9020 Datasheet, PDF (10/25 Pages) Fairchild Semiconductor – IGBT Basic II

Ate charge (Q_g) of the IGBT is greater, or smaller value of R_G has been chosen for faster oper-
ation of the IGBT, the peak value I_G current becomes greater. According to the value at the
peak, it may be necessary to amplify the power at the gate by using push-pull circuit. Q_g is
greater for devices with higher rated current, and the amount of necessary gate current
increases accordingly. When power amplification is necessary because the necessary value of
the current is large, careful selection in the device for push-pull is needed. This device must be
able to provide the amount of current demanded by the gate and must have quick response.
One of the most important values necessary in designing a gate drive is the maximum value of
I_G, which is provided by the gate drive. Average value of I_G provided by the gate drive can be
obtained easily by dividing the average current by the voltage, but the maximum value of I_G is
more meaningful. This is because the types and the current rating of the devices that make up
the gate drive are determined by the maximum instant current. In general, the amount of cur-
rent the gate drive must supply increases in relation to the operating frequency as shown in
the following equation, but a gate drive with a few Watts is enough for several Amp IGBT and
operating frequency of several 10s of kHz. The total power by the gate drive can be computed
with the following equation
P_{GD_TOTAL}= f × P_SW + P_{GD_INTERNAL}
P_{GD_TOTAL} : Total supplied power to the gate drive
P_SW : Sum of the charge and the discharge power respect to pulse
P_{GD_INTERNAL} : Power consumed by gate drive itself
P_SW = Q_IN × V_GG6
V_GG : V_GG+ - V_GG-
It is possible to use what is generally called gate charge (Q_g) for Q_IN, but if in this case, it is
possible to use the amount of current the gate drive must provide the IGBT in a single pulse.
As such, we can obtain the value by integrating the IGBT I_G curve on the oscilloscope. This
becomes a valid value in obtaining gate drive current through the above equation. Gate
charge can be obtained from gate capacitance, and its curve is available on the data sheet,
but its value would be smaller than Qg or Q_IN if gate capacitance is used simply with the equa-
tion Q = C×V. This is because gate capacitance has non-linear dependency on the gate and
the collector voltage.
Power consumed by the gate drive itself must also be considered as shown in the above
equation, and it must also take into consideration that the amount of power consumed
increases as the operating frequency increases. As such, there must be a safety margin in the
calculation of P_{GD_TOTAL}. In application, the maximum amount of power may only be a few
watts, but the maximum current may be more than a few Amps. The maximum current can be
obtained with the following equation:
I_G(max) = V_GG ⁄ R_G(min)
When the maximum current is actually measured, one must be aware that it may be less than
the calculated amount due to the falling voltage on the wire and stray inductance. When IGBTs
are connected in parallel and are operated at a low frequency, low RMS current could lead to a
mistaken complacency. However, under such situation, the maximum current would be twice
as large, since there are 2 IGBTs in operation, and one must be careful that it could lead to the
overload of the power supply of the gate drive. Wattage of the R_G can be decided with the
maximum calculated amount of current.
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Rev. A, April 2002