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MOTOROLA

SEMICONDUCTOR TECHNICAL DATA

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MMDF3N03HD

Motorola Preferred Device

DUAL TMOS

POWER MOSFET

4.1 AMPERES

30 VOLTS

RDS(on) = 0.070 OHM

CASE 751±05, Style 11

SO±8

Top View

Figure 1. On±Region Characteristics

VGS, GATE±TO±SOURCE VOLTAGE (VOLTS)

Figure 2. Transfer Characteristics

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 determined by how fast the FET input capacitance can be charged by current from the generator.

The published capacitance data is difficult to use for calculating 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 resistive 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 values 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 complicate 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 function 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 measure and, consequently, is not specified.

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 recovery 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 device, 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 ringing. 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 controlled by the device clearing the stored charge. However, the positive di/dt during tb is an uncontrollable diode characteristic 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 recovery 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 generated. In addition, power dissipation incurred from switching the diode will be less due to the shorter recovery time and lower switching losses.

The Forward Biased Safe Operating Area curves define the maximum simultaneous drain±to±source voltage and drain current that a transistor can handle safely when it is forward biased. Curves are based upon maximum peak junction temperature and a case temperature (TC) of 25°C. Peak repetitive pulsed power limits are determined by using the thermal response data in conjunction with the procedures discussed in AN569, ªTransient Thermal Resistance ± General Data and Its Use.º

Switching between the off±state and the on±state may traverse any load line provided neither rated peak current (IDM) nor rated voltage (VDSS) is exceeded, and that the transition time (tr, tf) does not exceed 10 μs. In addition the total power averaged over a complete switching cycle must not exceed

(TJ(MAX) ± TC)/(RθJC).

A power MOSFET designated E±FET can be safely used in switching circuits with unclamped inductive loads. For reli-

able operation, the stored energy from circuit inductance dissipated in the transistor while in avalanche must be less than the rated limit and must be adjusted for operating conditions differing from those specified. Although industry practice is to rate in terms of energy, avalanche energy capability is not a constant. The energy rating decreases non±linearly with an increase of peak current in avalanche and peak junction temperature.

Although many E±FETs can withstand the stress of drain± to±source avalanche at currents up to rated pulsed current (IDM), the energy rating is specified at rated continuous current (ID), in accordance with industry custom. The energy rating must be derated for temperature as shown in the accompanying graph (Figure 9). Maximum energy at currents below rated continuous ID can safely be assumed to equal the values indicated.

Figure 8. Capacitance Variation

Figure 9. Gate±to±Source and Drain±to±Source Voltage versus Total Charge

t, TIME (ns)