PV MOSFET driver reliability

=?UTF-8?B?TWlndWVsIEdpbcOpbmV6?= wrote...
>
>El 18/02/2015 a las 13:41, Piotr Wyderski escribió:
>> 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