Different leading-edge dimmers have different values for C S and L S which act to modify the current ring on the TRIAC. 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.
#Leading edge vs bleeding edge series
To reduce the ring, a damper circuit is added – in its simplest form a series resistance to reduce the amplitude of oscillation at the expense of reduced efficiency (and therefore more heat for the LED bulb enclosure to manage). Typical input current waveform for a power-factor-corrected dimmable bulb showing the oscillation caused by input current dropping below IH
The amplitude of the load ring is modulated by the surge protection capacitor C S, making the amplitude of the oscillation dependent on dimmer type.įigure 3. In the simplest form, the bleeder is a simple RC combination that insures a pulse of current when the input voltage is applied.Īn 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 (I F) (see figure 3).
#Leading edge vs bleeding edge driver
In order to insure that I L is achieved, a bleeder circuit is typically added to the LED driver input stage. Because the TRIAC-dimmer/incandescent-bulb interface is not sensitive to the L S and C S values, the values of these components are not constrained and vary significantly between different leading-edge dimmer designs.Īt turn-on, an LED load presents relatively high impedance, so input current may not be sufficient to latch the TRIAC dimmer. With no capacitive or inductive elements, the incandescent bulb does oscillate when presented with the voltage step of a dimmed AC sine wave. The bulb presents a very-low impedance during turn-on, latching the TRIAC (I F>I L) and once in conduction allows current to flow to zero crossing which holds the TRIAC in conduction (I F > I H) for almost the whole AC half-cycle. The installed base of TRIAC dimmers in use today are designed to work with an almost ideal resistance (an incandescent bulb). The TRIAC is protected against input voltage surges by a bypass capacitor C S and from high transient currents at switch-on by a series inductance (L S). It will continue to conduct until the forward current drops below a threshold (holding current). Unlike BJTs or MOSFETs the TRIAC will latch-on once it is energized (after forward current exceeds latching current). In leading-edge phase-cut dimmers, the switching element is typically a TRIAC. Why shimmering and flickering occurs in leading-edge dimmers and why leading-edge and trailing-edge dimmers respond differently Simplified schematic of a trailing-edge, phase-cut dimmer Simplified schematic of a leading-edge phase-cut dimmer (Including transient and surge suppression elements LS and CS)įigure 2. That-being-said, it is unlikely that the average consumer will know whether their fixture is controlled by a leading-edge or a trailing-edge dimmer, and so it is important that LED replacement bulbs work with both types.įigure 1. Leading-edge dimmers are typically lower cost and so are more widely used whereas trailing–edge dimmers exhibit lower EMI and are preferred in some markets (notably Europe) and noise sensitive environments. 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. 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. Phase-cut dimmers, either leading-edge or trailing-edge, make up the bulk of the dimmer market. This blog highlights how dimmer type determines both the selection of damper and bleeder circuits in LED drivers, and the switching topology needed to optimize operation. 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. The installed base of domestic dimmers supports the almost ideal resistance exhibited by the impedance of an incandescent bulb.