Cancelling source inductance - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2021, 09:18:26 am
content: So, I don't know that I would recommend this for general purposes, power electronics etc., but, just to show it's a thing that can be done --Equivalent circuit:Photo:Note the gate drive is looped under the source pin. This acts as one turn into two, which because of poor coupling, isn't going to be perfectly 1:2 in terms of EMF. Probably I have the gate loop just a little oversized compared to ideal, see below. (Don't mind the "sweaty" look of the 1 ohm resistor, it measures fine.)Waveforms -- no drain power:Ch1: drain, x100, invertingCh2: gate, x10The slight underdamping suggests the total gate drive path is around 50nH (resonant with the gate's ~3nF zero-bias equivalent), limiting peak current to about 4A. Probably a smaller loop would do, which will couple less from the source, but speed up the direct gate path. (The 30A gate driver is severe overkill, I just had it on hand for this test.)800V drain power:Note, the output is pulsed, because there's a pulse coupling network not shown in the schematic. The result is not quite differentiated, as the time constant is comparable to the falling edge rate; but you can see it's slowing down a lot as the transistor approaches saturation, which is why the pulse is much less than 800V peak.The apparent 40V peak at the gate pin, is not what's actually applied, G-S, internally. The source pin itself measures about 20V, as best I can probe externally -- with a 10x probe held vertically, making contact where the pin leaves the component body. Probably there's a similar amount dropped across the bondwires inside, there's just no way to see it.It would be interesting to solve for what the best compromise is, given some source inductance, some maximum coupling to it, some gate capacitance and drive resistance, solving for the gate loop, coupling, and how much benefit results. Sounds pretty doable, just don't feel like cranking it through at the moment.Tim

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 21, 2021, 09:44:24 am
content: What are you using to measure the drain that requires 50Ohm input in ch1?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 21, 2021, 10:22:04 am
content: Do I understand it correctly? : it adds some positive feedback to speed up the switching?If so, 'proportional base drive circuit' comes to mind.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 21, 2021, 10:56:00 am
content: And I did the opposite once in synch buck, deliberately adding 10mm of source inductance to slow down switching. Yes, measured reduced switch node voltage peaks and EMI. Is this a good way to slow down a mosfet, as other ways exist, too? I don't know, but it did the job. Nowadays I just try to do my best minimizing the loop and if that doesn't work out, add RC snubbers and/or increase gate drive R.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2021, 11:07:26 am
content: Quote from: MasterTech on February 21, 2021, 09:44:24 amThe (unspecified) pulse coupling network.It's not real easy to see in the photo, but there's:Drain pullup: 25k to +800VSeries 1+1nF (C0G 630V)25R to GND (TO-220 thin film, hiding behind the carbon comp)450R series (the 470R is a bit high, the 6.8k's in parallel bring it down to 450)BNC output into 50R scopeThere's also a 20dB power attenuator before the scope.So, the scope is terminated to read x100 correctly, and because the drain is pulling down, cap coupled, the pulse is negative-going. The 50ns time constant means the initial 10 or 20ns of the falling edge is pretty much equal between Ch1 and the actual drain, but it's increasingly out of sync from there.Quote from: doktor pyta on February 21, 2021, 10:22:04 amYes, indeed.Difference is this is in series (albeit, in series with still a very low impedance!), the ratio is small -- if N1:N2 = 2 but k = 0.5, the EMF ratio is 1:1 (or uh, something like that, I forget if there's a formula between k and EMFs..) -- and most importantly, the inductance is limited (ideally, very small). For the BJT case, inductance is high (pulse transformer), so base and load currents are tightly coupled and it gets driven in a constant-hFE mode.You could indeed still consider the "hFE" of a MOSFET -- although it's extremely high at DC (much like the gain of an opamp), it's rather low at HF, and indeed you could define an fT this way, i.e. where |Zg| = 1/|Yfs|.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2021, 11:20:19 am
content: Quote from: Siwastaja on February 21, 2021, 10:56:00 amIndeed, in power we tend to slow things down, because EMI.The answer depends; as EMC always does of course. The two prongs are voltage and current rate:Mutual source inductance (i.e. between gate and load loops) determines current rate.Miller effect determines voltage rate.You may find that, thanks to Crss being so small for modern devices at high voltages, the voltage is what kills you -- maybe include an R+C across D-G if you anticipate that being a potential problem.Similarly, we might use ferrite beads in various locations:Gate: can limit swing rate, but can also saturate under peak drive conditions so that switching is still fast but oscillation is dampened.Drain: likely saturates under load current, but provides damping between the, whatever load impedance is when off, and Coss.Source: acts to introduce delay and damping, during the low current portion of turn-on/off.One project I made, I used a one of those arch shaped shunt resistors, in the source circuit of a push-pull converter. Damn thing adds like 20nH, delaying turn-on by fully hundreds of nanoseconds (at full load i.e. ~20A peak)! Fortunately I could afford to drop Fsw on that, so it's not an inefficient mess. So glad I have a better collection of low value SMT chip resistors now.Tim

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 22, 2021, 11:22:31 pm
content: Hi Tim,I came across such device: LMG341xR050.It might interest You taking into account Your last experiments.