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PV MOSFET driver reliability

Started by Piotr Wyderski February 18, 2015
=?UTF-8?B?TWlndWVsIEdpbcOpbmV6?= wrote...
> >El 18/02/2015 a las 13:41, Piotr Wyderski escribi&Atilde;&sup3;: >> How reliable such devices are in real life? The most extreme, >> yet typical use case: a constantly opened N MOSFET (i.e. it >> should be closed only during exceptional scenarios), which implies >> constantly energized LED inside the coupler. For many^2 years. >> >> Is it an obvious no-go? One alternative is a transformer-coupled >> full wave rectifier driven by an oscillator I already have. Seems >> to be bulletproof, as there is not much to go wrong. The required >> isolation voltage is <50V, 1ms on/off is OK. >> >> A PV coupler is such a simple solution. Deceptively simple? >> >> The same question about regular optocouplers: do you use them >> if reliability of the device itself is important? >> >> Best regards, Piotr > > May be I missed something: if "opened" means "not conducting" > (as I interpret it, similar to relays or switches), the > PV coupler only will be powered in that exceptional scenario. > If you say "opened" thinking of a tap, pleaze ignore this message.
Whatever, is Piotr aware of depletion-mode MOSFETs? If he needs constant gate drive, removing it for a short interval, then change the MOSFET type and enjoy short drive intervals. -- Thanks, - Win
Piotr Wyderski wrote:

> How reliable such devices are in real life? The most extreme, > yet typical use case: a constantly opened N MOSFET (i.e. it > should be closed only during exceptional scenarios), which implies > constantly energized LED inside the coupler. For many^2 years. > > Is it an obvious no-go? One alternative is a transformer-coupled > full wave rectifier driven by an oscillator I already have. Seems > to be bulletproof, as there is not much to go wrong. The required > isolation voltage is <50V, 1ms on/off is OK.
Do you truly need isolation, or do you just need to operate a FET switch at some DC voltage? I have a device I build to switch power on a set of servo amps, up to 160 V. I have a capacitively-coupled voltage doubler that creates gate bias on the series-pass FET. I have a small, logic-level FET that switches the DC supply voltage at 1 KHz or so, and drives the capacitively-coupled input to thwe doubler. it gives a controlled ramp-up of the output voltage. Jon
On 2015-02-18 8:08 AM, Piotr Wyderski wrote:
> Joerg wrote: > >> Can't comment on that but imagine them to be pricey. > > The price doesn't matter, as there will be at most 3 of them and > the device is not intended for mass production. A hobby project, > but must be done right. > > > Why can't you use a high-side driver with integrated charge pump? > > Because these drivers steal power from the drain and the drain > terminal is connected to a rectified, but not filtered voltage, > i.e. it would be < U_min for a considerable fraction of the duty > cycle. If you power the driver from a separate supply, then the > Micrel's 20V source to drain absolute maximum limit applies and > I cannot guarantee that there will be no such spikes. > > Such drivers are good, but not applicable here. I need a floating > gate driver and this can be achieved by photovoltaics or inductive > coupling in this form or another. >
Then it probably boils down to whichever costs less. Your PV driver or this kind which costs around $5 in small quantities: http://www.analog.com/static/imported-files/data_sheets/ADuM6132.pdf Sometimes switching speed also needs to be considered. -- Regards, Joerg http://www.analogconsultants.com/
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. Best regards, Piotr
Jon Elson wrote:

> Do you truly need isolation, or do you just need to operate > a FET switch at some DC voltage?
The isolation is just for technical reasons, there are no safety requirements. The four transistors form two pairs of the mundane source-to-source AC switches and the input is 17*abs(sin x). At least for most of the time, wild spikes can happen. And here the floating driver is inherently much safer to the gate oxide.
> I have a device I build > to switch power on a set of servo amps, up to 160 V. > I have a capacitively-coupled voltage doubler that creates > gate bias on the series-pass FET. I have a small, logic-level > FET that switches the DC supply voltage at 1 KHz or so, and drives > the capacitively-coupled input to thwe doubler. it gives a controlled > ramp-up of the output voltage.
I was thinking about that, but a tiny toroidal transformer driven at 400kHz + 2 diodes + a resistor is so much simpler... Best regards, Piotr PS. I use the "open==conducting" meaning, if it should be the other way around, I'll adapt.
John Larkin wrote:

 > You need a transformer oscillator/driver, too.

Yes, but there often is one already, so can be (ab)used.

> If I want isolated power, I generally buy a SIP dc/dc converter, > instead of making the supply out of parts. There are tons of these in > the $3 to $4 range.
Agreed, but for MOSFET driving purposes several microamps are often enough (that's how much a typical PV coupler can yield), so the driver can be an MCU pin/a logic gate. Then a series capacitor, then the primary winding. This can safely scale upto low milliamps, which is more than needed if the switching can be slow. It obviously is a half-bridge dc/dc converter, but I don't consider it to be a power supply, since the power is so miserable. Best regards, Piotr
Den onsdag den 18. februar 2015 kl. 21.42.03 UTC+1 skrev Joerg:
> On 2015-02-18 8:08 AM, Piotr Wyderski wrote: > > Joerg wrote: > > > >> Can't comment on that but imagine them to be pricey. > > > > The price doesn't matter, as there will be at most 3 of them and > > the device is not intended for mass production. A hobby project, > > but must be done right. > > > > > Why can't you use a high-side driver with integrated charge pump? > > > > Because these drivers steal power from the drain and the drain > > terminal is connected to a rectified, but not filtered voltage, > > i.e. it would be < U_min for a considerable fraction of the duty > > cycle. If you power the driver from a separate supply, then the > > Micrel's 20V source to drain absolute maximum limit applies and > > I cannot guarantee that there will be no such spikes. > > > > Such drivers are good, but not applicable here. I need a floating > > gate driver and this can be achieved by photovoltaics or inductive > > coupling in this form or another. > > > > Then it probably boils down to whichever costs less. Your PV driver or > this kind which costs around $5 in small quantities: > > http://www.analog.com/static/imported-files/data_sheets/ADuM6132.pdf > > Sometimes switching speed also needs to be considered. >
I've used regular isopower icouplers as a drivers for fets it was before they had gate drivers so two was used with the output in series to get 10V gates drive it was used as a fast disconnect for a bunch of traics, they couldn't handle a dead short for a full cycle so something faster was needed -Lasse
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. -- Regards, Joerg http://www.analogconsultants.com/
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
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. Joerg, is ~1ms long enough to thermally damage a TO247 device? > 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. Best regards, Piotr