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MMDF3N06HD Datasheet(PDF) 4 Page  Motorola, Inc 

MMDF3N06HD Datasheet(HTML) 4 Page  Motorola, Inc 
4 / 10 page MMDF3N06HD 4 Motorola TMOS Power MOSFET Transistor Device Data POWER MOSFET SWITCHING Switching behavior is most easily modeled and predicted by recognizing that the power MOSFET is charge controlled. The lengths of various switching intervals ( ∆t) are deter mined by how fast the FET input capacitance can be charged by current from the generator. The published capacitance data is difficult to use for calculat ing rise and fall because drain–gate capacitance varies greatly with applied voltage. Accordingly, gate charge data is used. In most cases, a satisfactory estimate of average input current (IG(AV)) can be made from a rudimentary analysis of the drive circuit so that t = Q/IG(AV) During the rise and fall time interval when switching a resis tive load, VGS remains virtually constant at a level known as the plateau voltage, VSGP. Therefore, rise and fall times may be approximated by the following: tr = Q2 x RG/(VGG – VGSP) tf = Q2 x RG/VGSP where VGG = the gate drive voltage, which varies from zero to VGG RG = the gate drive resistance and Q2 and VGSP are read from the gate charge curve. During the turn–on and turn–off delay times, gate current is not constant. The simplest calculation uses appropriate val ues from the capacitance curves in a standard equation for voltage change in an RC network. The equations are: td(on) = RG Ciss In [VGG/(VGG – VGSP)] td(off) = RG Ciss In (VGG/VGSP) The capacitance (Ciss) is read from the capacitance curve at a voltage corresponding to the off–state condition when cal culating td(on) and is read at a voltage corresponding to the on–state when calculating td(off). At high switching speeds, parasitic circuit elements com plicate the analysis. The inductance of the MOSFET source lead, inside the package and in the circuit wiring which is common to both the drain and gate current paths, produces a voltage at the source which reduces the gate drive current. The voltage is determined by Ldi/dt, but since di/dt is a func tion of drain current, the mathematical solution is complex. The MOSFET output capacitance also complicates the mathematics. And finally, MOSFETs have finite internal gate resistance which effectively adds to the resistance of the driving source, but the internal resistance is difficult to mea sure and, consequently, is not specified. DRAIN–TO–SOURCE DIODE CHARACTERISTICS The switching characteristics of a MOSFET body diode are very important in systems using it as a freewheeling or commutating diode. Of particular interest are the reverse re covery characteristics which play a major role in determining switching losses, radiated noise, EMI and RFI. System switching losses are largely due to the nature of the body diode itself. The body diode is a minority carrier de vice, therefore it has a finite reverse recovery time, trr, due to the storage of minority carrier charge, QRR, as shown in the typical reverse recovery wave form of Figure 11. It is this stored charge that, when cleared from the diode, passes through a potential and defines an energy loss. Obviously, repeatedly forcing the diode through reverse recovery further increases switching losses. Therefore, one would like a diode with short trr and low QRR specifications to minimize these losses. The abruptness of diode reverse recovery effects the amount of radiated noise, voltage spikes, and current ring ing. The mechanisms at work are finite irremovable circuit parasitic inductances and capacitances acted upon by high di/dts. The diode’s negative di/dt during ta is directly con trolled by the device clearing the stored charge. However, the positive di/dt during tb is an uncontrollable diode charac teristic and is usually the culprit that induces current ringing. Therefore, when comparing diodes, the ratio of tb/ta serves as a good indicator of recovery abruptness and thus gives a comparative estimate of probable noise generated. A ratio of 1 is considered ideal and values less than 0.5 are considered snappy. Compared to Motorola standard cell density low voltage MOSFETs, high cell density MOSFET diodes are faster (shorter trr), have less stored charge and a softer reverse re covery characteristic. The softness advantage of the high cell density diode means they can be forced through reverse recovery at a higher di/dt than a standard cell MOSFET diode without increasing the current ringing or the noise gen erated. In addition, power dissipation incurred from switching the diode will be less due to the shorter recovery time and lower switching losses. t, TIME Figure 7. Reverse Recovery Time (trr) Standard Cell Density High Cell Density tb trr ta trr 

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