Synchronous rectifier control chips with malfunction warnings. Why use them? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 05, 2021, 10:26:23 pm
content: Hi,I have just come across a fantastic new current mode control chip for offline flybacks!..Its like the UCC28C43 but way, way better. Only thing is you need to add a “loop” in the PCB tracking near the chip, in order to create some stray inductance (4nH, or maybe more), which cancels out stray inductance involving the MOSFET bond wires etc………if you don’t add this stray inductance, then you will suffer malfunctional operation……..The amount of stray inductance that you need to add can vary depending on layout and mosfet package tolerance in its bonding wires, and indeed, varies with how long the TO220 leads are cut off at, etc etc.…….OK, if you read the above, you will no doubt be thinking that that controller is in fact extremely poor, and you’d wonder at the sanity of the semico that produced the chip...But here is such an SMPS control chip, which suffers this exact problem….its called the NCP4303, and is a synchronous rectifier FET driver….Pages 15 and 16 of the datasheet reveal the dire situation as decribed above.Does anyone know why people use these type of synchronous rectifier drivers that rely on the chip monitoring the secondary side switching node in order to control the switching of the synchronous FETs? I mean, they all have this kind of cut-throat, skull and cross-bones warning in their datasheets.Why are people not using chips like the ICE2HSO1G LLC controller? …which controls the synchronous FETs without any of this grief…and controls the synchronous FETs from the primary side, in coordination with the primary side fet switching. It even costs no more money.NCP4303 Synchronous rectifier driverhttps://www.onsemi.com/pub/Collateral/NCP4303-D.PDF

author: TimNJ
author_rank: Super Contributor
timestamp: January 06, 2021, 05:00:57 am
content: Because modern, adaptive synchronous rectifier controllers are very good, and saves you an entire custom magnetic part (pulse transformer) or at least some other form of isolated feed-forward mechanism.What topology are you doing?LLC:TEA1995T/TEA2095TMP6922AUCC24624Flyback:TEA1993TMP6902AUCC24612LD8526

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 06, 2021, 02:05:37 pm
content: Thanks, the thing is, if your synchronous fets go obselete, then you change them to fets which have possible different bonding wire inductance and malfuncations happen. Also, even the assemblers cutting the fet legs to different lengths effects it and causes malfunction, as the NCP4303 datasheet says.This is all assuming you manage to add the requisite amount of PCB track "compensatory inductance" in the first place, without too many PCB respins to make the project manager scrap the project.Especially with a digital isolator, the extra money is minimal, and is even less depending on the chip set chosen ICE2HSO1G plus isolator works out pretty much the same price as NCP4303 plus Primary controller.The "isolator" based solution also enables you to keep changing your fets to cheaper ones without need to keep re-checking the stray inductance.In cases of "design for maintenance", the "isolator" based solution , as you know, is far better, and risks less field failures, with all the costs involved with that.

author: TimNJ
author_rank: Super Contributor
timestamp: January 06, 2021, 03:54:52 pm
content: I think synchronous rectification (historically) was a hard thing to do right...but I think you may be overthinking it at this point in time. The chips I list above are pretty bulletproof, as far as I can tell. I have used all of them, never had any catastrophic malfunction. I'd say our PCB layouts were "okay" but not amazing. Only issues I've ever had with VDS sensing SR controllers is with very high currents >40A and trying to use TO-220 packages. In this case, you just need to be sure to sense directly across the MOSFET's drain-source, and try not to include any series package inductance in the measurement. The issues we had here were, again, not catastrophic but rather too short of an on-time, and thus too long of a body-diode conduction time. And/or use a PowerPAK/DFN style SMD package. Those have very low package inductance.Obviously, feel free to use a single package solution like the Infineon IC. It doesn't look bad. Just beware that it's a pretty old IC (as far as resonant control ICs go), and it might not have more modern power saving features, etc.I'm also not sure whether or not controlling the SR from the primary sideactuallyis best for performance. I think the secondary side waveforms don't always perfect match the primary side...so you might have to do some fiddling around to get it right anyway.TEA19161T + TEA1995T are very easy to use. There are tons of companies throwing power adapters together, with these types of chips, without heavy attention to detail, and they work fine.

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 06, 2021, 11:21:12 pm
content: QuoteThanks, i think youre right, but as we discussed, this is difficult with a TO220, especially since the leg length can mess things up.As you know, I'm not knocking NCP4303 by the way.......good on onsemi.com for 'fessing up about the issues involved here...these issues apply to all Vds sensing sec side synchronous rectifier controllers, certainly not just NCP4303. In fact, onsemi.com are confessing to the issue and offering a solution, so good on them. I just feel that with TO220's, there's going to be potential for a lot of fiddling about, which can be sidestepped with a pri-side SR controller like the ICE2HSO1G.I think rather than the suggested pcb track loop in the NCP4303 datasheet, i'd rather use a chip inductor package so i could vary the "stray inductance from say 4nH to 10nH.

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 08, 2021, 02:43:46 am
content: The attached is a Two Transistor forward , done with a Full bridge controller which has a synch rect driver output which can be used to drive the “freewheel” synch fet via a small Pulse transformer as in the attached LTspice sim and pdf schem. A “freewheel” synch fet is all that’s needed as the duty cycle can be made low such that the “power” synch fet can be avoided and so just a diode is OK here. [Vin=390v, vout = 250w and 24v]I am sure (?) many would agree that this is a perfectly satisfactory way to reduce secondary rectifier losses. Also, it avoids the dreadful problem of shoot through which can happen with those secondary side synch rect control chips which are afflicted by noise as they “look” at the noisy switching node. Also, the shown method here means you can slap in TO220 synch rects in parallel and not worry about lead or bonding wire stray inductance effects. No messing about trying to heatsink those “low stray inductance” SMD FET packages…..they are lousy as they need heatsinking through the FR4 PCB. (albeit with thermal vias but thermal vias are pretty lousy compared to a good solid metal TO220 tab screwed to a metal heatsink with just a little 100um thick insulating spacer)The LTC3723 assures that the synch rect drive is delayed and “clipped” such that there is no shoot through. This means less field failures and so the extra cost of the LTC3723 is worth while.This is also in the name of "design for maintenance" where SMPS's are designed and have to be maintained by engineers who do not have a decade of experience of SMPS design. I am sure you woudl agree its no good a design consultancy working some magic that the guys can't maintain.(By the way, the other reason to use a full bridge controller is that it has the “spare” output which could be used for pri side bootstrap high-side drive capacitor refresh, ..good in light load.)I am wondering why no semi-co’s are making chips like this? The LTC3723 can be “hacked” to do it as shown here but its expensive at approx $4.5 per 1000 pces. I am sure this could be done cheaper than the LTC3723 chip?The LTC3723 in this use-age has been "hacked" to do the job.LTC3723 is expensive but i am not using its full functionality here. I am surprised there is not a chip available that does what i am doing here?All it needs is a standard pwm controller, with an output which is the "delayed and clipped" inverse of the main gate drive. (the "delay and clip" to get the dead times)....i cant find anything off the shelf like this. Any body else?

author: TimNJ
author_rank: Super Contributor
timestamp: January 08, 2021, 04:39:29 pm
content: Again, I think you're over-complicating it. Do you have any other requirements other than 250W @ 24V? This is handled easily with a copy-and-pasted LLC app note design. (See below.) Your attitude seems to be that you know better than the top power supply and semiconductor manufacturers in the world. I'm not saying that they don't make dumb design decisions sometimes, but in general it's good, and well optimized. If you don't have a highly specialized requirement, go on Mouser and order a few modern 200-400W power supplies from Delta, Cosel, Meanwell, XP Power, Recom etc, and take some notes.There are good reasons why these brands typically converge on similar solutions for a given power level/form factor. Granted, there issomedegree of copying each other's features, but that's kind of how it works. Unless you really know what you are doing, trying to re-invent the wheel is just going to land you in a world of hurt with an overly complicated, overly expensive, non-competitive product.The secondary side controllers that I've mentioned are highly robust. I doubt you'll have any problems with reasonable layout, etc. And THT vs SMD...also not necessarily true. Low RDS(on) MOSFETs make it completely reasonable to use DFN5x6/PowerSO-8 style packages. The main benefit I see to through-hole at this point is allowing you to use vertical space, instead of relying on horizontal PCB area for heatsinking. If you have the bottom-side board space, I don't see a good reason to use through-hole.https://www.nxp.com/docs/en/nxp/user-guides/UM10972.pdf

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 10, 2021, 10:17:05 am
content: Thanks,That TEA19161/TEA1995 combo looks very good.It is strange though that page 26 of the application note schematic does not show any RC snubbers being used across the synchronous fets.ps://www.nxp.com/docs/en/nxp/user-guides/UM10972.pdfThe TEA1995 sec side synch rect driver for LLC looks interesting. It purports to be able to actually regulate the drain-source voltage of the synch fets to a certain on-state value. This would need some kind of very fast negative feedback loop, so it seems amazing that they have managed to do this.The benefit of this seems to be that it allows the synch fet to be switched off very quickly when switch off is needed. (at the end of every conduction period so every 5us or so.Presumably, when the synch fet has a high rdson, the TEA1995 is not able to regulate the drain-source voltage for much of the time and so the fet is presumably just driven fully on.Looks a great part, but I would still be nervous of putting this into a circuit with through hole FETs. I appreciate what you’ve said about SMD fets though.[Also, even using TEA1995 (and others like it) with say two or three paralleled SMD FETs makes me nervous]These TEA1995 style drivers dont seem to come with eval boards where there are SMD fets in parallel (and certainly not with thru hole fets)...in fact, very few of them have eval boards at all......i think this shows just how tricky these chips are......probably a lot of jiggling and re-laying out PCBs in the development cycle. (due to noise in the drian-source sense of the synch rect driver).I find with many contracts, re-laying out prototype PCBs is badly frowned upon, they want the first prototype pretty much right first time.

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 10, 2021, 07:55:14 pm
content: The LLC converter is a bit of a different case-in-point…..The LLC with synchronous rectifiers is actually better done with “reactive type” synch rect drivers, if possible. This is because when operating below the upper resonant frequency, a “normal” synch rectifier driver which simply turns the synch rectifiers on for intervals just inside the primary fet on_time, would suffer high reverse current through the synch rects. This reversing current isn’t damaging, but does significantly lower efficiency. This is as shown in the attached simulations…one with, and one without the reversing current in the secondary side.Obviously, having secondary side synch rect current sensing is needed to detect this reversing current. So “reactive type” synch rect drivers get a bit of “rite of passage” with LLC converters.Though again, they still need the SMD FETs.I must admit , give me a 2 transistor forward any day with a simple controller which simply spits out the (suitably cropped) freewheel synch rect drive signal and I will happily throw it across the isolation barrier with a pulse transformer and use it, with a bit of extra circuitry , to simply and solidly drive TO220 synch rects. No re-spinning the first prototype PCB because its too noisy or I did not add the requisite stray inductance as per NCP4303 datasheet. -And easy for non SMPS engineers to maintain.

author: opampsmoker
author_rank: Regular Contributor
timestamp: January 11, 2021, 09:00:40 pm
content: QuoteThanks, The TEA1995 Synch rect driver for LLC looks interesting. Its datasheet boasts that it has "No minimum on time".The NCP4303 and SRK2000 Synch Rect controllers are also for LLC...but they boast that they do have minimum on time. They say it is needed to prevent premature switch off of the synchronous rectifiers due to ringing of the stray cct inductance and mosfet junction capacitances.Do you know why TEA1995 says its advantageous to NOT have minimum on time of the synch rects?NCP4303:https://www.onsemi.com/pub/Collateral/NCP4303-D.PDFSRK2000:file:///C:/Users/andrew/AppData/Local/Temp/srk2000a.pdfTEA1995:https://www.nxp.com/docs/en/data-sheet/TEA1995T.pdf

author: TimNJ
author_rank: Super Contributor
timestamp: January 12, 2021, 01:30:36 am
content: I appreciate your efforts to really dive into the theory and try to understand it. At this point, I have not had the time to fully understand the entire control mechanism on a very deep level. It is a testament to the relative ease of use of these new controllers. So, unfortunately, I'm probably not of much help answering some of the deeper questions. But here's my attempt:Regarding RC snubbers: You will need a snubber in most applications. You can use two, one across each MOSFET, or you can just put one across the transformer winding (from drain of SR MOSFET #1 to drain of SR MOSFET #2 drain). This should be enough in most applications.Regarding the new style SR controllers, yes indeed they have a very fast control loop to regulate drain-source voltage to some level. Controllers from a few years ago were maybe 50-75mV level. Newer controllers are 20-50mV level. As I understand it, the point of regulating VDS via the gate voltage is to keep the MOSFET channel conducting near the end of the switching cycle, albeit at a higher effective RDS(on) than on the MOSFET's spec sheet. Still, it's better than conducting purely through the body-diode.In traditional LLC SR controllers, like NCP4303, the gate drive is either 100% on or off. The turn off threshold is a fixed value, and the controller is likely to shut off earlier (compared to newer controllers) to avoid negative current. The percentage of switching cycle through the body-diode may be too high, which leads to the I*V(body-diode) losses noted above. For the new style controllers, the VDS regulation voltage is on the order of 50mV, which might allow you to reduce the losses (near MOSFET switch-off) by up to 10x if you assume body diode drop of around 500mV.On TEA1995T, MP6922A, UCC24624 (and others), when dI(drain)/dt is negative (second half of the half-sine), instead of shutting off when VDS collapses to a set value, the gate drive voltage is reduced to push up the on state resistance of the MOSFET. With the controller modulating the RDS(on) , the drain voltage can be regulated to a "reasonable" level that is detectable via internal analog circuitry (i.e. ~ -50mV). The level is probably limited by sensitivity and noise performance of the IC's sensing circuitry (I presume). If it's too low, probably gets lost in the noise and then you get unreliable switching.I do not know why TEA1995T proclaims no-minimum on-time control...I presume it must have some? Maybe they're saying there's no need for you the engineer to worry about it. They've got it taken care of?Regarding SMD MOSFETs, on our low-cost 180W power supplies, we are using 2 x TO-220 with a single-side PCB with TEA1995T. Very average layout, nothing to write home about. The SR waveforms are all fine and have not seen any real issues with gate timing, and handles all fault events no problem. We are also using 2 x TO-220 for a 450W power supply...Only the 12V version with currents around 40A gave us trouble with TO-220. The rest are good. Originally used UCC24624 for the 12V version, but this controller has the disadvantage of a single source sensing connection for the two MOSFETs, and the body-diode conduction period was too long. We tried TEA1995T with separate source connections, and lo and behold, much better performance. Granted, I must admit that the layout is not ideal, and would probably have been okay with a better layout.Power dissipation wise, a PowerSO-8 (LFPAK56) style package can handle at least 200mW without any real PCB heatsinking, for acceptable temperature-rise, in my testing. Add some PCB heatsinking, and you can easily push 500mW+. So if you run the numbers, you'll probably see why it might be okay to run single MOSFETs for output powers 200-250W, maybe higher. I think the eval boards probably come with the SMD packages because itdoesmaximize SR controller performance, and they probably want you to see it working the best it can.