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Posted on 05 July 2019

Streetlighting Requires Large Numbers of LEDs

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Backlighting grows up to become streetlighting technology

The type of LEDs used in backlighting differs from that used for general purpose lighting. Whereas many general lighting applications use less than 10 LEDs of fairly high power - such as 1W each - backlighting tends to use hundreds, possibly thousands of small LEDs running at powers of 50 to 200 mW or so. This means that the type of LED drivers and systems architectures used have, so far, been much different.

By Christopher Richardson, Systems Applications Engineer for Lighting, National Semiconductor

 

With the advent of LED streetlighting (and parking lot lighting, warehouse lighting, etc.) the two worlds of general purpose and backlighting LEDs have taken a step closer to one another. This is because High Power Wide Area lighting, HPWA, of which streetlighting is big piece, requires much higher total output power than a light bulb retrofit or a fluorescent tube retrofit. The end result is that a large number of LEDs is needed. Backlighting LED drivers have tackled the challenge of controlling large numbers of LEDs in series-parallel arrays by providing a linear current source for each string and then improving power efficiency by using one switching power supply with a dynamically adjustable voltage output. Up until now such systems were limited in the current per channel to as much as 200 mA or so. National Semiconductor has taken this idea but expanded the power to as much as 500 mA per channel, as well as adding the control and protection features demanded by high reliability outdoor lighting such as streetlights.

Introduction

The first part of this article series seen in the July issue of Bodo’s Power Systems explained the principal challenges that the electronic drive engineer faces when designing an LED based streetlight that uses 50 to 200 1W LEDs. These are: controlling the total output voltage to within a certain limit for safety, matching the current from string to string in a series-parallel LED array, reliability in case of LED failures, and control of EMI, which becomes more and more difficult as total power increases.

The standard concept of using a buck regulator as the constant current source for each string of LEDs was introduced along with its advantages and disadvantages. Part I concluded by stating that the system architecture commonly used for backlighting could be applied to streetlighting if the power was increased. For reference, Figure 1 shows the LM3432, a six-channel backlighting controller that is capable of driving up to 40 mA per channel at an output voltage as high as 80V. Depending upon the maximum forward voltage, VF of each LED, this allows one LM3432 to power 20 to 25 LEDs per channel or 120 to 150 LEDs in total. This amount is typical in laptop LCD screen backlights, the target market for the IC.

LM3432 Can Power 120-150 LEDs

Dynamic Headroom Control

Each channel of the LM3432 is a linear regulator configured as a constant current sink. Linear regulators are not known for power efficiency, so to power the LEDs efficiently, the LM3432 is paired with a switching regulator (SMPS in Figure 1) which provides the power voltage for the LEDs, and more importantly, accepts a command from the LM3432 to dynamically adjust VO so that the voltage across each linear regulator is always minimized. The basis for adjustment of VO is the channel with the highest string voltage. Even LEDs binned for forward voltage exhibit some differences, and no binning exists for the drop in VF due to heat. The channel with the highest total LED string voltage is the channel closest to the dropout voltage of its linear regulator current sink. This channel commands the voltage from the Primary Power Supply to be just enough to stay out of dropout. The channel which is the ´master´ can and does change dynamically, hence the name Dynamic Headroom Control, or DHC. DHC puts the total system power efficiency above 90% and makes it competitive with a switching regulator that directly drives the LEDs.

Advantages Over Multiple Buck

A single large switching regulator with a variable output voltage feeding a series of linear regulators has several advantages over the Multiple Buck option detailed in Part I. In cell phones, laptops, and GPS units the physical space needed and cost are lower. Expanding the concept to streetlighting, where 50 to 200 1W LEDs are driven at a typical current of 350 mA makes a different advantage shine through: EMI and beat frequencies. Whereas the boost regulator that feeds a backlighting chip like the LM3432 takes a DC input which is already heavily filtered, streetlighting and HPWA applications are driven from AC mains. This makes the Primary Power Supply subject to a host of legal requirements. Safety and power factor correction are very important, but often the most challenging regulations of all when bringing an electronic product to market are those governing EMI. Figure 2 shows that for a system with four strings of 14 LEDs each (keeping total voltage under 60VDC) the Multiple Buck approach would require five switching regulators. Depending upon the total output power, the AC-DC portion could be as simple as a single-stage, power factor corrected flyback regulator. For efficiency purposes such regulators rarely exceed a switching frequency of 200 kHz. Each buck regulator is likely to run at a higher frequency such as 500 kHz to reduce the size of the output inductor. Two switching frequencies with differing filter needs already exist in the system, and as detailed in Part I, without frequency synchronization between each buck LED driver, the potential for beat frequency EMI exists, as each buck will run at a slightly different frequency.

Multiple Buck System with Multiple EMI Sources

The LM3464 is a new LED driver controller which combines the multiple channel, DHC technology of backlighting with much higher output currents. Each LM3464 controls up to four external power NMOSFETs as power linear regulators. The recommended maximum average current is up to 500 mA per channel. Figure 3 shows how the LM3464 can control the isolated, AC-DC offline Primary Power supply just as the LM3432 controls a DC-DC boost regulator. Even with drive current per channel at 350 mA, power efficiency of the LM3464 can be over 95%, and therefore easily on par with four well- designed buck LED drivers. One important difference between Figure 2 and Figure 3 is that the LM3464 introduces no new switching frequencies. The only switching noise comes from the AC-DC section.

The LM3464 is a High Power, Multi-channel Linear LED Driver

Total system efficiency also depends more upon the AC-DC regulator. The PFC flyback is economical but rarely exceeds 85% efficiency. As power levels exceed 50 to 75W a boost PFC pre-regulator followed by a forward converter is more common. As heat is a primary concern in LED performance and lifetime, and because heat generated and power efficiency are inversely proportional, a PFC boost followed by a resonant converter is finding use even at the 100-200W range.

Accuracy, Fault Reporting and Thermal Foldback

When one IC with a single reference voltage controls all the LEDs, it is easier to match currents from string to string. Given sense resistors with a 1% tolerance, the LM3464 guarantees that the currents in each string of LEDs will be within ±3% of one another. LEDs failing as open or short circuits are detected as shown in Figure 4 and can be configured to shut down only the affected channel or to shut down the entire system. As a further alternative, the LM3464 can be programmed to cycle continuously in a “hiccup” fashion until the fault is cleared. LED streetlights often include a system microcontroller which can interpret and respond to the fault signals. Advanced systems may even report a problem using powerline or wireless communication.

Integrated Fault Detection and Response

Another primary safety and reliability feature of the LM3464 is thermal foldback. Using an NTC thermistor or temperature sensor, typically placed in the center of the LED array, the system will gradually reduce the average output current of each channel through PWM dimming once the temperature exceeds a programmable threshold. Heat is the primary enemy of LED systems, and the humorous but nonetheless serious example of birds building nests on the heatsinks is one real-world example of why thermal foldback is needed. Even if a flock of seagulls takes up residence on a streetlight, the designer will often want to guarantee at least some light output from the LEDs to meet safety regulations on roadways. For this reason the LM3464 also allows the designer to choose between a thermal foldback loop which completely shuts off the system at a given temperature and a loop with a second breakpoint at a minimum drive current.

Thermal Foldback Loops - Right - No Minimum. Left - Minimum Output

Daisy Chains and Odd Numbers of Strings

Not all systems will have four channels, so the LM3464 can work in a daisy chain fashion as shown in Figure 6. The DHC control loop compares the drain voltages from each channel of each LM3464 in order to maintain high power efficiency. Similarly, should a system require three, six, or some other combination of channels not divisible by four, as many as three channels of any one LM3464 can be disabled.

Daisy Chaining and Disabling of Channels

Conclusion

Using a primary power supply and a multi-channel linear regulator with dynamic headroom control is an attractive choice for system designers who want a dedicated current source for every string of LEDs but have run into problems with using a buck regulator for each string. The LM3464 offers a smaller, less expensive, simpler option while maintaining high power efficiency, high reliability, and a high degree of control.

 

 

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