Application and environment design for ACPL-K34T needs to ensure that the junction temperature of the internal IC
and LED within the gate drive optocoupler do not exceed 150 °C. The equation (1) and (2) provided above are for the
purposes of estimating the junction temperatures. For example:
Calculation of LED and output IC power dissipation
LED power dissipation, PE = IF(LED) (Recommended Max) * VF(LED) (at 125 °C) * Duty Cycle
= 13 mA * 1.25 V * 50%
= 8.125 mW
Output IC power dissipation, PO = VCC (Recommended Max) * ICC(Max) + PHS + PLS
= 20 V * 4 mA + 53.3 mW + 32 mW
= 165.3 mW
where PHS = High side switching power dissipation
= (VCC * QG * fPWM)* RDS,OH(MAX) / (RDS,OH(MAX) + RGH) /2
= (20 V * 80nC * 200 kHz) * 4Ω/(4Ω+8Ω)/2
= 53.3mW
PLS = Low side switching power dissipation
= (VCC * QG * fPWM)* RDS,OL(MAX) / (RDS,OL(MAX) + RGL) /2
= (20 V * 80 nC * 200 kHz) * 2Ω/(2Ω+8Ω)/2
= 32 mW
QG = Gate charge at supply voltage
fPWM = LED switching frequency
RGH = Gate charging resistance
RGL = Gate discharging resistance
Calculation of LED junction temperature and output IC junction temperature at Ta=125 °C:
LED junction temperature,
T1 = (R11 × PE + R12 × PO) + TA
= (191°C/W * 8.125 mW + 68.5 °C/W * 165.3 mW) + 125 °C
= 138 °C < TJ(absolute max) of 150 °C
Output IC junction temperature,
T2 = (R21 × PE + R22 × PO) + TA
= (68.5 °C/W * 8.125 mW + 77 °C/W * 165.3 mW) + 125 °C
= 138 °C < TJ(absolute max) of 150 °C
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AV02-4229EN - September 16, 2013