POWER ELECTRONICS FEATURE
LEADING-EDGE VS. Trailing-edge dimmers
Andrew Smith, Senior Product Manager – LED at Power integrations Lighting, highlights how dimmer type determines both the selection of damper and bleeder circuits in LED drivers, and the switching topology needed to optimise operation
T
he installed base of domestic dimmers supports the almost ideal
resistance exhibited by the impedance of an incandescent bulb. These devices are increasingly called upon to support LED replacement lamps which offer challenges unanticipated by the designers of the dimmer systems, such as low current draw and very fast luminous response to minor power fluctuations. Phase-cut dimmers, either leading- edge or trailing-edge, make up the bulk of the dimmer market. After the input voltage rises following the zero crossing, leading edge dimmers inhibit for a period of time, controlling energy transferred to the lamp load and hence output brightness. Trailing-edge dimmers also regulate output by inhibiting for a period of time, however this is referenced to the negative going edge of the half-cycle. In leading-edge phase-cut dimmers, the switching element is typically a TRIAC. Unlike BJTs or MOSFETs the TRIAC will latch-on once it is energised (after forward current exceeds latching current). It will continue to conduct until the forward current drops below a threshold (holding current). The TRIAC is protected against input voltage surges by a bypass capacitor CS and from high transient currents at switch-on by a series inductance (LS). The installed base of TRIAC dimmers in use today are designed to work with an almost ideal resistance (an incandescent bulb). The bulb presents a very-low
impedance during turn-on, latching the TRIAC (IF>>IL) and once in conduction allows current to flow to zero crossing which holds the TRIAC in conduction (IF > IH) for almost the whole AC half-cycle. With no capacitive or inductive elements, the incandescent bulb does oscillate when presented with the voltage step of a dimmed AC sine wave. Because the TRIAC- dimmer/incandescent-bulb interface is not sensitive to the LS and CS values. At turn-on, an LED load presents
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resistance to reduce the amplitude of oscillation at the expense of reduced efficiency (and therefore more heat for the LED bulb enclosure to manage). The LED Bulb designer must add the
smallest amount of damping impedance at the input stage of the LED that will allow the LED bulb to remain above the minimum holding current. Different leading-edge dimmers have different values for CS and LS which act to modify the current ring on the TRIAC. The TRIAC in each dimmer type will see more ringing than would be seen at the bulb due to LS. Buck converters in particular have
Figure 1:
Simplified schematic of a leading-edge phase-cut dimmer (Including transient and surge suppression elements LS and CS)
relatively high impedance, so input current may not be sufficient to latch the TRIAC dimmer. In order to insure that IL is achieved, a bleeder circuit is typically added to the LED driver input stage. In the simplest form, the bleeder is a simple RC combination that insures a pulse of current when the input voltage is applied.
Figure 2:
Simplified schematic of a trailing-edge, phase- cut dimmer
LIGHT WITHOUT THE LOAD An LED lamp load does not exhibit incandescent-like pure resistance, and so, when presented with a step voltage the EMI filter and the bulk capacitance of the switching stage will cause an oscillation in the input current (IF). The amplitude of the load ring is modulated by the surge protection capacitor CS, making the amplitude of the oscillation dependent on dimmer type. To reduce the ring, a damper circuit is added – in its simplest form a series
challenges when supporting trailing- edge dimmers. Buck converters are very popular for LED lamp drivers due to their high efficiency and low component count. For a buck topology, when the input voltage falls below the output voltage, the switching circuit cannot draw any power from the AC rail (and is therefore unable to bleed down the switch voltage). In contrast, buck- boost, tapped-buck and flyback converters can draw current for the entire switching cycle. For this reason, buck-boost converters and tapped- buck drivers with ICs, which switch through the whole line cycle as the LYTSwitch-4 from Power Integrations, can pull down the dimmer voltage after it turns off and are therefore better able to support trailing-edge dimmers. Bleeder and damper circuits can be tuned to accommodate almost all leading-edge phase-cut dimmers. The designer trades off efficiency in order to achieve best possible dimmer compatibility but is not able to guarantee performance due to the variability of dimmer component values. Practical designs usually accommodate trailing-edge dimmers. In order to work with trailing-edge
dimmers, further compromise on efficiency (large bleed current) or even a change in topology may be required in order to achieve acceptable dimmer compatibility in a given bulb design.
Power integrations
www.powerint.com +1 (408) 414-9200
Enter 208 ELECTRONICS | OCTOBER 2014 19
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