Electronics > Beginners
AC switching mos-fets
Lusu:
Hi,
I'm studing a way to do trailing edge phase cutting for an AC dimmer (230V AC, absolute max. 5A load, not planned to be reached). I've found SI8751/2, see datasheet here: https://www.silabs.com/documents/public/data-sheets/Si8751-2.pdf
Particullay I'm looking at schematic from 3.2 (on page 6) and I need to find out suitable transistors for the requirements:
- must turn fully on at ~9V G-S (minimum gate output voltage of the chip)
- must handle 230V AC (well... 330V DC equivalent, so ideally >500V S-D for headroom as AC can go over 250V in normal operation)
- must handle minimum 5A without heatsink (will be inside the wall in a closed enclousure)
- must have as low as possible RDSon resistance
- THT desired, SMD will do
a) I am not familliar with such transistors, so any recommendation is appreciated.
b) I am unsure if the voltage divides between the two transistors or each of them should handle the full voltage.
c) What is the datasheet parameter that specifies the "full-on" gate voltage?
Thank you.
Jeroen3:
a) You could see if ST's MDmesh M5 can fit your specs, they have very low Rdson. But I think the voltage it tricky. Maybe you'd need more near 800V fets.
You can also look at a triac solution. The only problem for a thyristor based solution is 1 Watt per Ampere.
Perhaps for fet the STW65N80K5. But it needs 12 Vgs. Then you lose 3.5 Watts on those with 5A.
Or the STY50N105DK5, that is open at 9V, but is €20 per part...
b) Both must take the full voltage. They never switch theoretically simultaneous.
c) A graph with Vgs/Ids at certain Vds usually tells you if you are in the saturation region with minimum Rdson.
See attached.
Lusu:
Thanks for the info, I will check the recommended FETs.
I can't use triacs, because they do leading phase cutting, I want trailing edge. My circuit is already using a triac, but I want to change the solution to FETs. This will allow me to do whatever... leading edge, trailing edge, PWM (within limits)...
Jeroen3:
TK31V60X is indeed a nice part.
David Hess:
Practically any power MOSFET which meets the voltage and current requirements will work. 600 volt parts are very common for 240 volt AC applications. I did a quick survey and plenty of TO-220 parts exceed your requirements by twice the current for less than $2.
--- Quote from: Lusu on February 20, 2020, 12:05:58 pm ---c) What is the datasheet parameter that specifies the "full-on" gate voltage?
--- End quote ---
The full enhancement voltage is given as a parameter for specifications like on-resistance but 9 volts will fully enhance almost all power MOSFETs. 10 volts is common but some "logic level" parts will fully enhance with only 5 volts.
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