LED Boost converter efficiency improvement - Page 1
Forum section: electronics / projects

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: December 15, 2019, 01:50:58 am
content: So i have just built a constant current led driver for 30-60V LEDs that can drive them at up to 3.5A from a ~20V source. The chip i use is the LT3795.The circuit is identical to the one at the top of the datasheet (obviously with adjusted component values), which should have efficiencies of >90%, but at only 400mA output current.So far the design is working well, but i am only getting ~80% efficiency. One thing that i found was that there is a very odd gate drive waveform of the Low side mosfet (see below), if i understand correctly the biggest losses occur while the mosfet is changing from on to off, which is taking longer than it should (i think). I have no idea where it is coming from; i have checked FB, IntVcc and LED Current Sense for a possible fault conditions, that would cause the chip to force the Gate low, but have not found anything.So now i am wondering if that reduction in efficiency is even because of that switching weirdness of if i just got pulled down the rabbit hole. Also i am not sure if the efficiency at this point is just a fundamental limit, after all this is a 200W boost converter.EDIT: i have just checked and the chip also momentarily disables the PFet at the same time as the glitch on the NFet (i have no picture of that though), no idea what to make of that though

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2019, 02:51:11 am
content: ...Circuit and layout?Tim

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: December 15, 2019, 11:34:47 pm
content: Quoteright... kinda forgot about thatHere it is (down below), please be easy on me about the design, i haven't been doing this for too long.EDIT: as a temporary mod i have also added a 10n capacitor across R4, as the noise caused the chip to go into output voltage limit...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2019, 11:59:20 pm
content: Huh, why use Hall effect sensors for monitoring, when the controller has them built in (ISMON, IVINCOMP)?Trace lengths to the gates are pretty long, and unsupported by ground pour. Switching loop could be tighter. Could definitely afford to be on 4-layer build, would give better thermals too.I might guess the hiccup on the edge is measurement error, and not necessarily in the actual circuit. That's not exactly a good thing, as it means you've got at least as much common mode noise dropped across ground. (I hope this thing is going in a filtered metal box?)Tim

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: December 16, 2019, 12:15:09 am
content: Quotethat is just a carry over from an older design. i just copied over the temperature and current sensing parts to this. But using the internal ones is certainly a better idea.Four layer is a good idea. This PCB has only two, because i wanted a quick and cheap-ish prove of concept. The issue is that i can't make it any wider because of the enclosure i have for it.Quotei measured that edge on the pins of the mosfet, with a ground spring if thats what you meanThe thing will go into an aluminium pipeThanks for the quick reply

author: Someone
author_rank: Super Contributor
timestamp: December 16, 2019, 09:29:20 am
content: Quote from: TmaxElectronics on December 16, 2019, 12:15:09 amSo you used a single ended probe? Check what the signal looks like when you probe that ground point.

author: StillTrying
author_rank: Super Contributor
timestamp: December 16, 2019, 10:57:24 am
content: Quote from: TmaxElectronics on December 15, 2019, 01:50:58 amThat gate voltage looks quite normal or a least usual to me.Switching fully on in 40ns is quite fast.

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: December 16, 2019, 03:27:12 pm
content: QuoteI did consider that, too. but with the chip turning off the PFet, that is used to do PWM dimming and protect the led from faults, i think it might actually be an issue.If i understood the datasheet correctly these low side glitch and the high side one must be related, as i don't see any condition in which only the PFet is turned off.I will try to measure all of the inputs to the chip again later. maybe i will find something that way.

author: Marco
author_rank: Super Contributor
timestamp: December 16, 2019, 03:32:12 pm
content: Why not just short the low FET out temporarily and measure your efficiency?

author: StillTrying
author_rank: Super Contributor
timestamp: December 16, 2019, 05:29:44 pm
content: The first big dip in the gate voltage will be from the drain voltage starting to fall very fast, I don't think there is an unusual 'glitch' there myself.Using a 5uH6 inductor instead of the data sheet's 22uH might explain some of the 92->80% efficiency loss due to more switching?

author: TmaxElectronics
author_rank: Regular Contributor
timestamp: December 17, 2019, 04:52:31 pm
content: OK so a quick update:i have changed the inductor to a 11.2uH one (just two of the old ones in series) and reduced the switching frequency to ~150kHz.That improved efficiency to ~ 87% at 150W and ~85% at 210W (max LED power) with 25V input, making my circuit dissipate 10W lessHowever there is still the same waveform on the NFet (which might be correct after all), and it still switches off the PFet for around 300ns every cycle, not sure if that is really an issue though.At this time i can live with the efficiency (and the mosfets too) so thanks for the help everyone (more tips are of course appreciated).right now i can run the lamp with 10A converter input current limit (not actually limiting at 25V in), with the inductor getting to 75°, the bottom side mosfet at 89° (right on the tab) and the LED at 74° (C, of course). and if i improve the thermal resistance between the PCB and the heatsink, I think I can improve those temps a little more