Forums

PV MOSFET driver reliability

Started by Piotr Wyderski February 18, 2015
Den torsdag den 19. februar 2015 kl. 02.05.30 UTC+1 skrev John Larkin:
> 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!
yeh, a table of fusing current says a 1.8mm copper wire will melt in 10 seconds at that current
> > They also do impossible heat sinking, like testing the fets in > flowing, boiling, unspecified liquids.
I seem to remember an app note on how the testing is done the liquid is some DuPont fluorocarbon with a boiling point of 25'C
> > Claiming 375 watts dissipation in a TO220 is criminal. >
depends on the time frame :) -Lasse
On a sunny day (Wed, 18 Feb 2015 15:41:43 -0800) it happened John Larkin
<jlarkin@highlandtechnology.com> wrote in
<sl8aeatsuttf8kfbordo5he94vcr3g9fhi@4ax.com>:

>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.
mm, he could bridge the MOSFETS by 2 schottkeys Schott.. U1 -----|>|-------- | | | | | | d s | g |------------- out | | | U2 -----|>|-------- | | | | d s g | This will limit the voltage across the FETS at all times to a few hunderd mV, at 50 A to a few watts, and gives time to switch.
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.
This may have been true years ago, but IXYS has made a nice selection of parts available, and Mouser has a good stock of them. Table 3.6 in AoE-III, page 210, is quite useful, with parts from Infineon, Clare, Supertex, NXP, Vishay and IXYS. -- Thanks, - Win
Piotr Wyderski wrote...
> > PS. I use the "open==conducting" meaning, if it > should be the other way around, I'll adapt.
Well, that's pretty unconventional, SFAICT. If you really do need large-die n-channel parts (consider IGBTs intead?), there are multiple ways to get reliable gate-drive power, as have been spelled out, but also including an attractive optical power source, see AoE-III, Figure 3.107, with 0.5A gate-drive for high-speed switching. Table 3.5 has these isolated switches working to 1800A pulsed, or 100A continuous. But beasts like this are likely ridiculously over-spec'd for your application. -- Thanks, - Win
Joerg wrote:

> 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. >
b'. An isolated, low mA-range, power supply of any kind and Si826x. The part is cheap, easily available, comes in a much better package than similar ADI parts, has an UVLO and a strong gate driver. I didn't like this option because of the need for a separate supply, but this thread has really taught me something new. Thank you very much! Best regards, Piotr
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. -- Thanks, - Win
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.
To review, in a robust application, you want fast switching time, well under say 5us, and under 1us even better. This is so you can turn the switch on and check to see if it's at its proper low Vds(on) voltage. If it's not, then you have a load fault, a short, and you want to turn it off. Preferably you want to do all this within say 10us, see the 20 and 35us junction-temp limits calculated above. There are driver ICs conveniently designed for this purpose. We discuss this in AoE III, e.g., in the Logic Interfacing chapter, page 847, Figure 12.87, which shows an opto-coupled driver IC with a DESAT function. This is an industry-standard method for dealing with short-circuit faults in a robust way. In Figure 12.87 we suggest Avago's HCPL-316J and ACPL-332J. Both of these driver ICs include an optically-isolated FAULT feedback path, which your driving logic can use to deal with the fault. One way to deal with this would be to retry the power switch after a short delay to let it cool, say a few seconds. If there's still a failure after a few tries, a general FAULT condition is signaled and the user can be notified to fix the problem. -- Thanks, - Win
On 2015-02-19 3:53 AM, Piotr Wyderski wrote:
> Joerg wrote: > >> 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. >> > > b'. An isolated, low mA-range, power supply of any kind and Si826x. > The part is cheap, easily available, comes in a much better package > than similar ADI parts, has an UVLO and a strong gate driver. >
Those are nice. Just keep in mind that they have a lowish UVLO. Also, they seem not to be widely stocked but for a one-off case that's ok.
> I didn't like this option because of the need for a separate > supply, but this thread has really taught me something new.
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.
> Thank you very much!
As a SW guy once put it, we are all here to serve :-) -- Regards, Joerg http://www.analogconsultants.com/
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" :-) -- Regards, Joerg http://www.analogconsultants.com/
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! -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com