MTD20P06HDLPOWER MOSFET SWITCHING Switching behavior is most easily modeled and predicted The capacitance (Ciss) is read from the capacitance curve at by recognizing that the power MOSFET is charge a voltage corresponding to the off−state condition when controlled. The lengths of various switching intervals (Dt) calculating td(on) and is read at a voltage corresponding to the are determined by how fast the FET input capacitance can on−state when calculating td(off). be charged by current from the generator. At high switching speeds, parasitic circuit elements The published capacitance data is difficult to use for complicate the analysis. The inductance of the MOSFET calculating rise and fall because drain−gate capacitance source lead, inside the package and in the circuit wiring varies greatly with applied voltage. Accordingly, gate which is common to both the drain and gate current paths, charge data is used. In most cases, a satisfactory estimate of produces a voltage at the source which reduces the gate drive average input current (IG(AV)) can be made from a current. The voltage is determined by Ldi/dt, but since di/dt rudimentary analysis of the drive circuit so that is a function of drain current, the mathematical solution is t = Q/I complex. The MOSFET output capacitance also G(AV) complicates the mathematics. And finally, MOSFETs have During the rise and fall time interval when switching a finite internal gate resistance which effectively adds to the resistive load, VGS remains virtually constant at a level resistance of the driving source, but the internal resistance known as the plateau voltage, VSGP. Therefore, rise and fall is difficult to measure and, consequently, is not specified. times may be approximated by the following: The resistive switching time variation versus gate tr = Q2 x RG/(VGG − VGSP) resistance (Figure 9) shows how typical switching t performance is affected by the parasitic circuit elements. If f = Q2 x RG/VGSP the parasitics were not present, the slope of the curves would where maintain a value of unity regardless of the switching speed. VGG = the gate drive voltage, which varies from zero to VGG The circuit used to obtain the data is constructed to minimize R common inductance in the drain and gate circuit loops and G = the gate drive resistance is believed readily achievable with board mounted and Q2 and VGSP are read from the gate charge curve. components. Most power electronic loads are inductive; the During the turn−on and turn−off delay times, gate current is data in the figure is taken with a resistive load, which not constant. The simplest calculation uses appropriate approximates an optimally snubbed inductive load. Power values from the capacitance curves in a standard equation for MOSFETs may be safely operated into an inductive load; voltage change in an RC network. The equations are: however, snubbing reduces switching losses. td(on) = RG Ciss In [VGG/(VGG − VGSP)] td(off) = RG Ciss In (VGG/VGSP) 2500 VDS = 0 V VGS = 0 V TJ = 25°C Ciss 2000 (pF) 1500 ANCE Crss ACIT 1000 Ciss C, CAP 500 Coss Crss 010 5 0 5 10 15 20 25 VGS VDS GATE−TO−SOURCE OR DRAIN−TO−SOURCE VOLTAGE (Volts) Figure 7. Capacitance Variationhttp://onsemi.com4
