AN2052
Application note
VIPower™: implementing frequency modulation
on the VIPer53-E to improve EMI emissions
Introduction
This report describes how the frequency modulation reduces t h e electromagnetic
interference on an SMPS using t h e VIPer53-E as primary PWM-switch. The comparison
of EMI measurement data are presented in the following sections.
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Contents
AN2052
Contents
1
2
3
4
5
6
Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Circuit and description of test configurations . . . . . . . . . . . . . . . . . . . . 5
Analysis and possible further development . . . . . . . . . . . . . . . . . . . . . . 8
Optimization of external oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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Test results
1
Test results
Figure 1. Original, time average at 200 kHz, 53.42 dBuV/m
Figure 2. FM configuration “A”, time average at 200 kHz, 48.07 dBuV/m
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Test results
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Figure 3. FM configuration “B”, time average at 200 kHz, 43.92 dBuV/m
As shown in
Figure 1, 2
and
3,
using the FM technique, an improvement is obtained.
When frequencies are lower than 1 MHz, the device switching is the primary noise
generator. Using the configuration “A” or “B”, a 5 dBuV/m drop is obtained for
frequencies lower than 200 kHz.
Between 1 MHz and 10 MHz, both configurations “A” and “B” make a dramatic drop
bigger than 15 dBuV/m. Above 10 MHz, FM technique benefits are reduced and
capacitive coupling between copper tracks in PCB is dominant.
Table 1. EMI comparison between original “A” and “B” configurations
<1 MHz
Original
“A”
“B”
Marginally pass
Pass
More margin
1 MHz to10 MHz
Pass
More margin
More margin
>10 MHz
Enough margin
Enough margin
Enough margin
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Circuit and description of test configurations
2
Circuit and description of test configurations
Figure 4. Original configuration (fixed frequency at 70 kHz, and no FM)
Figure 5. Configuration "A" (switching frequency from 63 kHz to 77 kHz FM cycle
20 ms, 50 Hz)
Figure 6. Configuration "B" (frequency from 63 kHz to 77 kHz, FM cycle 1.4 ms,
700 Hz)
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