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PV MOSFET driver reliability

Started by Piotr Wyderski February 18, 2015
On 2015-02-20 12:06 PM, John Larkin wrote:
> On Fri, 20 Feb 2015 11:19:45 -0800, Joerg <news@analogconsultants.com> > wrote: > >> On 2015-02-19 7:36 AM, Winfield Hill wrote: >>> Piotr Wyderski wrote... >>>> >>>> Joerg wrote: >>>> >>>>> We know it's 17V and 50A but we'd also have to know >>>>> which FET you are using. That info is (hopefully) >>>>> in the datasheet :-) >>>> >>>> I haven't bought the FETs yet, but was thinking about >>>> IRFB7430PBF in TO220 or something bulkier in TO247. >>>> >>>> I naively thought that the 1524A max pulsed drain current >>>> is enough to survive even a short-circuit if disconnected >>>> in the ms range... >>>> >>>> Best regards, Piotr >>> >>> Whoa, it looks like there's a misunderstanding here. >>> If you short a strong low-resistance voltage source, >>> you actually want less current flowing, not more. >>> >>> Let's do the math. The 1524A rating of the '7430 is >>> for the case of a very low voltage across the MOSFET, >>> fully turned on, with only a volt or two of drop. >>> In fact, as the die rapidly heats, it'll current >>> limit at around 1kA, Fig 4, assuming 10V gate drive. >>> Trying to short out 17V, it would dissipate 17 kW. >>> >>> Z_thJC = dT_J(max) / Pd = 150/17kW = 0.009 &deg;C/W. >>> >>> Fig 14, Effective Transient Thermal Impedance, says >>> maximum allowed square single-pulse duration = 20us. >>> >>> That's the thermal mass calculation for the '7430. >>> >>> Let's try a higher voltage rated MOSFET (large-die >>> HV parts will limit at lower currents), say the >>> original IRF1407 version, an IRF1407PbF. It has an >>> impressive 333W dissipation rating, nearly as good >>> as the 375W rating of the '7430. But its 175&deg;C >>> current limit will be about 400A, or 6.8 kW. We >>> get Z_thJC = 150/7kW = 0.021 &deg;C/W, and datasheet >>> Fig 11 tells us the single-pulse duration = 35us. >>> So the "weaker" part can handle a full short for >>> almost twice as long. >>> >> >> With these little TO-packages it is more likely that there would be a >> bright orange flash, followed by a loud bang, plastic pieces shooting >> off in all directions, and an "amperage stench" :-) > > Where are the sound effects? I want my sound effects! > >
Phhsssseee ... tic .. tic-tic ... tic-tic-tic-tic ... PHOOF ... *BAM* No kidding, happened a few days ago. A class-D driver decided to go berserk while endurance-testing a circuit. I could get over there fast enough to turn it off. It took a lot of other stuff with it. All the fancy over-current protection in there didn't manage to protect the stage from death-spiraling. Replacing ICs on 2oz copper is a bear. This time the stench wasn't too bad though. Just a wee electric scent. -- Regards, Joerg http://www.analogconsultants.com/
Joerg wrote:

> Those are nice. Just keep in mind that they have a lowish UVLO.
They come in 3 UVLO flavors. "B" is ~9.5V and "C" is 12V. > That extra supply isn't nice but as John Wayne said, "Man has got to do > what man has got to do". It doesn't have to be regulated so even a > simple gate drive transformer could be pressed into service to make an > isolated supply. That's exactly how my test circuit works now. :-) The quickly-wound transformer sucks in terms of leakage inductance, but it can be improved. Will brag about the results. ;-) Best regards, Piotr
On 2015-02-22 10:56 PM, Piotr Wyderski wrote:
> Joerg wrote: > >> Those are nice. Just keep in mind that they have a lowish UVLO. > > They come in 3 UVLO flavors. "B" is ~9.5V and "C" is 12V. >
Good. There is a surprising number of engineers these day who take drive levels a bit lightly yet use standard-level FETs. Even manufacturers of IC aren't paying enough attention to that. Until one fine day ... tsssk ... PHUT ... *KABLAM*
> > That extra supply isn't nice but as John Wayne said, "Man has got to do > > what man has got to do". It doesn't have to be regulated so even a > > simple gate drive transformer could be pressed into service to make an > > isolated supply. > > That's exactly how my test circuit works now. :-) > The quickly-wound transformer sucks in terms of leakage > inductance, but it can be improved. >
That's why I wind most of mine on toroids. As RF engineers say, toroids don't talk. At least not much if they are of the ferrite kind.
> Will brag about the results. ;-) >
Please do so. It can help others getting out of a similar pickle, years down the road. I once explained in detail how to change the bulb in an Italian office desk lamp. The designer kind where this is difficult. Many years later I kept receiving emails from people in a similar situation because the manufacturer link I had posted back then had gone limp. -- Regards, Joerg http://www.analogconsultants.com/