In many consumer applications for lighting the cost of existing technology – namely incandescent light bulbs or fluorescent tubes – is so low that LED lighting’s many advantages simply cannot make up for the increase in initial cost.
Street lighting is notably different because the long life and high degree of control that solid-state lighting provides are coupled with an end-user – government – that evaluates cost of ownership along with initial cost.
This is where high-quality LEDs paired with good thermal management and robust drive electronics have real value. This article proposes an electrical drive solution that balances higher initial cost with long life – at least as long as the LEDs themselves.
DC bus voltages
It often takes 100 or more 1W LEDs to put out the thousands of lumens needed for a streetlamp. One way to drive 100 LEDs would be a single series chain.
This ensures an equal current through every LED, and since light output is proportional to current, it is the best way to guarantee equal light output from each device. The problem is that the DC voltage could easily be 400V. Such a voltage could be lethal, and also requires large, expensive components.
A second way to arrange 100 LEDs would be to use multiple parallel strings with a lower DC voltage. Well-known, cost-effective topologies such as the flyback converter 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).
DC bus voltages of 60V or less are common due to the 48V used in telecom applications and due to safety regulations such as the IEC’s definition of a safety extra low voltage. Because it is not as low as the logic voltages of digital circuitry and yet not as high as rectified offline voltage, a 48V distribution voltage is often referred to as an intermediate DC bus.
A DC-DC converter is the natural choice for the final stage in an LED power supply. The LED requires a DC current, so the voltage output is also DC.
The intermediate DC bus concept allows the designer to use cost-effective, non-isolated DC-DC converters because the previous stage has taken care of rectification, PFC and isolation.
Within non-isolated converters there are three main types:
? Step-down, or buck
? Step-up, or boost
? Step-up/down, or buck-boost.
Of these topologies, the buck regulator is by far the best suited to driving LEDs for several reasons.
First, the buck inductor is on the output, meaning that LED current and inductor current share the same average value. Furthermore, output current is always explicitly controlled thanks to the inductor.
Second, stepping down voltages is the most efficient form of power conversion, making the buck the most power efficient of all the switching converters. Third, the buck is the most economical of switching converters because the heaviest currents flow at the output, and the highest voltage is at the input.
It puts the least amount of voltage and current stress on the power mosfets and diodes which make up the power switches in switching converters. This means a wide selection of power switches, passive components and control ICs, all of which adds up to the most economical solution.
Arraying LEDs and picking a driver IC
For this example design, 100 LEDs will be used and each dissipates 1W. An intermediate DC bus of 48V is a good choice because off-the-shelf AC-DC power supplies are available with a wide selection of output power.
From 48V±5% a buck LED driver can be used to drive 10 LEDs in series. Ten such drivers make a robust lamp that can be designed to run all 100 LEDs without hazardous voltages.
Opto-semiconductor manufacturers bin their white LEDs for luminous flux, correlated colour temperature (CCT) and forward voltage. Binning for colour temperature and flux are important to maintain consistent colour and light output, but the more qualities an LED is binned for, the higher the cost.
When LEDs from various bins are used, the designs of LED lamps must accommodate a wide range of forward voltage. Therefore, each LED driver will be a 350 mA current source designed to work from an input of 45V to 51V over a range of output voltage from 30V to 40V, reflecting the potential variation of the VF of each LED from 3.0V to 4.0V.
The LM3402HV is an example of a buck regulator with an internal power N-mosfet.
Design challenges with buck regulators
When using a buck regulator to drive LEDs, the principal design challenge is the case when input voltage is at a minimum and output voltage is at a maximum.
Many switching regulators cannot turn on the internal power N-mosfet indefinitely. During each switching period the regulator must turn off for a minimum off-time of 300ns to refresh the “bootstrap” capacitor (C2), part of the circuit that drives the internal power FET.
Since the minimum off time is fixed, the maximum duty cycle that can be achieved decreases with increasing switching frequency. This is because those 300ns consume a larger and larger portion of the switching cycle.
Typical switching frequencies range from 50kHz to 1MHz, and 500kHz is often a good balance between the physical size of the power components, such as the inductor – which is smaller when switching frequency is higher – and power efficiency, which is higher when the switching frequency is lower.
In this case, 500kHz is not possible, so 370kHz will be used. This ensures that the LED driver has the smallest components possible while still being able to properly drive all 10 LEDs during worst-case input and output voltage conditions.
Christopher Richardson is systems applications engineer for lighting at National Semiconductor