On and Off, Conduction in a Mosfet

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  • Last Post 09 April 2019
Chris posted this 28 March 2019

This thread is dedicated as a Helper Thread for: Delayed Conduction in Bucking Coils.

A Mosfet has an On Time and an Off Time, and we can control this, with our machines.



To control the On Time and Off Time, Positive Voltage to the Gate ( 1 ) must reach the rated Voltage Difference between Source ( 3 ). This is seen as VGS on the Datasheet. IGSS is the Current that leaks from the Gate, to the Source, at a Specified Voltage, in this case 20 Volts. Most Mosfet's switch on at around 2-4 Volts but do not Conduct Fully until around 10 Volts.

Marathonman is correct, the Mosfet has an Internal Resistance: RDS(on) In this case, but this is when the Mosfet is On. In between this Resistance may be quite different. Turning the Mosfet on fully, and them off fully, is ideal to reduce heat problems!

Needing a Gate Voltage of around 8Volts+ to turn the Mosfet On properly.

This example is of the IRF730:


The Mosfet has an On Time and an Off Time Delay:



Turn On Time is td(on) and Turn Off Time is td(off). This is the Datasheet Rated times for the Mosfet. However, the Datasheet does not show the Circuit required to turn Off the Mosfet!

It is possible to Latch the Mosfet, meaning it will stay ON, the Mosfet will not turn Off, unless it has a proper turn off circuit. This is called, normally, a Pull Down Resistor.


Ref: Electronics Tutorials


RGS is the Pull Down Resistor, in this instance, Resistance ( R ) from Gate ( G ) to Source ( S ). The faster turn Off Time you want, the lower the value of the Resistor needed. The Mosfet has a Gate Capacitance. Power is required to Charge this Gate Capacitance, this means, this Power must be Discharged, from the Gate Capacitance, to turn the Mosfet Off.

Some Mosfet Drivers have the built in ability to turn the Mosfet off. The IC does this by dumping the Gate Capacitance Power to Ground. It does this very fast! Here is a basic Circuit showing this switching or discharging to Ground:


The Mosfet is turned on by this Circuit, through the 25 Ohm Resistor ( R2 ):



The Mosfet is turned Off, by the Diode ( D1 ) and the 1 Ohm Resistor ( R1 ):



The Mosfet Driver IC has an internal Diagram like so:



You can see, the IC MCP1403 in this case is dumping the Gate Capacitance Charge to Ground through the Diode D1, the Resistance R1 and through the Internal Mosfet seen in the Diagram above.

I have built one of my H-Bridges from this type of circuit:




For more information see: Reliable and Flexible Switching System

So, in this example, we can see, Conduction is a factor at which we can very easily change at will! We only need Voltage and a little bit of Current as a result.

I hope this helps!


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Marathonman posted this 29 March 2019


  Read a good article on hard and soft switching today that may be of use to others from On semi conductor.

Also the grey area you speak of causes a lot a heat when not in full conduction which is good to know in electronics.



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Jagau posted this 30 March 2019

Hi my friends

This very explicit little PDF can help you to meet almost any situation with a mosfet.

I used it regularely.




Attached Files

Jagau posted this 30 March 2019

Hi my friends

As you ca see optimal gate resistor shows an example of how to use different for gate resistors for turn-on and turn-off.

Another important point is:

Protection against gate-emitter surge voltage with zener or resistors.

And last one and so more important:

-the higher the VGS, the lower the RDS(ON) value tends to become. that keep temperature of mosfet low

in order to reduce loss, it is important to increase Voltage Gate Source  above Vth

 P.S.  I must tell you that in a specific situation I have an experience that gives me better results just a little before Vth
This is what I call a paradox


Vidura posted this 02 April 2019

Here an observation that I made when I tried different configurations for switching a MOSFET using the voltage of the same line that is switched. Basically working on the simple circuit that Chris proposes in the delayed conduction thread. First testing has shown that the switching did not occur as expected or predicted by simulation. In the best case a fast ringing of voltage has been achieved in this configuration, and no current flow. IMO the gate charge is lost immediately when the MOSFET started conducting. The next testing I used two bucking zener diodes on the gate and a 100nF cap instead of the pull down resistor. This seemed to work, at least partially at certain parameters a small current started to flow. When I measured the gate voltage it was only around 2.5volt and looked unchanged , only zooming in there was a small bump in the scope trace when switching, only 50or 100mv or so above Vth. At the last and most successful test for this particular switching method y used two bucking zener diodes 18v each and a 4k7 pulldown resistor (the best value have to be found for a given frequency and switch parameters). In this case I got a clear marked voltage and current pattern when switching occurred, at least above a certain power level. Hopefully some of these results will be useful. Regards Vidura.

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Jagau posted this 02 April 2019

Hi vidura

As I said in a previous post, I repeat it is very very rare to see a diode between the drain and the gate of a mosfet.
When Vz is reached on the side of the drain, the gate of the mosfet puts the mosfet in constant conduction much longer than the normal signal which activates the gate of the mosfet. That why you have very low output on your scope.


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Jagau posted this 02 April 2019

Hi friends

Chris makes a lot of effort to give us ideas a course of action in order to reach a goal that we all seek.
As we are in the general public section it is up to us to force our heads to find the circuit that will satisfy us.
It is not necessary to copy paste the proposed circuits, it is necessary to search and search. I think that's what we're doing here.
He is an excellent guide and I thank him for doing so much effort and giving me so many ideas for my experiences.


Chris posted this 02 April 2019

Thank You Jagau,

I have been un-well for days now, I hope to return soon with a clear head.

Yes, you're right, It is really important to connect the dots, make the link between the TVS that the MEG Team used and other switches that can be made to do the same job! Graham Gunderson the exact same thing, but with a series of IC's in a Ring called the Chain Oscillator.

They all do the same thing!


Chris posted this 03 April 2019

My Friends,

The switch time is regulated by the Output.

The Coil has become a Battery, an Active Element. It must be treated as such! Splitting the Voltage, across the load and the Coil is basically a Voltage Divider:


All I did was move the Load, to the Mosfet Source, so the Mosfet is now switching on the High Side of the Load. High Side Switching is what Graham Gunderson was doing also!


Think Switching, timing, finding and then catching the Wave!


Aetherholic posted this 04 April 2019

High side switching is what I used when experimenting with resonant bucking bifilar pancake coils and at extremely low duty cycle between 1% and 4%. The output power is there when the timing is correct.

Aetherholic - One truth, One field

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alohalaoha posted this 08 April 2019

Hard switching.PNG

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YoElMiCrO posted this 09 April 2019

Hi all.
With this formula you can calculate the resistor
series to the gate of the mosfet to obtain the speed of fall / rise
they need, as long as the limit imposed by the mosfet
be less than desired.
We look in the datasheet for the Qg parameter (Total gate load)
normally expressed in nC and ...
Ig = Qg / t
Where Ig in amper, Qg in nC and t in nS.
If you need different rise / fall times with a fast diode
style 1N5819 to separate the load / unload circuits from the
capacitor parasite of the gate is sufficient, as it represents
Chris in the previous photos.

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