Kapanadze Cousin - DALLY FREE ENERGY

[John.K1]

Anybody ,who understand the electronics - , I have a question- do we really need that mosfet gate driver?  Why not to connect SG directly to the Mosfet's gate? What extra actually that gate driver offers?  Is it some sort of Smith trigger making clean and sharp signal?  Or is there something else? Protection? The price of driver is comparable with my inexpensive AD9850, which has quite nice and sharp output.  Actually 2 outputs of square waves in 180 deg phase - so one output to one gate and the other output to the gate of the 2nd mosfet? 

Please comment

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[John.K1]

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[Dog-One]

Anybody ,who understand the electronics - , I have a question- do we really need that mosfet gate driver?  Why not to connect SG directly to the Mosfet's gate? What extra actually that gate driver offers?  Is it some sort of Smith trigger making clean and sharp signal?  Or is there something else? Protection? The price of driver is comparable with my inexpensive AD9850, which has quite nice and sharp output.  Actually 2 outputs of square waves in 180 deg phase - so one output to one gate and the other output to the gate of the 2nd mosfet? 

To get a MOSFET (switch) to flip quickly and efficiently (without heat build-up), you need considerable current on the gate to overcome the internal capacitance.  At low frequencies, below a couple kHz, it isn't really necessary to have a gate driver since you aren't switching that many times per second and the MOSFET can dissipate the heat.  Once you go above a few kHz, the MOSFET really begins to generate considerable heat with slow (low current) gate drive.  If you continue to push the frequency into the MHz range, the MOSFET will simply melt down as it will be spending most of its time in the transition level between being fully on and fully off.

Bottom line is:  The MOSFET gate acts like a capacitor to the input signal and with that has a capacitive ramp-up.  A MOSFET really isn't designed to run partially turned on or off like a bipolar transistor.  It needs to be on or off; anything else and it is a very inefficient piece of silicon.  If you look at the schematic symbol of a MOSFET, you will see there is no direct connection between the gate and the drain or source.  It's a plate, just like a capacitor symbol.  That's not by accident.  That's truly how it is working inside.

For this application where most of us are running between 8 and 30 kHz, it's just smart to use a gate driver.  So when you find a gate driver to put in front of the MOSFET, look at the peak amperage the gate driver can supply without blowing up, then use ohms law to calculate what size resistor will limit the current to that value or slightly lower and place the resistor between the output of the gate driver and the gate of the MOSFET.  That's all it takes and you should be good to go.

[Jeg]

Why not to connect SG directly to the Mosfet's gate?

SG doesn;t have enough output peak current to charge the capacity at mosfet's gate input quickly. So there are limitations at the fully on/off speed.

[verpies]

I have a question- do we really need that mosfet gate driver? 

For low frequencies and slow rise times - no if the driving signal has greater voltage than the V_{GS_th}.

Why not to connect SG directly to the Mosfet's gate?

You can connect the SG directly to the MOSFET's gate if the SG peak output voltage exceeds V_{GS_th} of your MOSFET.
But don't count on the SG being able to charge the MOSFET's gate quickly or rapidly.

What extra actually that gate driver offers?

Gate voltage up to 20V and gate charging current from 3A to 15A.
The high current is needed to charge and discharge the gate quickly because the gate is a 1nF to 10nF capacitor.
While the MOSFET is switching (is "in-between" ON and OFF) it dissipates a lot of heat so it is advantageous to minimize the "in-between" time as much as possible.

A 10mA current will charge a 10nF capacitance to 10V V_{GS_th} in 50μs which is enough for a 10kHz triangular wave, but not for a 10kHz square wave.

Is it some sort of Smith trigger making clean and sharp signal?

That, too but it is a minor reason.
The other more important reasons being: voltage level shifting and sometimes signal inversion.

The price of driver is comparable with my inexpensive AD9850, which has quite nice and sharp output.  Actually 2 outputs of square waves in 180 deg phase - so one output to one gate and the other output to the gate of the 2nd mosfet? 

180º will not give you the dead time and as a consequence the 2 MOSFETs in a push-pull driver will be both ON for a short time creating a periodic short-circuit that will burn up your MOSFETs.  A push-pull controller like the TL494 chip provides 2 outputs with an adjustable dead-time between them so that does not happen.

You can try it for an educational experience.  If you have a scope, monitor the signal between the MOSFET's gate and source terminal and see what happens as you increase the driving frequency.