Controlling Vgs of a MOSFET with Zener Diodes

[Rey]

Yep, I'm still working on that flyback project..

I settled on using a 555 timer to drive the flyback, as a ZVS driver will destroy my rectified flyback.

My problem here is that in order to fire the MOSFET, the grounds of the 12 volt supply (To the 555) and the DC output of my variac (The voltage that is actually driving the flyback) have to be common. This introduces another problem though: the drive voltage cannot exceed the Vgs limit (30 volts in this case) of the MOSFET, which is incredibly low on most MOSFETS (Care to show me any that have higher Vgs limits?).

I heard that you can use two Zener Diodes, with the cathodes paired, between the gate and source of the MOSFET, to shunt excess voltage. I need to know if this will work, and, are there Zener Diodes that can handle what I am trying to use them for?

 

[smanches]

Zeners on gates are for shunting transients, not controlling supply voltage. Can you post a schematic. I don't understand where your voltage problem is coming from sharing a common ground.

 

[Sceadwian]

They could be for controlling supply voltage, if the switching speeds required aren't high and the gate capacitance isn't overly much a resistor to keep the zener from blowing up is all you need to add. But then the RC time constant of the gate/resistor affects switching speed. For transients in that case you'd still need another zener a few volts above the first one with no resistor to 'take one for the team' in the case of transients. The resistor that protect the first zener would also limit it's protection ability. No reason a zener couldn't do both really, again as long as the parasitics weren't too much.

 

[Rey]

They could be for controlling supply voltage, if the switching speeds required aren't high and the gate capacitance isn't overly much a resistor to keep the zener from blowing up is all you need to add. But then the RC time constant of the gate/resistor affects switching speed. For transients in that case you'd still need another zener a few volts above the first one with no resistor to 'take one for the team' in the case of transients. The resistor that protect the first zener would also limit it's protection ability. No reason a zener couldn't do both really, again as long as the parasitics weren't too much.

It does require a high switching speed, in the range of 11 to 40 khz. Are there any other options?

 

[Sceadwian]

You have a common ground and a 12 volt supply, what's the problem? 12V should fully turn on most mosfets. As smanches request can you post a schematic at this point?

 

[Rey]

I think you misunderstood the problem I was having. I thought that there was too much voltage turning on the MOSFET. I know that this makes no sense, so forget about it, because I realized that I was totally wrong.

The real issue here is that the MOSFET is unable to avalanche all the induced voltage when it gets turned off, so I'll just make a new thread to ask for help about this.