Datasheet IRLR8743PbF, IRLU8743PbF (Infineon) - 8

IRLR/U8743PbF
Power MOSFET Selection for Non-Isolated DC/DC Converters Control FET Synchronous FET
Special attention has been given to the power losses The power loss equation for Q2 is approximated in the switching elements of the circuit - Q1 and Q2. by; Power losses in the high side switch Q1, also called the Control FET, are impacted by the R of the * ds(on) P = P + P + P MOSFET, but these conduction losses are only about loss conduction drive output one half of the total losses. P = I 2 × R ( ) loss rms ds(on) Power losses in the control switch Q1 are given by; + Q ( ×V × f) g g P = P + P + P + P ⎛ ⎜ ⎞ loss conduction switching drive output + Qoss × V × f + Q ( × V × f ) ⎝ 2 in ⎠ rr in This can be expanded and approximated by; *dissipated primarily in Q1. P = I 2 ( × R ) loss rms ds(on ) For the synchronous MOSFET Q2, R is an im- ⎛ ds(on) Q ⎞ ⎛ Q ⎞ portant characteristic; however, once again the im- + I ⎜ × gd × V × f⎟ + I ⎜ × gs2 × V × f⎟ portance of gate charge must not be overlooked since ⎝ i in ⎝ i in g ⎠ g ⎠ it impacts three critical areas. Under light load the MOSFET must still be turned on and off by the con- + Q ( ×V × f) g g trol IC so the gate drive losses become much more ⎛ significant. Secondly, the output charge Q and re- oss + Qoss ×V × f ⎞ verse recovery charge Q both generate losses that ⎝ 2 in ⎠ rr are transfered to Q1 and increase the dissipation in that device. Thirdly, gate charge will impact the This simplified loss equation includes the terms Q MOSFETs’ susceptibility to Cdv/dt turn on. gs2 and Q which are new to Power MOSFET data sheets. The drain of Q2 is connected to the switching node oss Q is a sub element of traditional gate-source of the converter and therefore sees transitions be- gs2 charge that is included in all MOSFET data sheets. tween ground and V . As Q1 turns on and off there is in The importance of splitting this gate-source charge a rate of change of drain voltage dV/dt which is ca- into two sub elements, Q and Q , can be seen from pacitively coupled to the gate of Q2 and can induce gs1 gs2 Fig 16. a voltage spike on the gate that is sufficient to turn Q indicates the charge that must be supplied by the MOSFET on, resulting in shoot-through current . gs2 the gate driver between the time that the threshold The ratio of Q /Q must be minimized to reduce the gd gs1 voltage has been reached and the time the drain cur- potential for Cdv/dt turn on. rent rises to I at which time the drain voltage be- dmax gins to change. Minimizing Q is a critical factor in gs2 reducing switching losses in Q1. Q is the charge that must be supplied to the out- oss put capacitance of the MOSFET during every switch- ing cycle. Figure A shows how Q is formed by the oss parallel combination of the voltage dependant (non- linear) capacitance’s C and C when multiplied by ds dg the power supply input buss voltage. Figure A: Q Characteristic oss 8 www.irf.com