Datasheet

t
I
in
(t)
0
Potential Misfire
Triac Fires Æ Inrush Spike
www.ti.com Design Calculations
12.8 Inrush Limiting and Damping
Inrush
With a forward phase dimmer, a very steep rising edge causes a large inrush current every cycle as
shown in Figure 36. Series resistance (R5, R9) can be placed between the filter and the TRIAC to limit the
effect of this current on the converter and to provide some of the necessary holding current at the same
time. This will degrade efficiency but some inrush protection is always necessary in any AC system due to
startup. The size of R5 and R9 are best found experimentally as they provide attenuation for the whole
system.
Figure 36. Inrush Current Spike
Damper
The inrush spike can also excite a resonance between the input filter of the TRIAC and the input filter of
the converter. The associated interaction can cause the current to ring negative, as shown in Figure 36,
thereby shutting off the TRIAC. A TRIAC damper can be placed between the dimmer and the EMI filter to
absorb some of the ringing energy and reduce the potential for misfires. The damper is also best sized
experimentally due to the large variance in TRIAC input filters. Resistors R5 and R9 can also be increased
to help dampen the ringing at the expense of some efficiency and power factor performance.
13 Design Calculations
The following is a step-by-step procedure with calculations for a 230V, 6.5W flyback design.
13.1 Specifications
f
L
= 50Hz
f
SW(MIN)
=72kHz
V
IN(NOM)
= 230VAC
V
IN(MIN)
= 180VAC
V
IN(MAX)
= 265VAC
I
LED
= 245mA
Δv
OUT
= 1V
Δv
IN-PK
= 35V
23
SNOA555C–April 2011–Revised May 2013 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver
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