Design configuration
35-W T8 tube
LED driver
Primary output voltage
The European standard EN61000-3-2 sets tight restrictions on lighting devices that consume more than 25 W with regard to total harmonic distortion (THD). These lighting devices also need to follow the power factor requirements and therefore include active power factor correction (PFC) to keep the input current in phase with the input voltage.
LED applications are becoming more and more popular, and they’re now used in retrofits, industrial and commercial illumination, street lighting, and more. They have proven their reliability in terms of efficiency and lifetime, so associated power supplies must fulfil these requirements too. National Semiconductor’s first design proposal comprises two stages: front-end PFC and the LM3445 LED driver.
The design proposal doesn’t require any galvanic isolation, improving efficiency (Fig. 1). Total system efficiency depends more upon the ac-dc isolated transformer. The PFC flyback is economical but rarely exceeds 85% efficiency. In this first design proposal, the isolation is located between the heatsink and the LEDs, using an insulating tape or a ceramic layer. As the transformer doesn’t require isolation, the efficiency improves.
The power supply’s main purpose is to convert the rectified ac input to dc regulated current. It is optimized for driving 30 high-brightness white LEDs in series at 350 mA with a maximum power capability of 35 W. It also provides protection for the LEDs, limits the transient input voltage, and protects against inrush current at plug-in. Its overall power-supply conformity (e.g., mains harmonics, mains interference, and international safety standards) meets all the applicable European Norms (EN).
Now, consider a 35-W T8 tube replacement (Fig. 2) using a ballast. The ballast is protected against fault conditions such as an LED string open circuit, a short circuit, or excessive load. No component will overheat or burn if a fault condition occurs, making the design very robust. The T8 and ballast feature:
• European input voltage but can be extended to a wide range from 85 to 265 V ac
• 0.98 power factor and 350-mA output current
• Output voltage depending on LED forward voltage is 100 V ±20%
• Main harmonics meeting EN61000-3-2 Class C
• Electromagnetic interference (EMI) (conductive) according to EN55022
• EMI (radiated) according to EN55022 in progress
• 87% efficiency
• Safety standards in progress
• Passive cooling method
• Temperature range of –20°C to 65°C
• Long lifetime by using polymer capacitor
• T8 tube from Osram using 90 LED LCW_G5GP-GX-6S in three strings with current sharing
First Stage: PFC
Many basic ac-dc power supplies generate harmonic distortion in the input line and have poor power factor, making it difficult to meet the European standard EN-61000-3-2. The solution is to use a PFC circuit to make the input current waveform appear sinusoidal, like the input voltage waveform.
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For the National Semiconductor ballast, the applicable standard to follow is the European standard EN-61000-3-2 Class C. This includes all lighting products, including dimming devices, with an active input power higher than 25 W.
The PFC works in critical conduction mode and acts as a boost converter. It provides a relatively stable output voltage (380 V dc) that will be the input voltage for the LED driver. The LED driver works as a step-down converter with constant current control, suiting it for use with rectified input voltage.
As the LED driver is performing with high input ripple, a low-value capacitor will be used on the 380 V dc. To maintain long lifetimes, no electrolytic capacitors are allowed due to their failure rate.
This ballast uses film capacitors from Epcos instead of electrolytic capacitors. A derating has been defined for all components following internal guidelines, minimising failure rates and increasing the lifetime of the complete system.
Second Stage: LED Driver
The LM3445 is an adaptive constant off-time ac-dc buck (step-down) constant-current controller designed to be compatible with triac dimmers and pulse-width modulation (PWM) signals. It provides a constant current for illuminating high-power LEDs. The dim decoder allows wide-range LED dimming.
Figure 3 shows in detail the drain source voltage and current of Q3 of one complete ac line cycle for the LED driver. The cycle can be divided into four different phases as shown on the plot: the switch-on phase; the conducting phase; the switch-off phase; and the off phase, with energy released into the load.
This LED driver uses a constant off-time control to regulate current through a string of LEDs. While the MOSFET is conducting, the LED current increases through the inductor until it reaches a peak defined by the reference voltage and the current sense resistor. With this peak current reached, the MOSFET turns off and the diode conducts during the period TOff.
Several revisions have been made to drive higher numbers of LEDs, including a successful test with 60 LEDs on one string that delivered a total efficiency of 92% with a 70-W output. It is also possible to use several strings of 30 LEDs or more by adding extra LM3445 stages in parallel, but more cable will be used to connect LEDs.
Figure 4 illustrates a different approach, where the primary output voltage of the power supply is less than 60 V, which fulfils the maximum voltage limit of the UL1310 Class 2 requirement. When an isolated system with a limited secondary voltage is desired, the only choice is to have more than one string of LEDs.
On the secondary side, the LM3464, which is a new LED driver controller from National, combines multiple channels. Each LM3464 controls up to four external power N-MOSFETs as power linear regulators. Therefore, multiple strings up to 15 series-connected LEDs are possible.
The recommended maximum average current is up to 500 mA per channel. Figure 4 shows how the LM3464 can control the isolated ac-dc offline primary power supply by accepting a command from the LM3464 to dynamically adjust the output voltage (VOut) so the voltage across each linear regulator is always minimised.
The basis adjusting VOut is the channel with the highest string voltage. Even when driving at 350 mA per channel, the power efficiency of the LM3464 can be more than 95%. One important difference between Figure 1 and Figure 4 is that the LM3464 introduces no new switching frequencies. This is a major advantage to control EMI, something that becomes more and more difficult as total power increases. The only switching noise comes from the ac-dc section.
Conclusion
There are different solutions for driving high numbers of LEDs. One solution, which uses a PFC as a standard boost to drive a string of 30 LEDs with an isolated heatsink, is an attractive choice for engineers who need strings with a high number of LEDs.
Our first example focused on an LED tube replacement, and complete details of the LED tube are available on request. Beyond the high power efficiency, the LED tube and ballast have long lifetimes, when properly designed. Maintenance costs are always a large investment with traditional lighting, so using LED tubes will minimise maintenance costs.
The second solution used a primary power supply and a multi-channel linear regulator with dynamic headroom control. It is an attractive solution for engineers who want a dedicated current source for every string of LEDs but have run into problems with using a buck regulator for each string.