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GaN FETs for 55V to 13.5V Synchronous Buck?

cupoftea

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Hi,
We need to do two Buck converters, 17-32Vin, 13.5Vout, 25A out.
Also, 17 – 55Vin, 13.5Vout, 50A out.

I looked at GaN FETs for this, thinking that even if a little more expensive, their far lesser heatsink requirements would mean the overall solution would be cheaper. However, not so.

However, would you agree that the reverse recovery problem of Si FETs with the 55Vin spec (above) will mean GaN will indeed be cheaper? When the top FET switches on into a bottom Si FET, there is a big reverse recovery spike, (especially when Vin is 55v) and it’s the top FET that gets heated up most by this current spike. So would you say, for the 55Vin spec above, a GaN FET solution will indeed be cheaper?

As you know, the bottom FET of a sync Buck shouldnt be GaN, since the "Internal diode" drop is too high , and the bootstrap cap thus gets overcharged, and damages the driver IC. But the Top FET can be GaN.
 
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Thanks, i believe for virtually any position, the GaN FETs (with antiparallel Schottkys) for Synch Bucks, give a more pricey solution than Si (with antiparallel Schottkys) , but perhaps GaN is a tad more efficient.......

************************^^^^^^^^^^^^^^^^^^^^^^******************************

****.....Also, do you know why the LM5143 (Synch Buck controller) was used for this Synch Buck spec (PMP31210 ref design)
of 20-58vin and 13.5Vout and 50A out?
Surely they should have just used the same_priced LM5119 instead?

The LM5143 has a max gate drive voltage of just 4.5V, meaning they are restricted to logic level FETs and their Rds(on)
is much higher at the 80V rating needed.
The LM5119 has a gate drive voltage up to 15V...meaning far lower Rds(on) FETs are available , and so FETs would be much cheaper.

Unfortunately we dont have an official company address, and so cant ask on the ti.com forum.
You would think they would advertise products in their suitable useage case, rather than in a totally unsuitable case?

I mean, 58Vin Synch Buck, with 700W output, and they are using a gate drive of 4.5V ??? When they could have gone with 12V gate drive for
same price solution.???

Maybe the SMPS is operated in sub zero temperature buildings and the extra heat dissipation is useful? -only plausible explanation.

PMP31210 Ref Design:

LM5143:

LM5119:
 
Yes i agree, but there are certain decisions that dont come under that...eg, you are doing an "offline converter for 500W, so you are not even going to consider a flyback".
"The Logic threshold FET for 700W with 58vin" decision is similar to that.

The only real reason to use logic level FETs in an SMPS is that your Vin is below say 7V, and you dont want to bother with a bias booster so you just put up with logic level FETs. As you know, people are not using logic level FETs because of any advantage of them. They have no advantages over "normal" threshold FETs. Especially in a synchronous buck, logic level FETs are a poor choice, because with their low gate threshold voltage, they may get spuriously turned ON by the other FET turning ON and pulsing a current spike through the drain gate capacitance. Hot logic level FETs have a ridiculously low VGS(th)......as you know, they are a very poor choice, and one that you "put up with".

Not least is the fact that if you choose a logic level gate driver, then you are restricted to only logic level FETs...so you likely end up paying more for your FETs, and being more susceptible to FET nil-stocking.

Also, how does a logic level hi side gate driver supply (bootstrapped) in a synch buck chip ever do "UVLO" of the driver?...as you know...it cant really do it......when VGSth is that low you cant really do it....if you have a 12V gate driver, then you can have it "UVLO out" at say 7V.

Logic-level devices, also, have higher gate charge than standard-level parts of similar V(BR)DSS and RDS(on)
 
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...Thanks, though GaN is "off the menu" for the western world now, as China has put a ban on Gallium exports to the West.
Infineon can get them from Canada now.



But hey why suffer
 
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Thanks, does anyone know if there is a way to find NFETs for Synchronous Bucks?...How to search them?....As you know, a NFET for a Sync Buck must have Cgd at least 100 times less than Cgs. There is no way to search based on Cgd or Cgd/Cgs.
We need them for 17-32VIN, 13V5 OUT, 30A out, and for 17-58Vin, 13v5 out and 50A out.
We also need 4V5 gate drive.
 
Thanks, so anyway, for Sync Buck Bottom FET, Cgd must be << Cgs. But as we know, if the d(Vgd/dt) is high then Cgd appears to be higher since Cgd(apparent) = i.(d(Vgd)/dt). So yes, its not just Cgd that we must look at...but Qgd. And we must review Qgd at the actual Vin. The higher that the Buck's Vin is...the lower must be the ratio of Cgd/Cgs.
In the attached sim we see how the ringed capacitor being lower reduces the spurious turn-on of the low side FET.

And of course, tying in with the top post......Logic level FETs are much more susceptible to this spurious turn-on, simply because their Vgs(th) is so much lower. And when logic level FETs are used in Synch Bucks with Vin's > 30v, then things just get worse and worse.

Basically if you are doing a Synch Buck with Vin > 30V, then you really want to be throwing logic level FETs into the Bin. They are way to
susceptible to spurious turn ON when the top side FET turns ON.
 

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