View irf840as datasheet:

PD- 91901B IRF840AS SMPS MOSFET IRF840AL HEXFET® Power MOSFET Applications VDSS RDS(on) max ID Switch Mode Power Supply (SMPS) Uninterruptible Power Supply 500V 0.85? 8.0A High Speed Power Switching Benefits Low Gate Charge Qg Results in Simple Drive Requirement Improved Gate, Avalanche and Dynamic dv/dt Ruggedness Fully Characterized Capacitance and D2Pak TO-262 Avalanche Voltage and Current IRF840AS IRF840AL Effective Coss Specified (See AN 1001) Absolute Maximum Ratings Parameter Max. Units ID @ TC = 25°C Continuous Drain Current, VGS @ 10V 8.0 ID @ TC = 100°C Continuous Drain Current, VGS @ 10V 5.1 A IDM Pulsed Drain Current 32 PD @TC = 25°C Power Dissipation 125 W PD @TA = 25°C Power Dissipation 3.1 Linear Derating Factor 1.0 W/°C VGS Gate-to-Source