MMSF3350

http://onsemi.com

6

Figure 8. Gate−To−Source and Drain−To−Source

Voltage versus Total Charge

1

10

100

100

10

Figure 9. Resistive Switching Time

Variation versus Gate Resistance

18

3

0

0

2

0

12

10

20

40

30

QT

Q2

Q3

Q1

50

1000

8

6

9

12

4

10

6

15

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

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

the typical reverse recovery wave form of Figure 16. 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

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

controlled by the device clearing the stored charge.

diode characteristic and is usually the culprit that induces

current ringing. Therefore, when comparing diodes, the

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

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.