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

Body

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
 

---In [email protected], <s.friedrich@...> wrote :

 Hi Douglas,
 
 I use an opto driver TLP250 very simple.
 
 At the mosfet I use 1k resistors from gate to source.
 
 On TLP250 directly 1μF capacitor MKP and parallel 68μF Elko.
 
 I removed the 10 Ohm gate resistor because I could not realize short pulse widths below 1 ms with it, ferrite beads work just as clean scope image but no short pulse widths possible.
 
 In addition, a 10nF capacitor between gate and source connected to relieve the gate, thereby minimal improvement.
 
 I have to drive at least 4 amps at 300 volts.
 
 The parts are all oversized.
 
 I can shorten the gate lines a bit but if that's the solution I doubt it.
 
 regards
 
 Sven   Gesendet: Donnerstag, 05. Juli 2018 um 17:03 Uhr
 Von: "konehead@... [EVGRAY]" <[email protected]>
 An: [email protected]
 Betreff: [EVGRAY] Re: Push pull generator.
    
 Hi Sven
 Use thin magnet wire wrapped inside tiny ferrite tubes  just a few turns ,maybe 4 turns....use very thin magnet wire around 28 or 30 GA  thickness (thin)...then connect this wire to mosfet gate.
 Also they say to include in series a low ohm resistor too, such as 5 ohm 1/4W size to help dampen any ringing...the ferrite squelches the HF stuff, the resistor the LF stuff is what I understand.
 Maybe your mosfets do not turn off fast enough, this would make for excess heat that is for sure!....are you using a driver chip to the mosfets??
 Try 4421 or 4422 types...
 Also if no drivers feeding the gates, you better for sure have a 10K "pull sown" resistor stretching across mosfet  from gate to source on the mosfet....I like to include the 10K resistor even if using a 4422 or 4421 driver chip just to be double safe....
 Kone
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