LED lighting drive power circuit design technology application

The arrangement of the LEDs and the specifications of the LED light source determine the basic drive requirements. The main function of the LED driver is to limit the current flowing through the LED under certain operating conditions, regardless of the input and output voltage. The most common is the use of transformers for electrical isolation. The article discusses the factors that LED lighting design needs to consider.

First, LED driver general requirements

Driving LEDs face many challenges. For example, the forward voltage will change with temperature and current, and the LED forward voltages of different individuals, different batches, and different suppliers will also be different; in addition, the LED's color "Point" also drifts with current and temperature.

In addition, many LEDs are often used in applications, which involves the arrangement of multiple LEDs. Among various arrangements, it is preferable to drive a single string of LEDs in series, because this method provides excellent current matching performance regardless of how the forward voltage changes and how the output voltage (Vout) “drifts.”

Of course, the user can also use other arrangements such as parallel, series-parallel combination, and cross-connect (Fig. 1), for applications that require "matching each other" LED forward voltages, and gain other advantages. In a cross-connect, if one of the LEDs is open due to a fault, only one LED in the circuit will double its drive current, minimizing the impact on the entire circuit.

Common LED arrangement

The arrangement of the LEDs and the specifications of the LED light source determine the basic drive requirements. The main function of the LED driver is to limit the current flowing through the LED under certain operating conditions, regardless of the input and output voltage. The basic working circuit diagram of the LED driver is shown in Fig. 2. The so-called “isolation” indicates that there is no physical electrical connection between the AC line voltage and the LED (ie, input and output). The most commonly used is a transformer for electrical isolation. Non-isolated, however, does not use high-frequency transformers for electrical isolation.

It is worth mentioning that in the LED lighting design, AC-DC power conversion and constant current drive can use different configurations:

1) Integral configuration, that is, the two are integrated together and are located in lighting fixtures. The advantages of this configuration include optimizing energy efficiency and simplifying installation;

2) Distributed configuration, where both exist alone, this configuration simplifies security considerations and increases flexibility.

LED driver can use constant voltage (CV) output according to different application requirements, that is, the output voltage is clamped under a certain current range; it can also work with constant current (CC) output, and the output design can strictly limit the current; The constant current and constant voltage (CCCV) output operation may be used, that is, constant output power is provided, so the forward voltage of the LED as the load determines its current.

In general, LED lighting design needs to consider the following factors:

Output power: LED forward voltage range, current, and LED arrangement

Power Supply: AC-DC Power Supply, DC-DC Power Supply, Direct AC Power Supply

Functional requirements: dimming requirements, dimming methods (analog, digital or multi-level), lighting control

Other requirements: energy efficiency, power factor, size, cost, fault handling (protection characteristics), standards to be complied with, reliability, etc.

More considerations: mechanical connection, installation, repair/replacement, life cycle, logistics, etc.

Second, LED drive power topology

In LED lighting applications using AC-DC power supplies, power conversion building blocks include discrete components such as diodes, switches (FETs), inductors and capacitors, and resistors to perform their functions, and a pulse width modulation (PWM) regulator is used to control Power conversion.

The isolated AC-DC power supply with transformers is usually added to the circuit and includes topologies such as flyback, forward and half-bridge. See Figure 3, in which the flyback topology is the standard choice for low-to-medium power applications with power less than 30 W. The half-bridge structure is best suited to provide higher energy efficiency/power density. In the case of a transformer in an isolation structure, its size is related to the switching frequency, and most isolated LED drivers basically use “electronic” transformers.

Common isolated topology

In LED lighting applications using a DC-DC power supply, the LED driving methods that can be used are resistive, linear regulators, and switching regulators. The basic application diagram is shown in the resistive driving method. Adjust the current detection in series with the LED. The resistance can control the forward current of the LED. This driving method is easy to design, has a low cost, and does not have electromagnetic compatibility (EMC) problems. The disadvantage is that it depends on voltage, requires binning of LEDs, and has low energy efficiency.

Common DC-DC LED drive

Linear regulators are also easy to design and have no EMC issues. They also support current regulation and fold back and provide an external current set point. The problem is that the power dissipation problem is insufficient and the input voltage is always higher than the positive direction. Voltage, and energy efficiency is not high. The switching regulator controls the current flow by continuously controlling the switching (FET) on and off through the PWM control block.

Switching regulators have higher energy efficiency, are independent of voltage, and can control brightness. Insufficiency is relatively high cost, complexity is higher, and electromagnetic interference (EMI) problems exist. The common topologies for LED DC-DC switching regulators include different types of Buck, Boost, Buck-Boost, or Single-Ended Primary Inductor Converter (SEPIC).

The step-down structure is used when the minimum input voltage is greater than the maximum LED string voltage under all operating conditions. For example, 6 series LEDs are driven with 24 Vdc. Conversely, the maximum input voltage is less than the minimum output voltage under all operating conditions. Boost structure, such as driving 6 LEDs in series with 12 Vdc; buck-boost or SEPIC configuration when input voltage and output voltage range overlap, such as driving 4 series LEDs with 12 Vdc or 12 Vac , But the cost and energy efficiency of this structure are the least desirable.

The method of directly driving the LED using AC power has also achieved certain development in recent years. The schematic diagram of its application is shown in Figure 5. In this structure, the LED strings are arranged in the opposite direction and work in a half cycle, and the LED is at a line voltage greater than the forward voltage. Only conduction. This structure has its advantages, such as avoiding power loss due to AC-DC conversion. However, in this structure, the LED is switched at a low frequency, and the human eye may perceive flicker. In addition, LED protection measures need to be added to this design to protect it from line surges or transients.

Third, power factor correction

The United States Department of Energy (DOE)'s ENERGY STAR solid-state lighting (SSL) specification states that any power level must be forced to provide power factor correction (PFC). This standard applies to a range of specific products, such as downlights, cabinet lights, and table lamps, where the LED driver power factor for residential applications must be greater than 0.7, while in commercial applications it must be greater than 0.9; however, this standard is a voluntary standard. The EU's IEC61000-3-2 Harmonic Content Standard specifies the total harmonic distortion performance of lighting applications with power greater than 25 W. The maximum limit is equivalent to total harmonic distortion (THD) < 35%, and power factor (PF) )>0.94.

Although not all countries are absolutely obliged to improve power factor in lighting applications, some applications may have requirements in this regard, such as utility companies aggressively promoting the commercial use of products with high power factor in public utilities, in addition to utilities. When institutions buy/maintain street lights, they can also decide whether they want to have a high power factor (usually >0.95+) according to their wishes.

PFC technology includes passive PFC and active PFC. Passive PFC solutions are bulky and require additional components to better change the current waveform to achieve a power factor of about 0.8 or higher. Among them, in the lower power applications of less than 5 W to 40 W, the flyback topology that is almost a standard choice requires only passive components and minor circuit modifications to achieve a power factor higher than 0.7.

Active PFC is usually added to the circuit as a dedicated power conversion section to change the input current waveform. Active PFCs typically provide boost, with a wide input range of 100 to 277 Vac AC, and a PFC output voltage range of 450 to 480 Vdc. If the PFC section is properly designed, it can provide high efficiency from 91% to 95%. However, with the addition of active PFCs, specialized DC-DC conversions are still needed to provide current regulation.

Fourth, energy efficiency issues

The energy efficiency of LED lighting applications needs to be considered in conjunction with power output. The US “Energy Star” solid-state lighting specification specifies lighting fixture-level energy efficiency, but does not address the energy efficiency requirements of individual LED drivers. As mentioned earlier, LED applications using AC-DC power supplies can use a two-stage distribution topology, so it is possible to use an external AC-DC adapter to provide power.

The "Energy Star" does contain specifications for single-output external power supplies. The external power supply specification for version 2.0 came into effect in November 2008, requiring a minimum energy efficiency of 87% in the standard operating mode, and a minimum power efficiency of 86 in the low-voltage operating mode. %; In this specification, the PFC is required only when the power is greater than 100 W.

U.S. Department of Energy's 2008 LED lighting efficacy research and development goals

In LED applications using AC-DC power supplies, providing AC-DC conversion energy efficiency involves a trade-off between cost, size, performance specifications, and energy efficiency. For example, using higher quality components and lower on-resistance (RDSon) can reduce losses and improve energy efficiency; lowering the switching frequency will generally improve energy efficiency, but it will increase system size. New topologies such as resonance provide more energy efficiency but also increase design and component complexity. If we limit the design to a narrower power and voltage range, it can help optimize energy efficiency.

Fifth, the driver standard

The LED driver itself is also evolving and focuses on further improving energy efficiency, increasing functionality, and power density. The "Energy Star" solid-state lighting specification of the United States proposes a lighting fixture-level energy efficiency limit that covers specific product requirements including power factor. The requirements of the European Union's IEC 61347-2-13 (5/2006) standard for LED modules powered by DC or AC include:

Maximum safe extra-low voltage (SELV) operating output voltage ≤ 25 Vrms (35.3 Vdc)

"Proper"/safe work under different fault conditions

No smoke or flammable in case of failure

In addition, the ANSI C82.xxx LED driver specification is still under development. In terms of safety, UL, CSA and other standards are required, such as UL1310 (Class 2), UL 60950, and UL1012. In addition, LED lighting design also involves product life cycle and reliability issues.