t
I
in
(t)
0
Potential Misfire
Triac Fires
Æ
Inrush Spike
Design Calculations - 120V, 30W
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(33)
This clamp method is fairly efficient and very simple compared to other commonly used methods. Note
that if the ringing is large enough that the clamp activates, the ringing energy is radiated at higher
frequencies. Depending on PCB layout, EMI filtering method, and other application specific items, the
transil clamp can present problems conforming to radiated EMI standards.
If the transil clamp becomes problematic at higher frequencies, an RCD clamp can be used to dampen the
ringing. Looking at the EMI Performance section, it is obvious that the evaluation board fails near 30MHz.
This would indicate an RCD clamp is indeed necessary for this design. C29 and R49, shown on the
Complete Evaluation Board Schematic can be populated as desired to improve the EMI signature. This
will degrade efficiency some.
Inrush
With a forward phase dimmer, a very steep rising edge causes a large inrush current every cycle as
shown in
Figure 24
. Series resistance (R39, R57) can be placed between the filter and the triac to limit the
effect of this current on the converter. This will, of course, degrade efficiency but some inrush protection is
also necessary in any AC system due to startup. The size of R39 and R57 are best found experimentally
as they provide attenuation for the whole system.
The inrush spike excites resonance(s) of the input filter, which can cause the current to ring negative, as
shown in
Figure 24
, thereby shutting off the triac. The RC damper of the first stage of the input filter
should be increased to dampen the worst-case ringing energy due to this edge. This can require a
significant increase in capacitance depending upon the dimmer tested (more than 10x the filter
capacitance). The resistance is then experimentally changed to create a ringing waveform that is most
contained. The objective is to prevent the input current ringing from crossing the minimum regulated
holding current thereby preventing misfires.
Figure 24. Inrush Current Spike
10
Design Calculations - 120V, 30W
The following is a step-by-step procedure with calculations for the 120V 30W Evaluation Board. The 230V
calculations can be done in the same manner. Many components are identical between both boards for
simplicity, therefore some components on the 120V board are over-sized.
10.1 Specifications
f
L
– 60Hz
24
AN-2150 LM3450A Evaluation Board
SNVA485B – June 2011 – Revised May 2013
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