MMDF2C01HD

7

Motorola TMOS Power MOSFET Transistor Device Data

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 14. 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.

N–Channel

P–Channel

VGS = 0 V

TJ = 25°C

0.3

0.4

0.5

0.6

0.7

0

1

2

VSD, SOURCE–TO–DRAIN VOLTAGE (VOLTS)

Figure 10. Diode Forward Voltage versus Current

0.8

0.9

3

4

VGS = 0 V

TJ = 25°C

1

0.4

0.6

0.8

1

1.2

0

0.5

1

VSD, SOURCE–TO–DRAIN VOLTAGE (VOLTS)

Figure 10. Diode Forward Voltage versus Current

1.4

1.8

1.5

2

VGS = 0 V

TJ = 25°C

1.6