Svart
Well-known member
anyone use photovoltaic isolators for HIGH side Nchannel drive? I'm thinking about designing a switch for 500VDC pulses for a input circuit protection test jig.
I'd like for it to be solid state, no relays/contactors/switches(directly inline with the pulse at least..).
Can't use SCRs or TRIACs because I do want this thing to turn off at some point in time.. :green:
Pchannels are out of the question due to the voltage levels.
This leaves only NIGBT or NMOSfets which need to reference a gate voltage to their sources. Those of you who have worked with these understand that you have to drive these to a Vgs of around +5v(preferably+15v) relative to the source to get them to turn on. This means that you have to hit the gate with a HIGHER voltage than that you are supplying to the drain. In this case it would be 500v+15v=515VDC.
The only a few ways to do this.
Charge-pumping the gate but I don't want to have to build a filter to ged rid of the ripple on the gate as I'm pumping it. I also don't want to have to build a pulse generator because this is supposed to be a quick test.. but I digress.
Transformer coupling is the same, I need a continuous pulse train and a filter network. I like simple so this is out as well.
Piezo coupling.. strange and somewhat exotic.. a piezo material transfers the energy as mechanical waves and then converts it back to an electrical pulse train.. again we need pulses..
So that leaves me with photovoltaic couplers. For those who don't know, these are little buggers who create voltage from light. Like little tiny solar cells they take an LED's light and turn it into a voltage, usually around 8-10v. This is enough to charge a gate enough to turn it on. I've used these buggers plenty of times as low side drivers that needed isolation but never on high side or at such voltages.
I plan on referencing the part to the 500VDC so that it sees this voltage as it's "ground" and thus outputs above this voltage to the gate. I'll need to ensure that the gate voltage never gets above 515vdc so there will be a zener protection circuit and a pulldown resistor to 500vdc.
anyone interested so far?
I'd like for it to be solid state, no relays/contactors/switches(directly inline with the pulse at least..).
Can't use SCRs or TRIACs because I do want this thing to turn off at some point in time.. :green:
Pchannels are out of the question due to the voltage levels.
This leaves only NIGBT or NMOSfets which need to reference a gate voltage to their sources. Those of you who have worked with these understand that you have to drive these to a Vgs of around +5v(preferably+15v) relative to the source to get them to turn on. This means that you have to hit the gate with a HIGHER voltage than that you are supplying to the drain. In this case it would be 500v+15v=515VDC.
The only a few ways to do this.
Charge-pumping the gate but I don't want to have to build a filter to ged rid of the ripple on the gate as I'm pumping it. I also don't want to have to build a pulse generator because this is supposed to be a quick test.. but I digress.
Transformer coupling is the same, I need a continuous pulse train and a filter network. I like simple so this is out as well.
Piezo coupling.. strange and somewhat exotic.. a piezo material transfers the energy as mechanical waves and then converts it back to an electrical pulse train.. again we need pulses..
So that leaves me with photovoltaic couplers. For those who don't know, these are little buggers who create voltage from light. Like little tiny solar cells they take an LED's light and turn it into a voltage, usually around 8-10v. This is enough to charge a gate enough to turn it on. I've used these buggers plenty of times as low side drivers that needed isolation but never on high side or at such voltages.
I plan on referencing the part to the 500VDC so that it sees this voltage as it's "ground" and thus outputs above this voltage to the gate. I'll need to ensure that the gate voltage never gets above 515vdc so there will be a zener protection circuit and a pulldown resistor to 500vdc.
anyone interested so far?