By: J. David DeLeonardo, Regional Analog & Power Specialist and Paul McGinnis, Advanced Engineer, Future Electronics
One important specification of any LED driver that claims to be dimmable by TRIAC is whether it causes unwanted LED blinking. This can be especially hard to avoid at the low end of the TRIAC dimming range.
READ THIS ARTICLE TO FIND OUT ABOUT
• The characteristics of a digitally-enhanced power analogue controller
• Why the value of a TRIAC’s holding current is a crucial parameter
• How software can be used to enhance the operation of a TRIAC dimming circuit
This article presents an LED driver design using Microchip’s MCP19111, a digitally enhanced power analogue controller, which is capable of driving an LED load across an exceptionally wide TRIAC dimming range of 1mA to 750mA with absolutely no blinking of the LED load. Microchip’s MCP19111_EVAL board, part number ADM00397, provides the hardware platform for the design, which drives a single LED load.
Characteristics of a hybrid power controller
This design, then, takes advantage of features of the new MCP1911x family of hybrid power-conversion controllers from Microchip. This new product family consists of three pairs of digitally enhanced power analogue products, each aimed at a specific set of power-conversion topologies. The MCP19111 supports the synchronous buck-converter topology.
As the ‘hybrid’ tag suggests, these devices offer microcontroller-like features as well as power-conversion capability, due to the embedded 8-bit PIC® MCU that they contain. In the MCP1911x family, these features include a watchdog timer, 11 GPIO pins, a 12-channel, 10-bit ADC, and an I2C peripheral with SMBus/PMBus™ compatibility.
The LED lighting design described here uses the MCP19111 to drive a single high-power LED, a LUXEON Rebel LED from Philips Lumileds, from from a 12V AC input subject to TRIAC dimming. The power-conversion topology is a synchronous buck. The typical application circuit for the MCP19111 taken directly from its datasheet shows clearly how this part can be best used to implement a converter design, as shown in Figure 1.
The MCP19111’s pins are nearly evenly divided between those related to the PIC MCU, on the left, and those related to the synchronous buckconverter elements, on the right. It should also be noted that most of the pins associated with the PIC MCU core are multi-functional.
Thus, the MCP19111 can be used to run a simple application, or to dynamically change the performance of the synchronous buck converter in the application under the direction of a system host via a PMBus or I2C interface. All of the pins associated with the synchronous buck converter, however, must be used as in Figure 1.
This shows that the MCP19111 is essentially a digitally configurable analogue synchronous buck controller running under the supervision and control of an independent MCU that is available to run a separate application. Once the programmable elements of the synchronous buck converter are configured, the converter can run without further involvement from the PIC core.
Off-the-shelf hardware platform
The application described below runs on the MCP19111 evaluation board without any modification to the hardware whatsoever. The code that was developed to run the application takes advantage of the existing ‘Buck Power-Supply Application’ code from Microchip. The modifications and additions required to implement this LED driver application are fully documented and available at: www.FutureElectronics.com/FTM.
IDesign Analyzer v1.1 is a software tool from Microchip that allows a user to optimise the board’s switching frequency, control-loop characteristics, resistors, capacitors and inductor to produce the desired output voltage and current. It is a five-tab spreadsheet that guides the user in a step-by-step process, and may be used when adapting the application described here.
In order to evaluate the operation of the application, a test set-up as shown in Figure 2 was used. The AC output from the TRIAC dimmer runs into a 10:1 step-down transformer (TR1) and then into a diode bridge. The rectified output is filtered via an aluminium electrolytic capacitor (C_1) into the input-voltage port of the MCP19111 evaluation board. Various TRIAC dimmers were tested to verify their compatibility with the system.
The load current through the TRIAC will be approximately 1:10 of the load current into the MCP19111 eval board due to the operation of the 10:1 step-down transformer. This means that, for this relatively lowpower application here, with <3W overall maximum power consumption, the load current will fall to less than the typical holding current of the TRIAC. This can be seen from the following calculation:
Power input = V_AC * I_AC so that I_AC = (3W)/(110V AC) = 35mA at maximum power into LED load
While there are TRIACs available with sensitive gates that have a holding current well below this value, in retrofit applications such a TRIAC might not be in place. This application therefore caters for the worst case, in which the TRIAC’s holding current is higher than the application’s load.
When the TRIAC load current falls below its holding current, the TRIAC will misfire after turn-on and effectively turn on and off repeatedly for the remainder of that AC half-cycle, as shown in Figure 4. In many LED driver circuits, this will result in visible flickering of the LED’s output, which is totally unacceptable to the user.
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Once the signal is passed through the diode bridge and out to the filter capacitor, the result is simply an AC voltage with a DC offset, see Figures 3 and 4. Of course, the resulting RMS voltage of the signal in Figure 4 is markedly lower than in Figure 3, but this does not matter: as long as the rectified and filtered signal is above a certain conversion threshold (V_In_Min_Conv), the MCP19111 will continue to draw current from the filter capacitor and deliver it to the load.Since the TRIAC can continue to misfire as in Figure 4 indefinitely, a very low load current can be delivered with no disturbances in either the load current or light output. In the test set-up, shown in Figure 2, load currents as low as 1mA are easily maintained.
Flow of the code
Figure 5 shows a flow chart of the code that was inserted into Microchip’s MCP19111 buck power-supply code immediately after initialisation and variable declaration. On start-up, the application code inhibits any switching of the FETs until V_in has risen above 5.0V for at least 100ms. After this condition has been met, the code enters Load Current Maintenance mode. Here, the output current is determined by multiplying the target value by the ratio of the V_Input_New, at the input to the board, by the expected nominal voltage (V_nom), as calculated by:
I (out) = I (Nominal) * [(V_Input_New)/(V_nom)]
From here, the value of the output-current set point is increased or decreased by one count or set to the minimum or maximum allowed value.
Clearly, from the above equation, it can be seen that no attempt is made to infer the TRIAC’s duty cycle: a change in the RMS value of the rectified and filtered line voltage will be treated the same way whether it is the result of a change in the TRIAC’s duty cycle or a change in the AC line voltage.
Extending the application’s capabilities
The application described here provides a base on which designers can build to provide more or better features,
using the capabilities of the MCP19111. For instance, the MCP19111 circuit and evaluation board may be customised to support much wider input- and output-voltage ranges, to provide for direct analogue dimming, and to correct for changes in the AC RMS line voltage that are independent of the TRIAC duty cycle.