MMDF2C03HD
http://onsemi.com
7
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
device, therefore it has a finite reverse recovery time, t
rr
, due
to the storage of minority carrier charge, Q
RR
, as shown in
the typical reverse recovery wave form of Figure 15. 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 t
rr
and low Q
RR
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 t
a
is directly
controlled by the device clearing the stored charge.
However, the positive di/dt during t
b
is an uncontrollable
diode characteristic and is usually the culprit that induces
current ringing. Therefore, when comparing diodes, the
ratio of t
b
/t
a
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 ON Semiconductor standard cell density
low voltage MOSFETs, high cell density MOSFET diodes
are faster (shorter t
rr
), 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.
Figure 10. Diode Forward Voltage versus Current
Figure 10. Diode Forward Voltage versus Current
I
S
, SOURCE CURRENT (AMPS)
V
SD
, SOURCE-TO-DRAIN VOLTAGE (VOLTS)
1.0
1.5
2.0
3.0
2.5
0.5 0.55
0
0.5
T
J
= 25°C
V
GS
= 0 V
0.6 0.65 0.7 0.75 0.8 0.85
0.5 0.7 0.9 1.1 1.3 1.9
0
0.4
1.2
1.6
2
V
SD
, SOURCE-TO-DRAIN VOLTAGE (VOLTS)
I
S
, SOURCE CURRENT (AMPS)
0.8
1.5 1.7
T
J
= 25°C
V
GS
= 0 V
N−Channel P−Channel
