Datasheet

(t)

Potential Misfire

Triac Fires Æ Inrush Spike

Design Guide

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Interaction with Dimmers

In general input filters and forward phase dimmers do not work well together. The TRIAC needs a

minimum amount of holding current to function. The converter itself is demanding a certain amount of

current from the input to provide to its output, and the input filter is providing or taking current depending

upon the dV/dt of the capacitors. The best way to deal with this problem is to minimize filter capacitance

and increase the regulated hold current until there is enough current to satisfy the dimmer and filter

simultaneously.

13.8 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 39. Series resistance (R5, R18) 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 R18 are best found experimentally as they provide attenuation for the

whole system.

Figure 39. 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 39,

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 R18 can also be

increased to help dampen the ringing at the expense of some efficiency and power factor performance.

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