EVLB001 Datasheet, PDF (28/35 Pages) ATMEL Corporation – Automatic microcontroller dimmable ballast

Conclusion
7.2 Appendix 2:
Small currents for the low voltage supply can be obtained from the AC line at
Capacitor
low loss by means of capacitor coupling as shown in the figures below. To esti-
Coupled Low
Voltage Supply
mate the required size of the coupling capacitor, use the following relationships
for current, charge, voltage and capacitance.
1.dQ/dt = I_DC
Figure 7-1. Negative Line Half Cycle
C1
- V_C1 +
V_D
AC
-V_PK
Ich1
V_D
C2
Vo
I_DC
“Negative” line half -cycle:
C1 charges to Vpk - V_D with polarity shown.
Figure 7-2. Positive Line Half Cycle
+V_PK
AC
C1
+ V_C1 -
Ich2
V_D
V_D
C2
Vo
I_DC
“Positive” line half -cycle:
C1 charges to Vpk - V_D - Vo with polarity shown.
1.dV = 2Vpk-Vo-2V_D
2.dQ = CdV or C = dQ/dV
For example, to obtain 15 mA at 20 VDC from a 220 Vrms 50 Hz line:
1.dQ/dt = (15 millijoules/sec)/(50 cycles/sec) or 0.3 millijoules / cycle.
2.Over 1 cycle, the coupling capacitor (C1) will charge from –220V x 1.4 to
+220V x 1.4 – 20V- V_D. dV = 2*Vpk-Vo-2V_D. dV ~= 600V.
3. The required C1 ~ 0.3 millijoules/600V or 0.5 uF
ATAVRFBKIT / EVLB001 User Guide
7-26
7597B–AVR–10/07