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Posted on 01 December 2019

Why Constant Current Drives Beat Voltage-Resistor Drives in LED Lighting

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Constant current boost converter improves the energy utilization

The design considerations for LED lighting systems powered by 1, 2 or 3-cell battery sources are to say the least numerous: battery energy utilisation, battery life, constant luminous flux output, the light signature (colour temperature), solution size, weight and of course total solution cost.

By Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors

 

In spite of such criteria, voltage-resistor drives (and even direct voltage drives) are often still employed for driving LEDs. When it comes to high power LED driving, such simple approaches don’t yield the best results. It’s worth recalling why.

As a function of both the manufacturing process and the operating conditions, LED forward voltage has a tendency to vary, and the use of a simple voltage-resistor drive with high power LEDs is not an ideal solution. Consider a high power white LED, its forward voltage can typically be anywhere between 2.95 to 4.25V. Not only does this change in response to LED current, it also changes with respect to temperature, at a rate of typically -2mV/K, so self-heating will also change the current.

To reduce the current variation the series connected limiting resistor will of course help but it will not solve the drive issues. To have as little LED current variation as is possible in the face of such a wide tolerance in forward voltage, the current setting resistor by necessity needs to be trimmed for each different LED forward voltage. Furthermore, in battery powered voltage resistor drives, a change in battery voltage adds to the LED current variation.

In practice, a single cell or dual cell NiCd/NiMH/Alkaline battery is not practical to use with a voltage-resistor drive for a high power LED; therefore limiting the drive’s use to batteries using 3-cells and above. Bear in mind too that at and below the LED forward voltage the battery just can’t drive the LED, requiring a change of battery or a whole unit replacement. This means the energy in the battery may not be fully utilized and the drive is therefore not an efficient and economical solution in the long term.

And a quick note on direct voltage drives: don’t use them! With a direct voltage drive, the LED current variation just can’t be determined and makes the drive simply impractical for use.

To maintain LED luminous flux and the light temperature signature throughout the lifetime of the LED, a constant current drive is required. With a constant current drive, the effects of the great variations in LED forward voltage and supply voltage become less of an issue. By comparing typical voltageresistor drives and constant current boost converter drives against exactly the same LED driving requirements, the advantages of dedicated constant current solutions are apparent.

Consider the Luxeon LXK2-PW12 LED and a nominal drive requirement of 350mA. If we set 10% and 50% reductions in light flux as points for comparison, then from Figure 1, it will be seen that these cut-off points occur at 300mA and 150mA respectively. Operating from a 4.5V supply (3-cell AAA alkaline batteries), a voltage-resistor drive for this type of LED with a typical forward voltage of 3.4V would then require a 3.14Ω current setting resistor.

Normalized luminous flux vs. LED current

Consider now Figure 2 showing the current in the same type of LED but with a forward voltage of 3.65V (rather than the typical value of 3.4V). The current obtained at 4.5V is 290mA, which doesn’t meet the 10% nominal flux cut-off criteria, highlighting the fact that resistor tuning is required for each different LED forward voltage. The discharge time to 150mA (50% reduction in light flux cut-off point), shown in Figure 3, is approximately 53 minutes. The battery voltage, Vbat, at this current is 3.86V. At this voltage, the battery will still have some energy remaining but the light output no longer meets the 50% light reduction cut-off point criteria.

LED current vs. battery voltage in 3-cell voltage-resistor drive; 3Ω resistor

LED current vs. battery discharge time for 3-cell voltage-resistor drive; 3Ω resistor

As alternatives to the previous 3-cell drive solution, Figures 4 and 7 show typical constant current boost converter circuits for a 2- cell battery system. The ZXSC400 operates from voltages down to 1.8V while the ZXSC310 operates down to 0.8V.

2-cell ZXSC400 circuit for 1W LED

LED current vs. battery voltage; 2 cell battery with ZXSC400

Figures 5 and 6 show the ‘LED Current vs. Battery Voltage’ and ‘Battery Voltage vs. Discharge Time’ curves for a 2-cell alkaline source used with the ZXSC400 driving an LED at 350mA nominal. At 300mA (10% reduction of luminous flux) the battery voltage is 2V and it can be seen that the time for the battery voltage to drop to this level is approximately 22 minutes.

Battery voltage vs. discharge time

2-cell ZXSC310 circuit for 1W LED

Figures 8 and 9 show the same curves for the ZXSC310. At 150mA (50% reduction of luminous flux) the battery voltage is 2V and the time for the battery voltage to drop to 2V is approximately 56 minutes.

LED current vs. battery voltage, 2 cell battery with ZXSC310

Battery voltage vs. time; 2-cell battery with ZXSC310

These results demonstrate that a 2-cell AAA battery used in conjunction with a ZXSC400 constant current boost converter provides a longer operational time than a 3-cell AAA battery used with a voltage-resistor drive and also provides a near constant luminous flux. For a 50% luminous flux cut off point, a 2- cell AAA battery based ZXSC310 system gives a similar run time to a 3-cell AAA battery used with a voltage-resistor drive. Therefore, use of a constant current boost converter improves the energy utilization, the battery life and/or product life, the total run cost of the solution and contributes to a ‘greener’ environment.

A constant current drive has the added advantage then of being able to remove the issue of variation in LED forward voltage, which voltage-resistor and direct voltage drives fail to achieve. With no need for forward voltage binning or resistor matching, the cost is further reduced. Use of a 2-cell rather than a 3-cell system also produces a significant reduction in the overall space and weight of the solution.

 

 

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