Mosfet AC SSR

[Zero999]

I think eneuro is talking about using a separate isolated power supply and the TLP351F to drive the MOSFETs.
http://www.toshiba.com/taec/components2/Datasheet_Sync/201410/DST_TLP351F-TDE_EN_22634.pdf

This will enable a very high switching speed at the expense of more complexity.

How often do you intend to switch the relay on/off? The VOM1271T  driver will be far too slow for any kind of power control.

Steady state wise, I think it'll be fine without a heat sink providing it's not in a hot environment. Why do you feel the need to avoid a heat sink? Is there not room in the enclosure for one?

[eneuro]

How often do you intend to switch the relay on/off? The VOM1271T  driver will be far too slow for any kind of power control.

Just trying to estimate how slow it could be with those mosfets choosen by @OC71 
We need such amount of charge AC mosfets switch gates from typical values for irfp4668pbf :
dQ= 2*Qg= 2*162nC= 324 nC
At 30mA VOM1271T says about 47uA gate current, while I=dQ/dt -> dt= dQ/I, so dt= 324nC/47uA ~7 ms  At 50Hz mains we have 20ms period, so it looks like full switch on could take 30% of 50H z period time ? 
Could someone verify this? maybe I've made some mistake there?

There is also something else VOM1271T open circuit voltage at 30mA iF is maximum ~9V, so we can not get Vgs even 10V? 

I was thinking it could be usable when we add zero crossing, but 7ms turn on and Vgs<10V, I do not know, I do not like it, so that is why I dismissed it.

[OC71]

@ Hero999,
It is for on/off control of a heater (I think it is called bang bang).
 --There will be no more than 30 per minute. Could that be considered steady state?
 --If I use the TLP351F,will I need to isolate its power supply?
 --I am trying to do it without a heat sink mainly just to see if it is possible.

@ eneuro,
With the switching speed you have calculated, can I use the VOM1271 for the
30 per minute on/ off control?

Is this graph appropriate for determining the suitability of a driver, or am I
Wrong on that ( by the way, the labels for the 10ms and 1ms curves appear to be
reversed)

[Zero999]

I think it'll be Ok.

When the MOSFETs turn on, at the peak mains voltage, the voltage across them will not be the full 170V. The load will also drop some of the voltage too. They will also not always turn on/off when the mains voltage is at it's peak. Sometimes the voltage will be nearer zero. It will come down to chance.

Yes, if the TLP351 is used, it must have an isolated power supply with the same level of separation as you would the mains, as the negative rail is connected to the MOSFETs which are connected to the mains.

[OC71]

Will this schematic provide the necessary isolation?

[Zero999]

Will this schematic provide the necessary isolation?

Yes, as long as you treat everything connected directly to the secondary of that transformer with the same precautions as mains.

[OC71]

Thanks hero.

[eneuro]

Will this schematic ...

Remember that gate driver output might need resistor to limit this current, especially when you have no any voltage regulator on driver PSU-TLP351F has Vmax 27V if good remember, IRFs gate +/-30V, so... have you ever looked at sample Mosfets gate driving circuits?

[OC71]

@ eneuro,
Can you please clarify, I do not understand what you are saying?

[Zero999]

No I don't understand either.

The MOSFET gate voltage will be below the peak AC voltage on the secondary of the transformer which should be below 20V for a 12VAC transformer.

[eneuro]

The MOSFET gate voltage will be below the peak AC voltage on the secondary of the transformer which should be below 20V for a 12VAC transformer.

Maybe power consumption in TLP351F will be high enougth to keep this gate driver "PSU" below peak AC voltage in the case of mains primary transients, but rather common thing have low current voltage regulator in PSU and gate resistors to limit gate current to levels safe for driver itself
I'm not sure which is RDSON of TLP351F internal mosfets, so adding gate resistor assuming it is quite low below 10 ohm, etc.....

[Zero999]

The MOSFET gate voltage will be below the peak AC voltage on the secondary of the transformer which should be below 20V for a 12VAC transformer.

Maybe power consumption in TLP351F will be high enougth to keep this gate driver "PSU" below peak AC voltage in the case of mains primary transients, but rather common thing have low current voltage regulator in PSU and gate resistors to limit gate current to levels safe for driver itself
I'm not sure which is RDSON of TLP351F internal mosfets, so adding gate resistor assuming it is quite low below 10 ohm, etc.....

The rectifier diodes will keep the voltage below the AC peak regardless.

And how do you know what the peak AC voltage is when he didn't even specify the secondary voltage rating of the transformer? I made the assumption it's something sensible such as 9V or 12V which shouldn't exceed the MOSFET's gate rating.