Problems with a back-to-back MOSFET dimmer for 230VAC

Question

I've been trying, unsuccessfully, to dim a 60W incandescent light bulb with two back-to-back MOSFETs, similar to this web page:

http://easy-electronics4u.blogspot.co.uk/2012/02/switch-ac-loads-using-mosfets-as-relay.html

My circuit looks like this: (there is also a 2A fast blow fuse and a 7D471K MOV on the mains input)

Below is the output from my scope:

• Channel 1: Live_230VAC
• Channel 2: DIM_GPIO
• Channel 3: Drain of Q5
• Channel 4: Source of Q5 and Q6

Q6 gets burning hot, Q5 stays cool. The lamp (obviously) flickers as we are only getting every other half wave. Thinking about the circuit, I feel that it will be impossible to get a Vgs voltage that is higher than the voltage at the source pin, since the source pin will see the peak of the 230VAC?

Is there a way (preferably simple & cheap :)) of solving this kind of double MOSFET dimmer?

Edit 1: Added revised schematic as per suggestions from @RoyC for consideration (and so that RoyC doesn't have to click my pasteboard.co link)

Answer 1

On the right of the diagram swap live and neutral. This is a safety thing although you should never assume neutral is anywhere near earth it is likely to be lower than live. Now your switch is floating on the neutral line not the live.

The drive voltage for your gates has to float referenced on the source connection between Q5 and Q6. Short out R19.

Remove the bridge rectifier it is not needed instead connect D7 to the point that you have labelled in your current diagram as Neutral_230VAc. The ground end uses the body diode of Q6.

Here is a rough diagram component values same as yours

Answer 2

It appears that your MOSFETs are not driven from a solid (i.e. low impedance) source.

Your driving voltage of 15 V, regulated by D8 and buffered by C5, is passed to Q5 and Q6 via the optotransistor, a 1 kΩ resistor and back to GNDA via a 220 kΩ (!!!) resistor. This means the resistors alone introduce 221 kΩ into your gate driving circuit. I wouldn't trust this configuration to properly drive a MOSFET's gate-to-source capacitance. It is good design practice to keep the impedance of a MOSFET driving circuit well below 50 Ω to avoid driving the MOSFET in the linear region, i.e. neither fully off nor fully on. I don't see a reason for putting a 220 kΩ resistor between the MOSFETs' sources and GNDA.

In order to fix this circuit, I suggest the following:

• Make sure you don't kill yourself with the mains voltage, I will not cover this very important part of the job here, because it can't be done Q&A-style - so please ask a professionally trained person to assist. At the very least, use a safety transformer to prevent yourself from getting a shock or blowing your scope when testing.

• Think about the driving circuit: Do you get a stable supply of 15 V? Do you get proper gate voltages at Q5 and Q6? What's the job you're trying to achieve with R19? How are the MOSFETs sources bouncing with regard to GNDA?

I know I'm using some questions to answer, but I am sure that these questions are of better help than just answering with a snotty "gate driving circuit is flawed".