Constant Current Switching Regulators are Better than Voltage Resistor Drive for LED Drive

Introduction

For the design of an LED light system powered from a low voltage supply such as 1-, 2-, 3- cell battery sources, the total cost of solution, battery life, the battery energy utilization, reasonably constant luminous flux output, the light signature (color temperature), the solution size and the weight are some of the important considerations, in applications such as flashlights, signage and illuminations. Switched mode power supplies outperform a voltage resistor drive for the above concerns.

 

Constant Current Drive vs Voltage Drive

A constant current drive is the recommended method of driving an LED and is the key to maintaining the LED luminous flux and the light temperature signature; increasing the efficiency, energy utilization and life; as well as reducing the overall cost of a LED light system.

LEDs have a wide variation in forward voltages, e.g. the forward voltage of a high power, white LED is typically 2.95 to 4.25V. The forward voltage also changes with the LED current. Additionally, the forward voltage changes with temperature at the rate of typically -2mV/K. Therefore the self heating will also change the current.

Sometimes, a series resistor is used with a voltage source to set the LED current and to lower the range of current variation, but this does not solve the drive issues. To minimize the LED current variation in the voltage resistor drive, the forward voltage needs to have a very small tolerance and requires the current setting resistor to be trimmed for each different LED forward voltage. Further, in battery powered voltage drives, a change in battery voltage changes the LED current, thus adding to the LED current variation.

With a constant current drive, the effects of variations in the LED forward voltage and the supply voltage are less of an issue. Low cost ZETEX ZXSC400 and ZXSC310 are such constant current boost converters.

 

Switching DC-to-DC Converter vs Voltage Resistor Drive

It is important to compare drive methods with regard to the system efficiency, battery life and/or the product life, the total unit and the running cost of solution, the size and weight of the system and the conscious effort for a ‘greener’ environment.

With a direct voltage drive, the LED current variation is undermined and impractical for use, and therefore should not be used. With a voltage-resistor drive, at and below the LED forward voltage the battery is no longer able to drive the LED, thus requiring the change of battery or replacing the whole unit. This means the energy in the battery may not be fully utilized and is therefore is not efficient (with regards to total energy utilized) and economical in long term overall solution. Single cell or dual cell NiCd/NiMH/Alkaline type battery solutions are not practical with a voltage-resistor drive for a high power LED; thus limiting it to 3-cells and above batteries.

In a voltage-resistor drive for a LXK2-PW12 LED, with a forward voltage of 3.4V typical, at 350mA, from 4.5V supply (3-cell AAA alkaline batteries), a 3Ω current setting resistor is needed.

For a nominal drive of 350mA, if we set a 10% and 50% reduction in light flux as comparison points then these cut off points occur at 300mA and 150mA respectively. Figures 1 and Figure 2 show a LED current for a 3-cell AAA battery when driving a LXK2-PW12 LED with a forward voltage of 3.65V (other than typical value of 3.4V) with a 3Ω current setting series resistor. The current obtained at 4.5V is 290mA, which highlights the fact that resistor tuning is required for each LED with a different forward voltage. The 290mA does not meet the 10% light flux reduction cut off design criteria right from the start for this LED with forward voltage of 3.65V with 3Ω series resistor. The discharge time to 150mA (50% reduction in light flux cut off point), from Figure 2, is approximately 53 minutes. The battery voltage, Vbat, at 150mA from the battery discharge curve in Figure 3 is 3.86V. At 3.86V, the battery will still have some energy remaining but the light output no longer meets the 50% light reduction cut off point.

Figure 1: Normalized luminous flux vs LED current	Figure 2: LED current vs battery voltage in 3-cell voltage-resistor drive; 3Ω resistor
Figure 3: LED current vs battery discharge time for 3-cell voltage-resistor drive; 3Ω resistor	Figure 4: 2-cell ZXSC400 circuit for 1W LED

 

Low cost, constant LED current DC-DC converters help to solve issues related to LED drive, battery life and overall cost. Figures 4 and 7 show typical ZXSC400 and ZXSC310 boost converter circuits for a 2-cell battery system. The ZXSC400 operate from voltages down to 1.8V while ZXSC310 operate down to 0.8V.

Figures 5 and 6 show a 2-cell alkaline ‘LED Current vs Battery Voltage’ and ‘Battery Voltage vs Discharge Time’ curves for ZXSC400 driving a LED at 350mA nominal. At 300mA (10% reduction of luminous flux) the battery voltage is 2V. From Figure 6, the time for the battery voltage to drop to 2V is approx. 22minutes.

 

Figure 5: LED current vs battery voltage; 2-cell battery with ZXSC400

Figure 6: Battery voltage vs discharge time; 2-cell battery with ZXSC400

 

Figures 8 and 9 show a 2-cell alkaline ‘LED Current vs Battery Voltage’ and ‘Battery Voltage vs Discharge Time’ curves for ZXSC310. At 150mA (50% reduction of luminous flux) the battery voltage is 2V. From Figure 10, the time for the battery voltage to drop to 2V is approx. 56 minutes.

 

Figure 7: 2-cell ZXSC310 circuit for 1W LED	Figure 8: LED current vs battery voltage; 2-cell battery with ZXSC310
Figure 10: Battery voltage vs time; 2-cell battery with ZXSC310

 

The results shows that a 2-cell AAA battery with ZXSC400 constant current boost converter gives longer operational times for high performance, near constant luminous flux than with a 3-cell AAA battery with a voltage-resistor drive. For 50% luminous flux cut off point, a 2-cell AAA battery ZXSC310 system gives a similar run time than 3-cell AAA battery with a voltage-resistor drive. Therefore, use of a switch mode boost converter improves the energy utilization, the battery life and/or the product life, the total run cost of the solution and the improvement towards a ‘greener’ environment. A constant drive also gives an advantage that the variation in forward voltage of the LED is no longer an issue; no forward voltage binning or resistor matching are required reducing the binning and matching cost. Use of 2-cells instead of a 3-cell system also gives significant reduction in overall space and weight of the solution.

 

 

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