IRAUDPS1_15 Datasheet, PDF (31/35 Pages) International Rectifier – 12V System Scalable 250W to1000W Audio Power Supply

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Gate Drive Current required
The Peak Gate drive current from IRS2085 = (V_CC / R_GATE ) x 2 outputs = (10V/ 22 ohms)
x 2 = 0.9A
The average current required to drive each gate depends on the switching frequency
and Qg of the selected MOSFET, which in this case Qg is 50nC (nano-coulombs) from
data sheet, there are two FETS in parallel per gate drive.
Average Gate Current = I_GATE = 2Qg x Fs = 2 x 50E-9 x 50kHz = 5mA
Total Average Gate Current required = 0.005A x 4 devices = 0.02A
MOFETS Power Dissipation losses
The power dissipation at DC can be calculated as following:
57A / 4 devices = 14.25A
DC Power dissipation per device = I² x RDS_ON / 2
Note RDS_ON at 100C from Data sheet Fig 5, is divided by 2 because it is 50% duty cycle
Power dissipation per device = (14.25)² x 7.5mOhms / 2 = 0.76W
Total power dissipation = (57)² x ¼ 7.5 mOhms = 3249 x 1.875 = 6.091 watts
MOSFET Switching loses
The MOSFETS switching losses can be calculated as following:
Switching losses = Turn ON_LOSSES + Turn OFF_LOSSES + Gate Drive _LOSSES
From IRF6648 data sheet T_{(RISE TIME)} = 29nS and T_{(FALL TIME)} = 13nS and Q_GD = 14nC
Losses contributed by the size of the gate series resistor
Gate drive series resistors actually slowdown the turn ON and turn OFF timing
(See Fig 2, R18-R21)
Delay losses contributed by the gate series resistor = G_{RES Delay} = Q_GD / ((V_CC – V_ML )/
R_GATE )). V_ML is the miller effect plateau voltage of gate charge curve. It is 5.5V for
IRF6648.
G_{RES Delay} = 14E-9 / ((10V-5.5V) / 22 ohms ) = 14E-9 / 0.2A = 70nS
The delay time that caused by large gate resistor is much longer than the rise time that
defined in IRF6648 datasheet. Thus gate resistor delay time will be used to calculate
MOSFET switching losses.
Turn ON_LOSSES = F_OSC x ½ x (G_{RES Delay}) x I x 2V_DS = 50kHz x 0.5 x 70nS x 14.25A x 28V
= 0.7 watts per device
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