Electronics > Projects, Designs, and Technical Stuff
AC Rectified - DC Mosfet dimmer - Hints please
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Seekonk:

--- Quote from: Kees van Gelder on March 03, 2017, 04:12:59 pm ---
For the average MOSFET that is in the range of 10pF. Hence your RC time being practically zero. Which means that the FET or IGBT will switch very fast.


--- End quote ---

I'd sure like to get some of those FET!
Benta:

--- Quote ---I'd sure like to get some of those!
--- End quote ---

Me too!!

@Kees van Gelder:
I assume you are commenting on my first reply (#3)?

Concerning the cap: the original idea is a 310 VDC supply using a huge 100 uF, 400 V cap (do you realize the physical size of such a thing?)
My suggestion is to make a 16 V DC supply by placing the cap at the R4/R5 junction instead. 100 uF, 25 V is absolutely sufficient and can be smaller with no problems. And at 16 V, no Zener is needed.

The series resistor between cap and optocoupler is to prevent too much discharge of the cap when the coupler is on.

But quite frankly, an even better solution is to place the coupler transistor in series with the gate, plus a pull-down resistor on the gate. This would provide "positive logic" to the circuit and eliminate the "always on" issue.

As to switching losses: a circuit as simple as this is not laid out for 50 kHz switching. I'd switch it at perhaps 500 Hz. And the relatively slow transistions (due to the gate resistor) are actually a blessing if you think about EMC radiation.

Seekonk:
This is the FET gate driven by one of those ebay 4 channel opto boards with a 5.1K pullup, just opposite of the pull down.  So, a 22K would be quite problematic.
Benta:

--- Quote ---This is the FET gate driven by one of those ebay 4 channel opto boards with a 5.1K pullup, just opposite of the pull down.  So, a 22K would be quite problematic.
--- End quote ---

I've no idea what eBay boards you are talking about, and I've no idea what your trace shows.

But let's put a few facts on the table:
IRF730 input capacitance: 700 pF typ.

This gives a time constant (at 22 kohm gate resistor) of: 22^3 x 700^-12 = 15.4 us

Now tell me that PWM'ing this circuit at 500 Hz is a problem. Not!

Seekonk:
I never directly answer any one persons issue directly.  That FET is pretty dinky.  I'm doing 480Hz quite often.  My gates are at least six times that capacitance.  I also don't allow short pulses at either end of the duty cycle.  20KHz was mentioned. There doesn't seem to be any practical reason to have a resistance that high.  As long as you are aware.  This is for those that might consider copying parts of your circuit.
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