LED lighting testing technology analysis

In recent years, LED as a new energy-saving light source has won high investment enthusiasm and great attention in the world and China, and has penetrated from outdoor to indoor lighting application market. China has also seen the emergence of tens of thousands of LED lighting companies of all sizes. The main reason for the success of LED lighting is that it has the characteristics of energy saving, environmental protection, long life, easy control, and maintenance-free.

Ironically, we often hear that LED lighting fixtures are not "long-lived" due to the life of the LED driver itself, which greatly increases the maintenance/use cost; or the low efficiency of the driver results in the energy efficiency conversion ratio of LED lighting fixtures not being as high as expected, or the output current ripple is not well controlled, affecting the luminous quality, which greatly reduces the green energy-saving advantages of LED lighting and even affects market popularity.

Therefore, the LED industry chain is perfected and mature, and the driver power supply is also an important part of it. However, the current situation is that the design and quality limitations of LED driver power supplies are increasingly becoming the "hindrance" to the development of the LED industry. Therefore, power module manufacturers and lighting manufacturers are paying more and more attention to the use of advanced test and measurement technologies and solutions. In response to this situation, Tektronix is ​​unique in that it goes deep into the customer base, deeply understands the actual needs of the site, and develops a variety of solutions for LED vertical applications, aiming to help more Chinese LED lighting manufacturers, driver power suppliers and related quality inspection/certification agencies obtain more accurate test results, ensure the quality of LED driver power supplies, and thus promote the healthy and prosperous development of China's LED lighting industry.

Figure 1: Tektronix offers a broad portfolio of test solutions specifically for LED lighting applications.

The reliability and energy efficiency of LED driver power supply are the key to testing

So, what characteristics should a truly high-quality LED driver have or what requirements should it meet? Tektronix has summarized the following main aspects:

1. High reliability and life: The life of the driver power supply must be compatible with the life of the LED, especially for the driver power supply of LED street lights, because it is installed at high altitude, maintenance is inconvenient and the maintenance cost is also high;

2. High efficiency: This is especially important for the structure where the power supply is installed in the LED lamp. Because the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The high efficiency of the power supply means that it consumes less power and generates less heat in the lamp, which reduces the temperature rise of the lamp and is beneficial to delaying the light decay of the LED.

3. High power factor: As society's requirements for power supply quality continue to increase, people are paying more and more attention to the power quality and harmonics problems caused by electrical equipment. For example, the European Union has issued a standard that stipulates that power supply equipment with a power greater than 25W must have a power factor correction circuit. It is said that many other countries may soon have certain indicators for power factor for 30~40W LED driver power supplies.

4. Constant current drive: There are two common methods: one is to use a constant voltage source to supply multiple constant current sources, and each constant current source supplies power to each LED separately. This method is flexible in combination, and the failure of one LED does not affect the operation of other LEDs, but the cost is slightly higher. The other is to directly supply constant current, and the LEDs are connected in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and the problem that the failure of a certain LED does not affect the operation of other LEDs must be solved.

5. Appropriate output ripple: Output ripple will affect the light output effect of the LED. However, reducing ripple requires the use of capacitors with higher quality and capacity. In order to increase the overall service life of the power supply, designers tend to adopt a capacitor-free solution. Engineers must determine a compromise solution on the output ripple indicator.

6. Surge protection: LEDs have poor surge resistance, especially reverse voltage resistance. Some LED lights are installed outdoors. Due to the start-up of the grid load and the induction of lightning strikes, various surges will invade from the grid system, and some surges will cause damage to the LEDs. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage.

7. Protection function: In addition to the conventional protection function of the power supply, it is best to add LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

8. Protection: For external installation structures of lamps, the power supply structure should be waterproof and moisture-proof, and the outer shell should be sun-resistant.

9. It must comply with safety regulations and electromagnetic compatibility requirements.

To achieve high-quality drive power design standards, comprehensive testing, measurement and analysis are required. As shown in Figure 1, LED lighting testing mainly includes five parts: power quality, harmonics, power switching device measurement, modulation analysis and drive output parameter testing.

Figure 2: Key test items for LED lighting driver power supply.

Comprehensive and fast analysis of power quality, harmonics and power devices

There are many ways to measure the input power, power factor and harmonics of the drive power supply. These power quality measurements can be easily achieved using Tektronix oscilloscope products with power analysis functions. Use one channel to measure the input voltage and one channel to measure the input current. With the built-in PWR power analysis software of the oscilloscope, all power quality related parameters can be directly displayed on the screen.

Figure 3: A Tektronix oscilloscope with the PWR power analysis module can directly display various measurement parameters, helping power engineers easily complete development testing.

The efficiency of the driving power supply is directly related to the switching loss inside the power supply. Reducing the loss of power devices is not only a requirement for improving power supply efficiency, but also a key factor in improving system thermal stability and reliability. However, it is not easy to measure the switching loss of power devices. Since the voltage amplitude of the device in the on and off working states is very different, general oscilloscope products cannot accurately measure the tiny voltage in the on state under such a large dynamic range, and the conduction loss cannot be measured.

For high-speed switching power devices, the power can only be calculated by multiplying the voltage and current to get the transient power. At this time, the voltage and current must be waveforms at the same time, otherwise the error will be huge. However, the principles and structures of voltage probes and current probes are different, and the time delay for high-speed switching signals is also different. If this deviation cannot be accurately corrected, the measurement results of the oscilloscope are unreliable. Tektronix's oscilloscope power measurement solution not only has comprehensive switching loss test functions, but also has a powerful probe calibration function to completely eliminate time delay errors. The unique waveform search function can easily find the moment when the maximum loss occurs when the power supply or load changes.

Specifically speaking of models, DPO/MSO4000B and DPO/MSO3000 series oscilloscopes are good choices for R&D process testing with high requirements; while DPO/MSO2000 series can fully meet the requirements for design optimization and parameter testing applications in factories and production processes, which can further reduce costs. These oscilloscopes have a long storage length per channel, can capture the entire cycle of drive power supply measurement at a high sampling rate, waveform navigation facilitates power system troubleshooting, and advanced mathematical calculation functions. Users can customize comprehensive mathematical expressions and automatically calculate power and other test results, thereby greatly improving test efficiency.

Table 1: Three levels of oscilloscope selection that can improve LED driver power supply test efficiency and increase test accuracy.

In addition, for excitation sources, Tektronix currently offers the PWS4000/PWS2000 series of programmable DC benchtop power supplies, which provide precise, high-resolution constant current and constant voltage outputs for driving LEDs.

PWM analysis and output ripple measurement help enter the high-end market

The real energy-saving household LED lighting products must have dimming function in the end. It is also the demand of high-end markets such as Europe and the United States. It will gradually become popular in other countries and markets in the future. The latest technology of LED dimming is PWM, which controls the ratio of power device on and off by generating PWM signals with different pulse widths, so as to adjust the output luminous flux. Compared with other dimming methods, PWM dimming does not change the current flowing through the LED, the lamp will not have color cast, and it also avoids the energy loss caused by resistor dimming, maintaining the high efficiency of LED lighting.

Tektronix's AFG3000 arbitrary waveform generator can easily generate PWM signals. Users can arbitrarily change the form of PWM changes, the ratio of pulse width output and the change law. All operations only need to press a button on the panel. The generator's built-in impedance adjustment function can adapt to more circuit needs.

Ripple is the AC voltage added to the DC output voltage, and is also an important measurement parameter in LED power supply testing. The larger the ripple current, the lower the power supply cost, but the light output will be affected, and the LED junction temperature will increase, thus affecting the LED performance, and even seriously reducing the LED's service life (experience shows that the service life is shortened by half for every 10°C increase in junction temperature). Therefore, it is very important to accurately measure the ripple. The Tektronix oscilloscope power supply test solution has a ripple measurement function, which can complete the measurement of ripple parameters with one click. For high-end applications, Tektronix's TCPXX series current probes can provide ripple current measurement accuracy as low as 1mA. For low-cost applications, there are also corresponding cost-effective probes to choose from.

In addition, if engineers want to complete the ripple measurement task with a simple digital multimeter, Tektronix can also provide a corresponding solution. The DMM4040/4050 digital multimeter has a dual parameter measurement function and can display the AC voltage or current component at the same time. Its dual degree function, trend chart and histogram function are convenient for users to detect output changes over a long period of time; through the front and rear panel input function, users can alternately measure two sets of data for easy comparison.