TYPICAL APPLICATIONS
EMC AND EMI RECOMMENDATIONS
No SR capacitor is used. Therefore, the obtained
switching times are the maximum values. A capacitor of
1000 mF is connected between VPWR and GND.
GND
Out
V
PWR
33981
Figure 25. 33981 Initial Configuration
CONDUCTED MEASUREMENTS
TEST SETUP
Key
1
EUT (grounded locally if
required in test plan)
8
Biconical antenna
2
3
Test harness
–
–
Load simulator (placement
and ground connection)
10 High quality double-
shielded coaxial cable
(50 Ω)
To perform a conducted emission measurement in
accordance with the CISPR 25 standard, the test bench in
Figure 26, Conducted Emission Test Setup, on page 24 was
developed.
4
Power supply (location
optional)
11 Bulkhead connector
5
6
Artificial Network (AN)
12 Measuring instrument
13 RF absorber material
Ground plane (bonded to
shielded enclosure)
Power Supply
LISN
7
Low relative permittivity
support (ερ ≤ 1.4)
14 Stimulation and monitoring
system
Measurement
Point for
Conducted
Emission
Figure 24. Test Bench for Radiated Emission
EUT
EMC RESULTS AND IMPROVEMENTS
Non-Conductive
Material
The 33981 OUT is connected to an inductive load (0.47 Ω
+ 1.0 mH) switching at 20 kHz with duty = 80%. The current
in the load was 17 A continuous.
Load (1.0 mH + 0.47 Ω)
Optical PWM Signal
BOARD SETUP
The initial configuration of our 33981 board is represented
in Figure 25.
Figure 26. Conducted Emission Test Setup
EFFECTS OF SOME PARAMETERS
The conducted emissions level rise with the duty cycle.
When the duty increases the di/dt on the VPWR line is higher.
The device has to deliver more current and provide more
energy. Figure 27 describes the effect of duty cycle increase
on the VPWR current waveform. The conducted emission
level rises with the output frequency. This is due to the
increasing number of commutations.
33981
Analog Integrated Circuit Device Data
Freescale Semiconductor
24
FREESCALE [ Freescale ]
