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PV MOSFET driver reliability

Started by Piotr Wyderski February 18, 2015
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
On a sunny day (Wed, 18 Feb 2015 13:41:58 +0100) it happened Piotr Wyderski
<peter.pan@neverland.mil> wrote in <mc21am$idb$1@node1.news.atman.pl>:

>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
mm, if your application has any microprocessor with FLASH memory, then that is likely to go long before the LEDs in optos. _constantly_???? How about a relay? That can replace the MOSFET too. Reaction time? Temperature? Voltage? Budget? all depends.
On 2015-02-18 4:41 AM, 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. > > 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? >
Can't comment on that but imagine them to be pricey. Why can't you use a high-side driver with integrated charge pump? http://www.micrel.com/_PDF/mic5018.pdf This example is for logic level FETs, not standard level. -- Regards, Joerg http://www.analogconsultants.com/
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. Best regards, Piotr
On Wed, 18 Feb 2015 13:41:58 +0100, Piotr Wyderski
<peter.pan@neverland.mil> 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. > >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
SSRs and optocouplers seem to be very reliable, used prudently. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
> >A PV coupler is such a simple solution. Deceptively simple? > > >
Can you overdesign it from both ends? Design so that the proper FET state is achived with only 1/20 the rated LED current. And design so in normal operation the current is 1/2 the rated current and this will still be 10x more than needed. The low operating current will reduce the rate of degredation of the LED and you will have a large 10x margin. Mark
On Wed, 18 Feb 2015 08:36:12 -0800 (PST), makolber@yahoo.com wrote:

> >> >A PV coupler is such a simple solution. Deceptively simple? >> > >> > >Can you overdesign it from both ends? > >Design so that the proper FET state is achived with only 1/20 the rated LED current. And design so in normal operation the current is 1/2 the rated current and this will still be 10x more than needed. The low operating current will reduce the rate of degredation of the LED and you will have a large 10x margin. > >Mark >
Usually all you need to do is stay well below the LEDs rated current. Some device data sheets have lifetime vs current and temperature curves. One trick is to overshoot the LED current during transients, but back off steady-state. That gives fast switching but low average current. Sort of like this: https://dl.dropboxusercontent.com/u/53724080/Circuits/Optos/Faster_Opto_Totem.JPG -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
John Larkin wrote:

> One trick is to overshoot the LED current during transients, but back > off steady-state. That gives fast switching but low average current.
That's a very interesting idea. OTOH, I know that a low power pulse transformer + two Schottky diodes will last for eternity, as there is no wear-out mechanism. The VOM1271's datasheet says "high reliability", but what exactly is that "high reliability"? 30 years of always on with I_F=5mA? :-/ Best regards, Piotr
On Wed, 18 Feb 2015 18:37:43 +0100, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>John Larkin wrote: > >> One trick is to overshoot the LED current during transients, but back >> off steady-state. That gives fast switching but low average current. > >That's a very interesting idea. OTOH, I know that a low power pulse >transformer + two Schottky diodes will last for eternity, as there is >no wear-out mechanism. The VOM1271's datasheet says "high reliability", >but what exactly is that "high reliability"? 30 years of always on >with I_F=5mA? :-/ > > Best regards, Piotr
You need a transformer oscillator/driver, too. 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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
El 18/02/2015 a las 13:41, Piotr Wyderski escribi&oacute;:
> 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. -- Saludos Miguel Gim&eacute;nez