TYPICAL APPLICATIONS
POWER DISSIPATION
DC MOTOR 200 W
INFLUENCE OF SR CAPACITOR
A concrete example is the 33981. A 200 W DC motor, a
frequency of 20 kHz, and an ambient temperature of 85ºC are
chosen. The 33981 is evaluated using the following board.
The thermal impedance of the board is in the range of
15ºC/W.
The SR capacitor value has an impact on these switching
losses. Figure 41 illustrates the percentage of the switching
losses versus the total power dissipation for the same load
conditions as Figure 38. The higher the SR capacitor value,
the higher the switching losses. They can be more than 50%
of the total power dissipation in the 33981 with a 4.7 nF
capacitor and is a basic applications trade-off. A compromise
should be found between the power dissipation and the
electromagnetic capability (EMC) performance.
6
Pswitching
Pon
5
4
3
2
1
0
0
2.2
3.3
4.7
Figure 39. 33981 Evaluation Board
POWER DISSIPATION
Csr (nF)
Figure 41. Power Switching vs. SR Capacitor
RECIRCULATION PHASE
Figure 40 illustrates the power dissipation in the 33981.
The conditions are listed in Figure 38. Maximum power
dissipation of 3.1 W is obtained with a duty of 95%.
Figure 42 illustrates the power dissipation for the two
recirculation approaches, diode or low-side MOSFET. The
power dissipation gain for the entire system when using the
low side instead of the diode can reach up to 1.5 W with a
duty cycle of 50%.
MC33981 Power Dissipation
3.5
Pon_state
P switching
3.0
Ptotal
Total Board Power Dissipation
2.5
2.0
1.5
1.0
0.5
0
4.5
4.0
3.5
3.0
Power HS
Power Diode
2.5
Power Total Board with Diode
2.0
Power LS
Power Total Board with LS
1.5
1.0
0.5
0.0
0
10 20 30 40 50 60 70 80 90 100
Ratio PWM %
0
10
20
30
40
50
60
70
80
90
100
Duty Cycle (%)
Figure 42. Total Board Power Dissipation
Figure 40. Power Dissipation (Pon and Pswitching) vs.
Duty Cycle
JUNCTION TEMPERATURE
The junction temperature of the 33981 versus duty cycle
for the condition listed in Figure 38, is given in Figure 43. The
maximum obtained junction temperature is 132ºC with a duty
33981
Analog Integrated Circuit Device Data
Freescale Semiconductor
29
FREESCALE [ Freescale ]
