High-performance LED street light driver design

0 Introduction

Flashlights, MR-16 bulb upgrades, emergency lights, and virtually any low-power white light lighting application are already using LED technology.

Streetlights may be the next area where LED technology will be widely used. Compared with flashlights and low-power applications, LED streetlight applications also pose greater challenges.

1 Design requirements

LED street lighting will not be achieved overnight, as there are still major technical challenges to overcome. With a few exceptions (such as solar cells), street lamps are powered by AC power (usually called "offline"), mostly 120V or 230V AC. For fluorescent tubes and high-pressure discharge lamps, a wide range of ballasts are available for offline operation. But the circuits are simple because the number of light-emitting elements is small. Few fluorescent lamps have more than four tubes, and high-pressure discharge lamps use at least one element. LEDs are very different, however, and most of them are only 0.5W to 5W, even including "high-power" LEDs. Although there are some exceptions, for street lamps, it is usually necessary to use 100 or more 1W LEDs to produce the thousands of lumens required.

LEDs are current-driven devices, and a 1W white LED driven at 350 mA typically has a forward voltage VF of 3.0 V to 4.0 V. LEDs are PN junction diodes with very low dynamic resistance. Applying voltages greater than three times VF to the diode results in uncontrolled current flow. If an LED is connected directly to an off-line AC voltage, it will glow brightly and then fail quickly. The term "driver" is used to describe the power regulation circuit that converts the off-line voltage into a controlled DC current. A flashlight is likely to be lost long before it is worn out. The application requirements of street lamps are obviously different, so long-term reliability and product life are the main considerations for street lamps. LEDs have been promoted as the longest-lasting commercial light source, but if the lamp can last for tens of thousands of hours, the driver that matches it must be able to last the same amount of time. This means paying more attention to every aspect of the power driver, from the system architecture to the selection of each circuit component.

2 DC bus voltage

One way to drive 100 LEDs is to use a single series chain, as shown in Figure 1. This ensures that the same current flows through each LED.

And because light output is proportional to current, this is the best way to ensure that every device emits the same light output. The problem, however, is that the DC voltage can easily reach 400 V. Such high voltages can be lethal, and also require large and expensive components.

Figure 1 All 100 LEDs are connected in series

Another way to drive 100 LEDs is to use a lower DC voltage. It is well known that cost-effective topologies such as flyback converters make good AC-DC stages (often called offline converters) because they can combine the step-down function with galvanic isolation and power factor correction PFC. The DC bus voltage is usually 60 V or less, partly because of 48 V in telecommunication applications and partly because of safety regulations (such as the IEC definition of safety extra-low voltage). The 48 V distribution voltage is higher than the logic voltage of digital circuits and lower than the rectified offline voltage, so it is often called the "intermediate DC bus".

3 DC-DC LED driver topologies

(1) When V IN > Vo, buck is used and the output capacitor is optional, see Figure 2 (a); (2) When V IN < Vo, boost is used and the output capacitor is required, see Figure 2 (b); (3) When V IN and Vo overlap, buck-boost is used and there are many topologies, see Figure 2 (c).

Figure 2 Three main types of non-isolated converters

The DC-DC converter is the natural choice for the final stage of an LED power supply. LEDs require DC current, so the voltage output is also DC. Since the previous stage has already taken care of the rectification, PFC and isolation, using an intermediate DC bus allows the designer to use a cost-effective non-isolated DC-DC converter. There are three main types of non-isolated converters: step-down or buck, step-up or boost, and step-up/step-down or buck-boost. These three types are depicted in Figure 2. Of these topologies, the buck regulator is by far the most suitable for driving LEDs for the following reasons: First, the buck inductor is at the output, which means that the average value of the LED current and the inductor current are the same; moreover, the output current is always clearly controlled by the inductor; second, step-down voltage is the most efficient form of power conversion, which makes the buck the most power efficient of all switching converters; third, the buck is the most economical switching converter because the highest current is at the output and the highest voltage is at the input. As a result, the current and voltage seen by these power conversion devices across the switching converter composed of power MOSFETs and diodes are minimal. This means a wide selection of power switches, passive components and control ICs to create the most economical solution.

4 Arrange LEDs and select driver IC

For this example design, 100 1 W LEDs will be used. Choosing a 48 V intermediate DC bus is a wise choice because there are readily available AC-DC power supplies with a wide range of output power options. A 48 ? (1 # 5%) V buck LED driver can be used to drive 10 LEDs in series. Ten such drivers can create a bright lamp that can be used to run all 100 LEDs without using dangerous voltages. Semiconductor manufacturers classify their white LEDs by luminous flux, correlated color temperature (CCT), and forward voltage. Classification by color temperature and luminous flux is important for maintaining consistent color and light output, but the higher the specification for classifying the LED, the higher the cost. When using a variety of LED grades, the LED lamp must be designed to accommodate a wide range of forward voltages. Therefore, each LED driver will be designed as a 350 mA current source that can generate an output voltage range of 30 V to 40 V from an input voltage of 45 V to 51 V, making the possible variation range of VF of each LED between 3.0 V and 4.0 V.

Figure 3 Simplified system architecture

The LM3402HV is a step-down regulator with an internal power N-MOSFET that operates up to 75 V. Due to its minimum overtemperature current limit of 530 mA, it is also well suited for 350 mA output current, sufficient to drive LEDs with a wide ripple current range if necessary. Figure 3 shows the system architecture and Figure 4 shows the complete circuit for each LM3402HV.

Figure 4 Detailed LM3402HV circuit

5 Design Challenges Using Buck Regulators

When using a buck regulator to drive LEDs, the primary design challenge is how to handle the situation where the output voltage is highest when the input voltage is lowest. Like many switching regulators, the LM3402HV cannot turn on its internal power N-MOSFET indefinitely. During each switching cycle, the regulator must be off for 300 ns (the minimum off time) to refresh the bootstrap capacitor, which is part of the circuit that drives the internal power FET. The minimum off time is fixed, and because 300 ns becomes a larger and larger portion of the switching cycle, the maximum duty cycle that can be achieved decreases as the switching frequency increases. The following example calculates the highest possible switching frequency, fSW-MAX, based on a VO-MAX of 40 V and a VIN-MIN of 45 V. The following equation can be used to calculate fSW-MAX.

The typical switching frequency range of the LM3402HV is 50 kHz ~ 1MHz, and using 500 kHz can usually achieve a good balance between the physical size of power components (such as inductors, which will become smaller when the switching frequency is higher) and power efficiency (which will become higher when the switching frequency is lower).

In this example, 500 kHz is not available, so 370 kHz will be used.

This will ensure that the LED driver components are as small as possible while still being able to drive all 10 LEDs properly during worst case input and output voltage conditions.

6. Avoid the series-parallel trap

Many engineers will consider a series-parallel array driven by a current source, as shown in Figure 5. For this example, the circuit will be a single current source outputting 3.5 A at the same 30 V to 40 V output voltage.

Figure 5 Cross-connected series-parallel array

This is not a practical solution. First, even with cross-connections as shown in Figure 5, the natural differences in VF between different LEDs means that the 3.5 A from the driver will never be evenly distributed between the different LEDs. While it is possible to improve the current mismatch by binning the LEDs very strictly by VF, this improvement is only effective when the LED die temperature is 25 ? (the temperature at which binning is done). Once the die temperature rises, VF starts to drop. And like VF itself, the voltage of different LEDs varies differently with temperature. An array that is perfectly current matched at 25 ? will become unbalanced again when it reaches thermal steady state. Worse, there is a positive feedback loop between the LED currents, and as the forward voltage drops, the die temperature rises.

Those LEDs whose VF drops more will draw more current, causing their die to heat up more, which in turn causes their VF to drop further.

Figure 6: Poor fault response of series-parallel array

The second reason street light designers do not use the series-parallel approach is that system reliability becomes poor when an LED fails. When an LED fails and becomes an open circuit, the current source shown in Figure 6 will continue to output full current, causing increased current to flow through the remaining path. LEDs can also become short circuits when they fail, which can cause the voltage of the array to drop significantly, causing imbalance. Any imbalance in current can cause other LEDs in the array to overheat, reducing light output in the short term and reducing lumen maintenance in the long term, which can cause the lamp to dim prematurely or fail. Therefore, in order to obtain a reliable LED light source, each LED string should have its own dedicated current source (or current bank).

7 Summary

In many consumer lighting applications, the cost of existing technologies such as incandescent bulbs and fluorescent tubes is so low that the many advantages of LED lighting cannot make up for the higher initial purchase price. The situation is significantly different for street lighting. Because LED street lighting has a long life and high controllability, it is very suitable for government needs and also makes it easier for governments to evaluate the cost of ownership and initial purchase price of LED street lights. With good heat dissipation equipment and powerful driver circuits, this is the value of LED street lighting solutions. The driver solution proposed by the author strikes a good balance between higher initial cost and extended service life. Each street light can control its light output, respond to and report faults, and communicate with neighboring street lights, providing efficient and reliable service to the community.