Re: Aw: [EVGRAY] Re: Push pull generator.

Body

Yahoo having a bad day again.

Hi Sven,

Don't put ferrite beads on the gate circuit as they add inductance. Try to avoid inductance by making the distance between the gate driver and the gate as small as possible an minimize any area between the wires that act like a coil winding adding inductance. The gate-source is a capacitor. Adding inductance makes it an LC-tank which makes oscillations when storing energy. The oscillations are amplified at the drain. Also the inductive kickback from any inductance can go above the gate-source breakdown voltage which degrades and destroys the MOSFET sooner or later. By adding extra capacitance in parallel to the gate-source capacitance slows down the MOSFET as the drain signal follows the gate signal. The series resistor at the gate is for absorbing the energy of the formed LC-tank to prevent oscillations of the gate and thus also oscillations on the drain.

Opto couplers are slow devices. The TLP250 has a delay of 0.5 us and a max frequency of 25 kHz.

A fuse normally isn't fast enough to protect semiconductors from destruction. Use an electronic protection that removes the gate signal to shut off the device. For over voltage protection use some kind of snubber active or passive. A crowbar protection made by using a thyristor (SCR) that shorts out the voltage supply rails and blows a fuse is an effective way to prevent over voltages.

'Fundamentals of MOSFET and IGBT Gate Driver Circuits:' (http://www.ti.com/lit/ml/slua618/slua618.pdf http://www.ti.com/lit/ml/slua618/slua618.pdf).
Just follow those rules to make a good gate driver. If not doing that all kinds of trouble may arise.

If the impedance of the capacitors are less than that of B2 the current mainly takes the path shown. If R2 is omitted this may cause ringing when charging the gate capacitance. R2 has to absorb the energy to prevent oscillations in the tank formed by the inductance of the two loops and the capacitance of the gate-source capacitor. For fast response R2 is chosen for critical damping which means no ringing.

Regards
Ole