Forums

PV MOSFET driver reliability

Started by Piotr Wyderski February 18, 2015
On Wed, 18 Feb 2015 15:18:58 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2015-02-18 2:44 PM, Piotr Wyderski wrote: >> Winfield Hill wrote: >> >>> Whatever, is Piotr aware of depletion-mode MOSFETs? >> >> Yes, I use them sometimes to limit the short-circuit current >> (IXYS has parts able to go as high as 16A@500V) but: >> >> 1. It is next to impossible to buy them in small quantities and they >> cost an arm and a leg. >> 2. Even if one manages to get them, their R_DS_ON is much higher >> than that of the enhancement-mode N MOSes. >> 3. The property of "start disabled" is highly desired, as >> the initial state is much better defined. >> >> What I am trying to do is a (nearly) lossless device >> for switching between two rectified out-of-phase AC >> power supplies 17V/50+A. 4 1mOhm N-MOSFETs seem to be >> more reliable than a high current relay and they allow >> me to add overcurrent protection easily. >> > > >If they have to switch while 50+ amps are flowing you better swing the >gates around darn fast. I don't know if your PV coupler can do that. >Else a transformer plus UVLO (or driver with UVLO in there and input >tied high) on the other side may be better. That would require some >serious thought.
Yeah, a fet might pass through an embarassing zone of the SOAR curve if you switch it slow. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
John Larkin wrote:

> Yeah, a fet might pass through an embarassing zone of the SOAR curve > if you switch it slow.
So it seems that the PV driver might not to be the reliability bottleneck, but the super-durable 400+A FETs themselves, if driven improperly. Thank you all very much for pointing it out. What reasonable switching speed should I aim at? Best regards, Piotr
On 2015-02-18 3:34 PM, Piotr Wyderski wrote:
> Joerg wrote: > > > If they have to switch while 50+ amps are flowing you better swing the >> gates around darn fast. > > If you mean cross-conduction, then it can't happen. > But if you want me to leave the linear region ASAP > in order not to fry the FETs, it's a concern.
The latter is what I mean.
> Joerg, is ~1ms long enough to thermally damage a TO247 device? >
Probably it causes damage. You'd have to look in the SOA graph of your FET. It'll tell you how long it can spend in the linear region at around 50 amps and cruising through your voltage range. I'd be surprised if 1msec were allowed.
> > I don't know if your PV coupler can do that. > > It can take long (<1ms) to make the FET fully on, > but the gate discharge circuit can be fast. >
It has to be fast in both directions if there is a tight SOA limit. But only the datasheet will tell. -- Regards, Joerg http://www.analogconsultants.com/
On 2015-02-18 3:49 PM, Piotr Wyderski wrote:
> John Larkin wrote: > >> Yeah, a fet might pass through an embarassing zone of the SOAR curve >> if you switch it slow. >
Oh yeah, and it can go with an impressive kablouie and lots of smoke. I just had a similar event, an UVLO didn't come and rat-tat-tat .. *POOF*
> So it seems that the PV driver might not to be the reliability > bottleneck, but the super-durable 400+A FETs themselves, if driven > improperly. Thank you all very much for pointing it out. What > reasonable switching speed should I aim at? >
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 :-) -- Regards, Joerg http://www.analogconsultants.com/
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
On Thu, 19 Feb 2015 00:49:25 +0100, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>John Larkin wrote: > >> Yeah, a fet might pass through an embarassing zone of the SOAR curve >> if you switch it slow. > >So it seems that the PV driver might not to be the reliability >bottleneck, but the super-durable 400+A FETs themselves, if driven >improperly. Thank you all very much for pointing it out. What >reasonable switching speed should I aim at? > > Best regards, Piotr >
The fet data sheet should have a SOAR (safe operating area) graph, which tells you how much time you can spend at a given voltage*current point before the silicon melts. To be safe, switch fast, as in very roughly 10 microseconds. That might imply a modest amount of gate current, 10s of mA maybe. You've got to figure out the total gate charge required, and the time you have to switch, to get the current. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Den torsdag den 19. februar 2015 kl. 01.15.50 UTC+1 skrev Piotr Wyderski:
> 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
afaict I will barely do 50A@17V for 1ms won't even do 2A@20V for 10ms -Lasse
On 2015-02-18 4:15 PM, 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. >
This one is quite impressive: http://www.irf.com/product-info/datasheets/data/irfb7430pbf.pdf Figure 10 shows the SOA. For 1msec 17V is at 10-20A but if the load is resistive you won't have that much while the load is zero. If capacitive and discharged you might exceed the limit. Then it may be a close call. I'd try to switch it faster, like in 100usec.
> I naively thought that the 1524A max pulsed drain current is > enough to survive even a short-circuit if disconnected in the ms range... >
Those are usually just marketing numbers :-) Anyhow, a millisecond is a long time in FET-land. -- Regards, Joerg http://www.analogconsultants.com/
On 2015-02-18 3:23 PM, Piotr Wyderski wrote:
> Joerg wrote: > >> http://www.analog.com/static/imported-files/data_sheets/ADuM6132.pdf > > Yes, I've already used iCouplers in my previous project. > Great parts in terms of speed, reliability and isolation, > but the 8-pin ADuM5241 eats 70mA when idling, which is insane. > The price of its internal simplicity -- no feedback channel. > >> Sometimes switching speed also needs to be considered. > > Yes, and then PVs are not an option. They are sloooow... > > Best regards, Piotr >
Then youhave two options that I can see: a. Use a gate drive transformer but build a reliable UVLO circuit on the high side. It must come on with a snap once you exceed 10V or so and when it drops below must instantly short the gate to ground. That will take a little engineering but should be fun. b. Next is the lazy method but quicker to develop. Buy a little iso module that converts 5V or whichever crircuit supply you have (or your 17V) and make a stable isolated 12V on the high side. Look over your shoulder to make sure nobody sees you doing this, hardcore analog guys can take someone's man card away for that. Place a sturdy gate driver with Schmitt input up there, Micrel or something. Then add an optocoupler to ferry your command signal across. That's pretty much it, should still be under $5, just not as elegant as method a. -- Regards, Joerg http://www.analogconsultants.com/
On Wed, 18 Feb 2015 16:37:25 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2015-02-18 4:15 PM, 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. >> > >This one is quite impressive: > >http://www.irf.com/product-info/datasheets/data/irfb7430pbf.pdf > >Figure 10 shows the SOA. For 1msec 17V is at 10-20A but if the load is >resistive you won't have that much while the load is zero. If capacitive >and discharged you might exceed the limit. Then it may be a close call. > >I'd try to switch it faster, like in 100usec. > > >> I naively thought that the 1524A max pulsed drain current is >> enough to survive even a short-circuit if disconnected in the ms range... >> > >Those are usually just marketing numbers :-) > >Anyhow, a millisecond is a long time in FET-land.
IR is infamous for claiming absurd currents and powers. Imagine a TO220 conducting 195 amps DC through its source lead! They also do impossible heat sinking, like testing the fets in flowing, boiling, unspecified liquids. Claiming 375 watts dissipation in a TO220 is criminal. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com