Re: Large IGBT need help

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 

Hi,

On Tue, 13 Apr 2004, Tesla list wrote:
 >  > Original poster: Fucian-at-aol-dot-com
 >  > Hmm what else could i use them for besides a tesla coil?Isnt the issue
 >  > with these IGBTs the factt hat they need alot of current to turn
 >  > on?What if i give them the cutrrent?How much do they need?
 >
 > Original poster: "Jim Mitchell" <electrontube-at-sbcglobal-dot-net>
 > The gate charge varies with voltage.  Go look at the datasheet and calculate
 > the gate capacitance.
 > Use the equation C = Q/V where C = capacitance [farads]
 >                            Q = charge [coulombs]
 >                            V = potential [volts]

To be more precise, gate charge depends mainly on the initial
drain/collector voltage before switching. The datasheet specifies
the maximum total gate charge Q for a given gate voltage and drain or
collector voltage.

Because charging the gate is like charging a capacitor, the gate and gate
driver are an R*C = R_driver * C_gate network, R selected according to how
fast you need to switch the igbt/mosfet on. Not too fast (HV spikes) and
not too slow (high IGBT/mosfet conduction losses).

  Q = I dt = (U/R) dt
  <=>  R_gate ~= (U_driver/Q_gate)*t_rise
  <=>  I_peak ~= U_driver/R_gate = Q_gate/t_rise

For example U_driver max gate voltage 15V, Q_gate 200nC, t_rise 100ns
  R_gate ~= (15V/200C)*100s = (15/2)ohm = 7.5ohm max gate+driver resistance
  I_peak ~= 200nC/100ns = 2A max peak driver current

These are just approximate, but should be quite close to the real values.

t_rise depends on your switching frequency. You could make it 5% of the
total switch period e.g. t_rise = 0.05/freq[Hz]

cheers,
  - Jan

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  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering