High Voltage LED Applications



This article describes how to create high voltage circuit designs using the MLX10803 for domestic and industrial use, or any application requiring high light levels from light emitting diodes.

The applications described in this article are for driving high power LED diodes. The described circuits can be used in other applications with similar circumstances as well, in case they fall within the specifications of the MLX10803. This is a conceptual description and in some cases, no component values are given. The applications described have in most cases been implemented.

It is not a trivial task to design for high voltage, and great care must to be taken in respecting the high voltages involved. The examples discussed have to be carefully designed and fabricated; slight errors can have devastating results. These examples are not recommended for the inexperienced or untrained. It is intended for electrical and electronic engineers with relevant training and experience designing and building high voltage circuits. The circuit board has to be designed according to high voltage rules. Depending on the user’s specific implementation, these examples can have lethal voltages present.

The circuits described in this article have a very good efficiency; converting efficiencies up to 98-99% are to be expected. This makes the circuit design fairly simple in respect of thermal handling. Remember that if you design lamps with up to 100 watts and more, even 1% loss generates 1 watt, and more heat.

Every kind of active current regulation generates ripple on the regulated output. This ripple can generate electromagnetic radiation (EMR), resulting in electromagnetic coupling to the surrounding electronics (EMC). The MLX10803 and the applications described in this article are designed to minimize EMR. Additional care has to be taken when designing the circuit board and the physical application. Melexis makes no claims about the suitability of any of these circuits for EMI/EMC and EMR compliance against international regulations. Compliance testing is recommended and it will fall to the user to conduct such testing prior to sale in specific countries and markets.

Design tools

High voltage design is more demanding than making low voltage applications using the MLX10803. Melexis strongly recommends using the following presented tools when making high voltage applications. Use these tools for your own safety, testing before you connect your LED application to a 110/220 V source.

Design tool 1: coil calculation program

This Microsoft Excel calculation sheet is a necessity when designing with high energies.

Find this program on the Melexis web site. Download the file and then open the coil_calc_(x).zip file [where (x) is the version number] and use the included Microsoft Excel spreadsheet.

The spreadsheet is self-instructive. Play with it and get a feeling about the relationship between coils, currents and sense values. The real mathematics behind the MLX10803 function is complicated; this sheet is a simplification. The tool gives only rough values, but it gives an impression what component sizes and settings are needed.

This tool will help you to find a rough value for:
  • The coil
  • The programmable sense voltage and the sense resistor
  • The mono flop time and the time to partly discharge the coil, which is a derivate of the oscillator frequency in the MLX10803

After finding out coil, reference resistor, and sense resistor values, as well as mono flop time, oscillator resistor values, and sense voltage settings with this tool, it is time to apply the component values to the MLX10803 LED driver circuit and try out the values in an application.

Design tool 2: evaluation board EVB10803

Testing your solution with the EVB10803 evaluation board is highly recommended. This will save you time and money and help you to understand the function of the MLX10803 chip. You should not design any high voltage application without an understanding of the function of the MLX10803 circuit.

Warning: The evaluation board EVB10803 is specified to function up to 32 V, or 80 V for half a second. It is not intended to be used at 220 V.

The LED supply can be separated from the circuit supply (VS/PWM) by removing the jumper for that on the EVB10803. See the manual for EVB10803 for instructions. When the supply to the LED is separated from the VS/PWM, the supply voltage to the LED is limited by the switching N-FET transistor parameters and the fly back diode. Please use this option with great care, and check carefully what type of transistor and fly back diode your EVB10803 is equipped with before applying any higher voltage than 32 V.

Applications: Simplest solution

Figure 1: Simple MLX10803 solution for driving LEDs with high voltage lines.

The Figure 1 circuit schematic illustrates the simplest possible high voltage solution utilizing the MLX10803. The coil and the RSENSE resistor have to be adapted to the type of diodes used and the number in the series. Additionally, the desired operational range has to be taken into account for the right size of this coil and resistor. The circuit is a down regulator, so at a given minimum operational voltage there are a maximum number of diodes possible in a series connection. This number of diodes depends on the forward voltage of the type of diode. The sum of the diode voltages must in all cases be lower than the input voltage.

Example: Typical white diodes have a forward voltage of 3.5 V. Twenty-five diodes in series results in 87.5 V in total forward voltage, which will give a margin on a 110 V supply. This is the recommended maximum number of diodes for 110 V in this case. The forward voltage varies with many factors and one among these is temperature. The difference between 87.5 V and 100 V is needed for these variations of forward voltage as well as variations of the supply voltage. Note that this design will also work for a much higher applied supply voltage and gives you a universal voltage lamp if so is desired. Figure 2 shows the estimated component selection for driving a series string of twenty LEDs.

Figure 2: Simple MLX10803 solution for driving 20 LEDs with high voltage lines component selection.

Applications: More efficient solution

Figure 3: More efficient MLX10803 solution for driving LEDs with high voltage lines.

The Figure 3 schematic describes a possible higher efficiency high voltage solution. This solution has the same limitations as the circuit in Figure 1 in that it regulates the voltage down from the input lines. Therefore, the number of LEDs that can be driven in a series configuration are similarly limited. Figure 4 shows the estimated component selection for driving a series string of twenty-two LEDs, two more LEDs than the previous circuit.

Figure 4: Higher efficiency MLX10803 solution for driving 22 LEDs with high voltage lines component selection.

Applications: LED lamp dimming solution

Figure 5: MLX10803 dimming solution for driving LEDs with high voltage lines.


The circuit in Figure 5 is basically the same as that in Figure 3 with the exception that the VREF pin of the MLX10803 is tied to potentiometer R3 instead of the VS/PWM pin. Operating the R3 potentiometer will act as a dimmer for the string of LEDs being driven by the circuit.

Summary

The Melexis MLX10803 high power LED driver can be configured in several ways to provide reliable driving current for a string of LEDs while being fed by 110/220 V main lines. The circuits presented here can be modified depending on the desired application and the number of LEDs that need to be driven and whether dimming is desired.

Disclaimer

Be aware that these circuit applications work directly with a 220 V or 110 V supply. There are national and international safety regulations for this type of electrical circuits. This article does not cover any safety issues for electric circuit designs.

The circuit applications in this document have been thoroughly tested by Melexis when not otherwise mentioned, and have worked satisfactorily in the described applications. However, Melexis does not assume any legal responsibility or will not be held legally liable in the use of these circuit applications, under any circumstances.

Reference:
  1. Melexis Application Note MLX10803 – High Voltage Solutions
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