In the circuit that you didn't consider the power of low side, when it will work(Vgs)?

are you thinking that its a p-channel fet on the bottom ? and asking how i am driving the gate?
If so: its an n-channel.

 

........................
The reason i am not using a triac for phase angle control is that i am using the bulb as a heating element to effect PID control.
That isn't so easy with a triac.

I don't see what using a triac has to do with PID control. The output of the PID can be a voltage that controls the phase angle firing of the triac, which controls the load power.

If the thermal time constant of the load is long enough you don't even have to use phase angle control, just turn the load on and off for several cycles of the line frequency with the on/off duty cycle determining the average power, sort of a low frequency PWM.

 

I don't see what using a triac has to do with PID control. The output of the PID can be a voltage that controls the phase angle firing of the triac, which controls the load power.

If the thermal time constant of the load is long enough you don't even have to use phase angle control, just turn the load on and off for several cycles of the line frequency with the on/off duty cycle determining the average power, sort of a low frequency PWM.

I wrote PID instead of PWM.
What i meant was that i wanted to use PWM control, as i am familiar with its function and implementation.
It also makes the soft PID easier to implement using my micro-controller.

 

Below is a simulation of an SSR circuit using a 4N25 opto isolator and two back-to-back N-MOSFETs to control the load. The switching looks good at a 25kHz control frequency and 60Hz AC power frequency. The input is isolated from the output (Rsim is just to avoid a floating node error in the simulation). Power for the MOSFET drive circuits is generated using a half-wave rectifier from the 12Vac supply.

You may have to add a driver if you want to drive the opto from a micro.

View attachment 75649

Crutschow, i can see from this post you have achieved the same function i am trying to implement at the moment.

I am struggling to get the circuit working properly, my bottom fet keeps heating up very fast and i am not too sure why.
Cheers

 

Crutschow, i can see from this post you have achieved the same function i am trying to implement at the moment.

I am struggling to get the circuit working properly, my bottom fet keeps heating up very fast and i am not too sure why.
Cheers

You need the isolation of an opto coupler as shown in the referenced post.

I explained why there's high current in the bottom FET in Post #10.

 

You need the isolation of an opto coupler as shown in the referenced post.

I explained why there's high current in the bottom FET in Post #10.

Thanks for the response.
I used a battery to charge the gates and had success, the hot fet was broken and got hot no matter what. it is now replaced.
Excuse me if i am irritating with my questions, but how does the opto-isolater change things?
i understand the premise and i am using a 4n26 for my PWM signal, however i cant see how it is providing isolation.
Thanks again

 

Did you not understand by explanation in post #10 as to why there's a short in your circuit when the bottom FET turns on?
Just replace the bottom FET with a short and draw the bridge diodes to see the short-circuit current path through the bridge diodes.

Using a single diode to generate the DC removes that sneak path.

The opto isolator is needed if the circuit is directly connected to the mains. If the AC supply is from a transformer than you don't need the opto and Rsim can be a short.

 

I understood, however i accidentally used a broken fet instead of a new one, so i believed the problem persisted - it didn't.
thanks for the tips!

 

Did you not understand by explanation in post #10 as to why there's a short in your circuit when the bottom FET turns on?
Just replace the bottom FET with a short and draw the bridge diodes to see the short-circuit current path through the bridge diodes.

Using a single diode to generate the DC removes that sneak path.

The opto isolator is needed if the circuit is directly connected to the mains. If the AC supply is from a transformer than you don't need the opto and Rsim can be a short.

I have modified the your schematic to allow me to understand its operation better.

From what i can tell, the capacitor voltage bumps up in unison with the positive source voltage to maintain the VGS potential for positive cycles.
However for the negative cycle, the capacitors negative terminal becomes -34V with its positive held at 34V. There is now 68V across the divider and +13V on the gate. VGS = 13(positive gate) + 34(negative source) = 47V.
The gate is adequately positive in relation to the source to allow the mosfet to conduct, but the VGS potential has surpassed the 20V the fet can tolerate.

Is this understanding correct?
I am struggling to see how i can manage this with the fets i have.

 

Your understanding is incorrect . You need to consider which reference point your voltages are referred to.
Here's a sim showing Vgs never exceeds 7V. I chose (arbitrarily) the cap negative terminal as "0V".

 

Your understanding is incorrect . You need to consider which reference point your voltages are referred to.
Here's a sim showing Vgs never exceeds 7V. I chose (arbitrarily) the cap negative terminal as "0V".
View attachment 79386

i need to get my thinking cap on.
Thanks for replying and spending the time to simulate this.
I have tried with Tina, however i end up wasting more time that it was worth.

Im going to study my errors and see why my perceptions lead me astray.
Thanks again

 

I'm not sure what you to trying to do, but the below is another thought.

 

Problem there, Scott. FET2 doesn't switch on as its Vgs is never positive. .

 

Problem there, Scott. FET2 doesn't switch on as its Vgs is never positive. .

Normally the Vgs of mosfet can be work as +/-20V, do you think it has problem?

 

A Vgs of -20V may not damage an N-channel MOSFET, but it won't turn it on .

 

I was tried to figure out the circuit as below, but it seems my brain was twisted and miss something, now I'm untie it, I hope the positive and negative cycle all can get the voltage for Vgs, I also designed another circuit, but it only can make one side work for the mosfet.

 

A Vgs of -20V may not damage an N-channel MOSFET, but it won't turn it on .

Yes, it won't turn on, I was tried to designed many circuits to find out which kind is the match one.

 

Unfortunately the post #36 circuit can't work either . On one half-cycle NFET2 short-circuits the supply via a bridge diode.

 

Unfortunately the post #36 circuit can't work either . On one half-cycle NFET2 short-circuits the supply via a bridge diode.

Thanks Alec_t.
I'm trying not to use the isolated DC To DC, but if there is no two power source, it still has the shorted problem as you mentioned, I can't figure out the others methods now.

The question is : does the DC To DC really isolated?

 

This rings a bell to another post. How many watts is the low ohm lamp??