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IRF7805A Datasheet(PDF) 3 Page - International Rectifier |
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IRF7805A Datasheet(HTML) 3 Page - International Rectifier |
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3 / 8 page  www.irf.com 3 IRF7805/IRF7805A Control FET Special attention has been given to the power losses in the switching elements of the circuit - Q1 and Q2. Power losses in the high side switch Q1, also called the Control FET, are impacted by the R ds(on) of the MOSFET, but these conduction losses are only about one half of the total losses. Power losses in the control switch Q1 are given by; P loss = Pconduction+ Pswitching+ Pdrive+ Poutput This can be expanded and approximated by; P loss = I rms 2 × R ds(on ) () + I × Q gd i g × V in × f + I × Q gs2 i g × V in × f + Q g × Vg × f () + Q oss 2 ×V in × f This simplified loss equation includes the terms Q gs2 and Q oss which are new to Power MOSFET data sheets. Q gs2 is a sub element of traditional gate-source charge that is included in all MOSFET data sheets. The impor- tance of splitting this gate-source charge into two sub elements, Q gs1 and Qgs2, can be seen from Fig 1. Q gs2 indicates the charge that must be supplied by the gate driver between the time that the threshold volt- age has been reached (t1) and the time the drain cur- rent rises to I dmax (t2) at which time the drain voltage begins to change. Minimizing Q gs2 is a critical factor in reducing switching losses in Q1. Q oss is the charge that must be supplied to the output capacitance of the MOSFET during every switching cycle. Figure 2 shows how Q oss is formed by the paral- lel combination of the voltage dependant (non-linear) capacitance’s C ds and Cdg when multiplied by the power supply input buss voltage. Figure 1: Typical MOSFET switching waveform Synchronous FET The power loss equation for Q2 is approximated by; P loss = Pconduction + Pdrive + Poutput * P loss = I rms 2 × R ds(on) () + Q g × Vg × f () + Q oss 2 ×V in × f + Q rr × Vin × f ( ) *dissipated primarily in Q1. Power MOSFET Selection for DC/DC Converters 4 1 2 Drain Current Gate Voltage Drain Voltage t3 t2 t1 V GTH t0 |
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