Tweet

Posted on 01 March 2019

Want To Dim Your LEDs with a TRIAC Dimmer?

Free Bodo's Power Magazines!

 

 

 

LEDs simply react a lot faster than conventional bulbs

Light dimmers for common bulb based lighting have been around for ages. The most common implementation of such a dimming circuit is based around a TRIAC (TRIode for Alternating Current).

By Ernest Bron, Field Applications Engineer, National Semiconductor Europe

 

Most TRIAC based dimmers are intended to connect directly to the 220V or 110V AC mains. Figure 1 shows the basic (simplified) circuit and associated waveforms of a TRIAC dimmer. The mains voltage is divided down by R1 and R2 and C1 is charged up. At some point, the voltage of C1 reaches the trigger voltage of the diac which then becomes conductive. The charge from C1 is dumped on the gate of the TRIAC which starts conducting. This then connects the mains AC signal to the load (typically a light bulb).

Basic circuit and the associated waveforms

The diac will remain conductive until the current flowing through it reaches a level close to zero. Effectively this means that the TRIAC remains conductive until the input mains AC voltage reaches zero. Since the input mains AC voltage is a repetitive sinusoid waveform, the net effect of the dimming circuit is that the load (light bulb) is exposed to the mains AC voltage for only a certain percentage of the waveform period. Less exposure of the voltage over the load, means less power dissipation inside the load (integrated over time), hence the dimming effect. Figure 1 shows the basic waveforms. The top one shows the mains AC signal coming in and the bottom one shows the actual signal presented to the load. The delay time from the start of the period to the TRIAC turning conductive is determined mostly by R1, R2 and C1.

A TRIAC based dimming circuit is simple, low cost and achieves the desired effect elegantly (you determine turn on time, but turn off is automatic). This circuit is by far the most popular for dimming lighting.

In order for TRIAC based dimmers to operate properly, they require the load to be resistive in nature. Since traditional lighting has predominantly been light bulb based, this has never posed a problem. With the introduction of fluorescent based lighting, other (more complex) types of dimming circuits were developed. However, the bulk of lighting continued to be light bulb based and TRIAC dimmers remained prevalent in the market.

Now we are on the verge of widespread introduction of new more energy efficient lighting solutions, many of them LED based. TRIAC based dimmers will not work with LED based lighting. Nevertheless, it is desirable to find ways to make TRIAC based dimmers work with LED lighting. Not only because there are a lot of installed TRIAC based dimmers, but also because suppliers of lighting systems want to offer complete solutions with all possible options, including dimming capability. Naturally it is preferable to use standard, low cost and off-the-shelf building blocks. A TRIAC based dimmer is just that.

TRIAC dimmable LED driver image

LEDs are mostly dimmed by either changing the current, or by turning them on and off quickly using a constant current (PWM dimming). So in order to hook up a standard TRIAC dimmer to LED based lighting modules, to translate the trigger point (or delay) into either a DC current or a PWM dimming signal. As trivial as this may sound, this is not simple to do. Using the inherent frequency of the AC signal coming from the TRIAC is not an option. Depending on the line frequency (50Hz or 60Hz) this will be either 100Hz or 120Hz. A light bulb responds only slowly to any change in power being dissipated in it and will inherently eliminate any flickering effect. But, switching any LED on and off at those frequencies will yield visible flickering effects. LEDs simply react a lot faster.

In order to hook up a standard TRIAC dimmer to LED based lighting modules, we need to translate the trigger point (or delay) into either a DC current or a high frequency PWM dimming signal. National Semiconductor has now introduced a new LED driver IC, the LM3445. This part integrates most of the functions needed to translate a TRIAC dimmer trigger point into an average current running through a number of LEDs. Figure 2 will be used to illustrate how the device achieves this. In figure 2 we see an example circuit diagram of the LM3445 being used to control and dim LEDs.

Circuit diagram of the LM3445 being used to control and dim LEDs

On the top left side we see the entry point for the main AC input. This AC power input is first rectified using a diode bridge (BR1). The rectified signal is translated to a lower voltage level signal by means of R2, D1, Q1 and is fed to the BLDR pin of the LM3445. This same lower level signal is used to provide a stable power level to Vcc via D2 and C5 (should the BLDR signal go too low, D2 and C5 will buffer it). The main function of R5 is to ensure that a minimal amount of current is drawn even at light loads (we want to make sure the TRIAC in the dimmer remains conductive).

The AC signal on BLDR is compared to an internal fixed voltage to determine the angle at which time the TRIAC is triggered. After level limit and some noise filtering this angle signal is fed to the ASNS pin where it is externally filtered by means of R1 & C3. The net result is an analog signal which is indicative of the duty cycle of the mains AC input. This signal then re-enters the LM3445 via its FLTR1 pin and is used by an internal dim decoder circuit to limit it to a level corresponding to duty cycle ratios between 25% and 75%. At the same time this duty cycle limited signal is output in two forms. One is a PWM style signal, output on the DIM pin, the other a DC level signal output on the FLTR2 pin (in reality, the dim decoder first creates one and then the other, details can be found in the datasheet).

The internally imposed duty cycle limits of 25% to 75% correspond to dimmer firing angles between 45° and 135°. Note, though, that dimming at angles above 135° is still possible. It is simply not done by the internal dimming detect circuitry. At those dimming ratios, there will be either voltage headroom or FET on-time limitations which will in effect cause the LEDs to dim.

The actual LED driver section of the LM3445 is based around a constant off-time control scheme. R4, Q3 and C11 generate a linear current ramp which is fed into the COFF pin and used to generate the off-time. With the off-time defined, the on-time of the DC/DC is determined by the peak current limit (i.e. Q2 is turned on until such time as the peak current limit is reached). It is the DC output signal of the dim decoder circuit (the same one as present on the FLT2 pin) which determines the peak current limit. So the on-time is directly controlled by main AC duty cycle. Varying on-time at constant off-time effectively means changing the duty cycle, which in turn means changing the average current through L2 and the LEDs. Hence we achieve dimming.

The circuit on the top right side of figure 2, made up of D4, D8, D9, C7 and C9, is a so-called valley-fill circuit and is used to provide the power needed to drive the LED string.

Multiple Strings

In addition to providing basic TRIAC firing angle detection and conversion into average LED current, the LM3445 also offers the ability to provide a master dimming signal which can be used to daisy-chain multiple LED drivers (either LM3445 or other types) in a master/slave setup. This allows for accurate dimming control over multiple strings and/or modules, making sure the overall visual effect of dimming over a large number of strings/arrays is uniform and smooth. Figure 3 shows how such a master/slave dimming circuit could look like when using multiple LM3445 devices.

Master-slave dimming circuit using multiple LM3445 devices

The first LM3445 is operated in master mode, while all the others are operated as slaves. Master mode operation is more or less automatic; a master PWM dimming signal is provided via the DIM pin. The one thing we do want to make sure though is that in the event of a sudden input voltage drop, the master device detects this before any of the slaves do so. To achieve this we place an additional diode in series on the Vcc circuit. Slave mode is achieved by connecting the FILT1 pin to Vcc. This disables the internal dim decoder and the DIM pin can be used to input an external dimming signal (provided by the master LM3445). Figure 3 shows an example where the valley-fill circuit is implemented separately on each device. In the case where there are many slaves it may be advantageous to combine this circuit into one larger circuit to be shared by all LED drivers.

Conclusion

The LM3445 makes it possible to create energy-efficient LED based lighting solutions that can be dimmed using standard off-the-shelf TRIAC based dimmers without visible flickering effects. With its ability to enable dimmable lighting products at a significantly improved energy efficiency level, the LM3445 is part of National’s PowerWise® family of products. More information on National’s energy-efficient solutions can be found under www.national.com/powerwise.

 

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.