november 2014 docid10909 rev 2 1/13 13 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 the viper53-e as primary pwm-switch. the comparison of emi measurement data are presented in the following sections. www.st.com
contents AN2052 2/13 docid10909 rev 2 contents 1 test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 circuit and description of test configurati ons . . . . . . . . . . . . . . . . . . . . 5 3 analysis and possible further development . . . . . . . . . . . . . . . . . . . . . . 8 4 optimization of external oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5 reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
docid10909 rev 2 3/13 AN2052 test results 13 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
test results AN2052 4/13 docid10909 rev 2 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 1 mhz to 10 mhz >10 mhz original marginally pass pass enough margin ?a? pass more margin enough margin ?b? more margin more margin enough margin
docid10909 rev 2 5/13 AN2052 circuit and description of test configurations 13 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) n 9 g g 9 g f , & |