AN2052 STMICROELECTRONICS | Alldatasheet
Document overview
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- PDF pages: 13
Technical content
Datasheet sections
- 1 Test results
- 2 Circuit and description of test configurati ons
- 3 Analysis and possible further development
- 4 Optimization of external oscillator
- 5 Reference
- 6 Revision history
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 the VIPer53-E as primary PWM-switch. The comparison of EMI measurement data are presented in the following sections.
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
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. frequencies lower than 200 kHz. capacitive coupling between copper tracks in PCB is dominant. Table 1. EMI comparison between original “A” and “B” 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 Figure 6. Configuration "B" (frequency from 63 kHz to 77 kHz, FM cycle 1.4 ms,
700 Hz)
3 Analysis and possible further development
Figure 11. Original peak Figure 12. Configuration “A” peak
Figure 13. Configuration “B” peak spectrum. In EMI test data of SMPS, it corresponds to a low frequency . measurements obtained are very similar to the spectrum envelope of Figure 11. measured through a narrow-band filter, over a relatively long period of time. Note that there is no regulation on the measurement average time.
Optimization of external oscillator AN2052
4 Optimization of external oscillator
T argeted to lower down the average value of EMI noise, the PWM must go through the whole frequency range within the testing time. In other words, the FM cycle is as short as possible. On the other hand, since the PWM signal is modulated by the FM cycle and switching cycle, to avoid audible noise, FM frequency should be lower than 2 kHz. In fact the average data of configuration “B”, 700 Hz oscillator , are lower than the configuration “A”, 50 Hz oscillator. Another drawback of configuration “A” is the dependency on AC mains, which can be an issue in the transformer design.
5 Reference
Figure 14. Oscillator with 2 BJT s, low cost and start-up voltage down to 3 V
6 Revision history
Table 2. Document revision history Updated the title in cover page.