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How to implement high current LED arrays

Posted: 22 May 2007 ?? ?Print Version ?Bookmark and Share

Keywords:high current LEDs? RGB LEDs? LED driver solutions?

By Mick Lovell Field
Micrel Inc.

Today's high-current LEDs are finding applications that replace conventional lamps including filament and fluorescent and metal-halide lamps as well as conventional LEDs. These new LEDs bring the advantages of high reliability, color control and extended lifetime as well as usually better luminous efficiency. They are finding applications in the home, industrial, portable devices and in vehicles.

The latest high current LEDs include multiple series connected LED strings on a single thermal/mechanical substrate. They are driven with current control to give matched light output on each LED. Typical mechanical configurations include star, string and ring. Driving these series of connected arrays of LEDs requires accurate control of the LED current to control illumination and power consumption as well as giving a long operational life.

This article summarizes the key requirements for driving Osram Ostar high-current LED arrays and shows how these requirements can be met with an easy-to-implement driver based on Micrel's MIC2196 chip.

Figure 1 shows an array of six outstanding brightness and luminance (up to 18cd/m?) white LED emitters in ThinGaN LED technology. These are mounted onto an ultralow thermal resistance substrate. The entire assembly is only 20mm in diameter and is capable of dissipating 27W. Moreover, the six LEDs occupy an area of only 2.1-by-3.2mm. The nominal operating current of this series connected array is 700mA with a peak current rating of 1A. Total voltage variation of the device is between 17V and 26V at full load current.

Figure 1: This entire assembly of six white LED emitters is only 20mm in diameter and is capable of dissipating 27W

LED arrays
Typically, these arrays of LEDs are required to work from a nominal 12Vdc supply which can have a significant output voltage variation. Dimming can be used to reduce light levels or to match mixed color RGB LEDs light output. Dimming requirements are complicated by the variations present in the chromaticity of high power LEDs with LED current. Two common methods of dimming are presently used: a continuous (analog) dimming in which the current in the LED is reduced in a linear manner and PWM dimming in which the LED current is turned ON and OFF by a switch to achieve an average current.

Compensation and optical feedback may be required in the case of mixed color RGB LEDs where the variations in chromaticity with load current exceed the perception level of the human eye. In terms of chromaticity, this is known as exceeding the four-step Macadam ellipse.

This level of optical compensation and feedback is beyond the scope of this article. PWM dimming frequency must be well above the persistence of vision threshold. An ability to turn the LED array completely off can also be considered part of a dimming scheme. The LED driver must have a small footprint and have high efficiency since it usually packaged with or very close to the lamp. Electrical noise on the input side must be minimized to reduce the amount of additional filtering required to meet conducted emissions specifications. Moreover, the driver must provide over-voltage safety protection during an open circuit LED. Finally, the driver electronics must also be easy to implement and flexible in application. (View Table 1 for details.)

Driver design
The LED Driver uses a Micrel MIC2196 400kHz boost controller as the power stage. MIC2196 steps up the input voltage using a boost switch-mode power cell. The power switch Q2 boosts the input voltage using L1, D1 and C1. Q2 is an N-channel MOSFET whose gate is driven by the MIC2196 controller. The high current capability of the MIC2196 FET driver reduces switching loss in the MOSFET, which is a major factor in determining overall efficiency. The high operating frequency means that the main energy storage components L1 and C1 can be small-value, small-footprint.

Overvoltage protection is implemented simply as Zener diode Z1. In case of an open circuit, the LED current feedback signal will be lost and the output voltage will rise because the PWM duty cycle will increase. During this event, the Zener Z1 will conduct at around 33V, limiting the maximum possible overvoltage by injecting current into the feedback network in such a way as to reduce the duty cycle.

The required Ostar current is measured by the current sense resistor R10 and used to control the duty cycle of the MIC2196. 330m for R10 gives 700mA nominal current with DIM pin not used (open-circuit). A value of 680m for R10 will give a nominal 350mA LED current. This current measurement is scaled to the error amplifier within the MIC2196 by means of R14 and R3. A 1 percent voltage reference, Z2, gives the required current setting accuracy without requiring an external current sense amplifier.

The two different dimming methods are shown in Figure 2a and Figure 2b, respectively. In Figure 2a, continuous dimming is achieved by adding the resistor R15 and inputting a DC voltage to the "Analog DIM" pin. Alternatively, a filtered PWM signal could be applied to the same pin. The dimming range of 5V to 0V gives a linear variation of 10-100 percent output current.

PWM dimming can be understood using Figure 2b. In this case, the Analog DIM resistor R15 is removed. The OSTAR current setting components remain the same: R10, R3 and R14. To achieve PWM operation, a low RDSON MOSFET Q3 is added. This MOSFET is the switch that allows PWM operation. Q3 ON resistance is low when compared to R10 so it does not significantly affect the current sensing network. A PWM drive signal of 100-300Hz and amplitude of 5V (for a logic level MOSFET) is applied to the gate of Q3. A pull-down resistor, R15, holds the MOSFET 'OFF' when no gate drive signal is present. Hence, this circuit can be simply adapted to give an OSTAR OFF capability.

The entire circuit is capable of power levels up to 30W can be implemented on a single sided 25-by-30mm PCB using SMD multi-source components.

Test results
The upper test waveform in Figure 3a, shows the input EMI with Vin=10V and Iout=700mA. The lower waveform is the drain-source voltage of Q2. The EMI is 200mVpp quasi-sinusoid with minimal high frequency content. This means very little, if any, additional filtering is needed. Figure 3b shows the start up with Vin=10Vin and Vout=20V and 700mA Ostar current. The upper waveform is the voltage across the Ostar and the lower voltage the drain-source voltage of Q2. Start-up takes

Figure3: (a) The EMI is 200mVpp quasi-sinusoid with minimal high frequency content. This means very little, if any, additional filtering is needed (b) Start-up takes 2ms and is well controlled.

In terms of the variation in Ostar current, with dimming voltage applied to the analog DIM pin, the Ostar current is proportional to dimming voltage and controllable down to 10 percent of full load current. (View Figure 4.)

In Figure 5, the analog DIM pin is stepped between 0V and 5V. In this configuration, R3 and R15, from Figure 2a, have been set to give a step from 700mA to 1A. The traces are for Vin=10V t (left) and Vin=16V (right). These waveforms illustrate the driver?s ability to work at 1A Ostar current. Moreover, it can be seen that PWM-ing of the analog DIM pin is possible. The response is dominated by the LC constant of the output filter.

PWM dimming in Figure 6 is substantially linear from 1 percent to 100 percent Ostar current over the input voltage range.

The waveforms in Figure 7show the Ostar current below and the driver output voltage above. The left hand waveform is at 1 percent duty and the right hand one at 5 percent duty. The ripple in both cases is due to the dynamic response of the driver. In the case of PWM, dimming the driver dynamic response has been optimized to achieve 1 percent dimming.

A MathCAD program was developed to calculate the current setting resistors; R3, R14 and R15.

Over the range of input voltage and Ostar voltage variation, the Ostar current varies from 680mA to 720mA. This corresponds to a tolerance of 3 percent from these causes. Taking into account all component temperature variations, as well as Z1, voltage reference tolerance indicates that the design goal of 10 percent will be easily achieved. ( View Figure 8 for details.)

Efficiency under all variations of input voltage and possible Ostar voltage meets the design goal of better than 90 percent. ( View Figure 9 for the graph.)

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