovery it looks, but I seem to remember more, maybe I'm looking at the wrong series, maybe they're obsolete now, dunno. Anyway, switching was up to 400kHz, inductive load, and yeah, no problem with recovery.It's hard switching that's the problem, a general class D amplifier will find this problematic, or in general a capacitive load condition -- anything where one transistor is carrying reverse (body diode) current when the opposing side switches on.In such a case, it can be better to set dead time to zero, or even slightly negative -- while intentionally increasing loop inductance to limit commutation dI/dt, and snubbing it to keep peak voltages and EMI down.And yeah, the advance since then has been this: old transistors scaled as Rds ~ Vds^2, i.e., as voltage goes up, performance is not only worse but quadratically worse. So MOSFETs of 600V rating say already performed so much worse than 400V. Performance being a figure of gate charge versus Rds(on), lower (in both) being better.Over the last decade, the SuperJunction process has been developed, reducing the scaling factor to Rds ~ Vds, i.e., performance is basically independent of voltage, it's plain old proportional, if you want more current or voltage, just scale up the die area proportionally. Or put another way: Qg is proportional to switching area, Id(max) * Vds(max).Now, the body diode drop doesn't scale nearly so fast (more like log(Vds) probably? Hmm, I never did look at it in detail, I wonder), so you are still disadvantaged, at high voltages, to achieve synchronous rectification -- out of a 12V supply, it's a hell of a lot easier (indeed just about mandatory) to get a 0.1V or even lower Vds(on), than it is to get less than 1V out of a 400V supply. For something of the same power level, say, a 1 HP motor speed controller or something -- in the first case the voltage drop must necessarily be low just as a consequence of keeping power dissipation manageable, in the other the diode drop isn't problematic at all (1V out of 400 is only a 0.25% efficiency hit in conduction loss!) and you have to work a lot harder (spend say $20 on transistors, and use somewhat stronger gate drivers) to eke that little bit more performance out of it.Put another way, the 400V inverter with synchronous switching, needs whatever, say 4 times more inverter current capacity, just to avoid a quirk of the transistor, whereas the 12V inverter just needs whatever it does.So it is kind of cruel I guess, that body diode recovery sucks worse at higher voltages, and is that much harder to avoid, through synchronous rectification at least. But we have other ways to solve it, and if nothing else, can employ the OP re-diodeing circuit to prevent it outright (at obvious expense to conduction loss, of course). So it's not a huge problem.Tim