技術 - LED調光
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 energized (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, the values of these components are not constrained and vary significantly between different leading-edge dimmer designs.
At turn-on, an LED load presents 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.
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) (see figure 3). 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 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. The designer must allow sufficient margin (give up efficiency) in the damper circuit to work with as many dimmers as practicable.
To further enhance damping, a bleeder is needed to compensate for, or mask the ringing below, the holding current. A simple RC bleeder is used across the input line or after the bridge rectifier. The bleeder is optimized with respect to the power rating of the LED driver. For lower power LED lamps higher bleed is required.Trailing-Edge Dimmers Present A Different Set Of Problems
The input voltage waveform from a trailing-edge dimmer is sinusoidal at the start of each half-line cycle. The MOSFET switch is driven by a controller which continually energizes the gate, making the dimmer less susceptible to current ringing.
However, the power supply in the LED will present a high impedance to the dimmer when the MOSFET switch is opened to cut power delivery. Trailing-edge dimmers require the input voltage of the LED driver to fall to zero each half-cycle to enable the dimmer controller to energize its own supply rails. This ensures that the zero-crossing detector will turn on the switch at the beginning of the next voltage half-line cycle. If there is insufficient impedance to bleed down the dimmers output voltage before the next AC cycle begins, then the dimmer may misfire causing shimmer and flicker.
Buck converters in particular have 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.
- Sources: Ledjournal
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. 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. The Yoswit smart dimmers support both Leading Edge and Trailing Edge dimming and can be switched case by case when connecting to different types of lamps through the app.
Deep DimmingRead More In Technical Explanation:
The typical dimming range requirement for leading-edge TRIAC dimmers is a 10:1 current step; for trailing edge dimmers the figure is above 5:1. To understand what controls dimming range and why the accepted performance for leading-edge and trailing-edge dimmers is different, it is necessary to understand how minimum output current is achieved in a phase-cut dimmer.
Trailing-edge dimmers use internal logic circuitry to control the dimming angle which requires power. This power is delivered to the driver when the TRIAC is turned-off (not delivering power to the lamp). To ensure this happens, trailing-edge dimmers tend to have higher dimming angle than equivalent leading-edge types. Typically this was not a problem with incandescent bulbs because their light output changes exponentially with power at low brightness levels (the bulb gives very little light output at low conduction angles) which means low brightness occurs significantly above the limit the power storage requirement imposes.
The red trace in figure one shows an LED driver output current that is directly proportional to the conduction angle. The LED load takes a high load current even at relatively low conduction angles, but does not reduce current sufficiently by the time the minimum conduction range for TRIAC dimmer is reached. The high load current delivered using this approach means that TRIACS will see high holding currents, reducing the likelihood of shimmer or flicker.
The designer can elect to increase the dimming-slope of the LED driver to arrive at a lower output current at a higher dimming angle (blue trace in figure 1). This allows the bulb to dim to a lower brightness but risks causing shimmering and/or flickering as the load current drops. In a practical design this means that a significant amount of extra power must be drawn by a bleed circuit to keep the TRIAC from misfiring (this region is shown in red area in figure 1). This reduces driver efficiency making the need for heat sinking or potting materials more likely in the final design.
An alternative approach is to employ an adaptive bleeder circuit such as shown in the driver in figure 3. The bleeder circuit draws more current in deep dimming to compensate for the reduced output current but preventing excess heating at full brightness
- Sources: Ledjournal
2-in-1 Triac & 0/1-10v Dimming
Sync Dimmer Brightness On Multiple Switches
Unlike the traditional manual dimmer, the Yoswit smart dimmer switch provides the app interface to display the output level on the screen for each of the switch. The output of the switch can be set as a scene and synchronized the lamp brightness on multiple switches with just a click.