driving power mosfet

[quioui]

Hi, I have a question about controlling power mosfets. Lets say I have some electronic circuit that is fed with a commercial switching mode power supply and I have a high voltage, full wave rectified AC signal (220V line). The ground of the electronic circuit and the diode bridge should be the same in order to control the mosfet but how? I tried connecting the two grounds together but a high voltage gets everywhere in the electronic circuit. What should I do?

 

[ronsimpson]

I tried connecting the two grounds together but a high voltage gets everywhere in the electronic circuit. What should I do?

'The high voltage gets everywhere'? What!

In a switching power supply the MOSFET and the PWM live on the power line. Informaiton (the error signel) goes through a opto isolator to keep the primary and secondary seperate.

I do have high speed isolators that can drive the gate of the MOSFET, but this is not often done.

 

[quioui]

Let me tell you what I am trying to do: I am trying to control the light intensity of a standard AC light bulb by pwming a full wave rectified 220V AC. The control circuit has its own power supply and it produces pwm signal between 10V and 0V. A mosfet is connected in series with a light bulb which is powered by the rectified 220V. The source of the mosfet is connected to the ground of the rectifier bridge. The problem is, the ground of the bridge and the ground of the power supply of the control circuit are different. That's why the 10V pwm signal that is fed to the gate of the mosfet cannot turn on the transistor. Vgate is 10V with respect to the ground of the control circuit but the source is at another ground (rectifier's). Connecting two grounds together does not work so I am asking how can I combine those grounds?

 

[crutschow]

Anytime you want to control something at the mains voltage you need galvanic isolation for safety. Typical ways to do this are either with a small pulse transformer or an opto coupler.

 

[quioui]

I do use optocoupler but when I short the grounds together, a high voltage (about 80V) propagates through the ground line and everything in the control circuit becomes 80V.

 

[crutschow]

I do use optocoupler but when I short the grounds together, a high voltage (about 80V) propagates through the ground line and everything in the control circuit becomes 80V.

The reason for the opto coupler is so that you don't have to connect the grounds together. Otherwise it serves no purpose. The opto coupler input common goes to the drive circuit common and the opt coupler output common goes to the common of the circuit being driven.

 

[quioui]

Oh, I see So I need to get 10V from 220V rectified signal and feed it to the optocoupler. Thank you!

 

[Mosaic]

has the OP ever considered using a (zero crossing) triac circuit as opposed to a HV MOSFET & PWM?

 

[johansen]

i was going to say this thread looked to be the same as the other one posted last week

https://www.electro-tech-online.com/threads/igbt-help-needed.127819/