Test Design of LED Driving Power Supply

However, it is ironic that we often hear that led lighting lamps are not "long-lived" because the service life of LED driving power supply itself is directly dragged down, which greatly increases the maintenance / use cost; Or the efficiency of the driving power supply is not high, resulting in the energy efficiency conversion ratio of LED lighting lamps is not as high as expected, or the output current ripple is not well controlled, which affects the luminous quality, greatly reduces the green energy-saving advantage of LED lighting, and even affects the market popularization.

Therefore, the driving power supply is also an important part of the improvement and maturity of the LED industry chain. However, the current situation is that the design and quality limitations of LED driving power supply have increasingly become the "hind leg" of the development of LED industry. Therefore, power module manufacturers and lamp manufacturers pay more and more attention to the use of advanced testing and measurement technologies and schemes. In view of this situation, Tektronix is unique. It goes deep into customers, deeply understands the actual needs of the site, and formulates various solution combinations for LED vertical applications, in order to help more Chinese LED lighting manufacturers, drive power suppliers and relevant quality inspection / certification institutions obtain more accurate test results and ensure the quality of LED drive power supply, So as to promote the healthy and prosperous development of China's LED lighting industry.

Figure 1: Tektronix provides a variety of test solution combinations for LED lighting applications.

The reliability and energy efficiency of LED driving power supply are the key to the test

So, what characteristics or requirements should a truly high-quality LED driving power supply have? Tektronix summarizes the following main aspects:

1. High reliability and service life: the service life of the driving power supply should match the service life of the LED, especially for the driving power supply such as LED street lamp, because it is installed at high altitude, it is inconvenient to maintain and the maintenance cost is large;

2. High efficiency: This is particularly important for the structure where the power supply is installed in LED lamps. Because the luminous efficiency of LED decreases with the increase of LED temperature, the heat dissipation of LED is very important. The efficiency of the power supply is high, its power consumption is small, the calorific value in the lamp is small, which reduces the temperature rise of the lamp and is beneficial to delaying the light decay of the LED.

3. High power factor: with the continuous improvement of social requirements for power supply quality, people pay more and more attention to the power quality and harmonic problems brought by electric equipment. If the European Union has issued standards, it stipulates that power supply equipment with power greater than 25W must have power factor correction circuit. It is said that many other countries may have certain index requirements on power factor for 30 40W LED driving power supply in the near future.

4. Constant current drive: at present, there are two kinds: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. The combination of this method is flexible. The failure of one LED will not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, LED series or parallel operation. Its advantage is that the cost is lower, but the flexibility is poor. It also needs to solve the problem that the failure of one LED does not affect the operation of other LEDs.

5. Appropriate output ripple: the output ripple will affect the light output effect of LED. However, reducing ripple requires the use of higher quality and capacity capacitors. In order to improve the overall service life of the power supply, designers tend to adopt the scheme without capacitance. The engineer must determine a compromise on the output ripple index.

6. Surge protection: the anti surge ability of LED is relatively poor, especially the anti reverse voltage ability. Some LED lamps are installed outdoors. Due to the start-up and swing of power grid load and the induction of lightning stroke, various surges will invade from the power grid system, and some surges will cause led damage. Therefore, the LED driving power supply should have the ability to suppress the invasion of surge and protect the LED from damage.

7. Protection function: in addition to the conventional protection function, 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 the external installation structure of lamps, the power structure shall be waterproof and moisture-proof, and the shell shall be sun resistant.

9. Meet the requirements of safety regulations and electromagnetic compatibility.

To achieve high-quality driving power design standards, it is necessary to carry out comprehensive test, measurement and analysis. As shown in Figure 1, LED lighting test mainly includes five parts: power quality, harmonic, power switching device measurement, modulation analysis and driving output parameter test.

Figure 2: key test items of LED lighting driving power supply.

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

There are many methods to measure the input power, power factor and harmonic of the driving power supply. These power quality measurements can be easily realized by using Tektronix oscilloscope products with power analysis function. Use one channel to measure the input voltage and one channel to measure the input current, and cooperate with the PWR power analysis software built in the oscilloscope, all power quality related parameters can be directly displayed on the screen.

Figure 3: Tektronix oscilloscope with PWR power analysis module can directly display various measurement parameters to help power engineers easily complete development and testing.

The efficiency of driving power supply is directly related to the switching loss inside the power supply. Reducing the loss of power devices is not only the requirement of improving power efficiency, but also the key factor to improve the thermal stability and reliability of the system. However, it is difficult to measure the switching loss of power devices. Because the voltage amplitude of the device in the on and off working states varies greatly, ordinary oscilloscope products can not accurately measure the small voltage in the on state in such a large dynamic range, and the on loss can not be measured.

For high-speed switching power devices, the power can only be calculated by multiplying the voltage and current to obtain the transient power. At this time, the voltage and current must be the waveform at the same time, otherwise the error is huge. However, the principle and structure of voltage probe and current probe are different, and the time delay of high-speed switching signal is also different. If this deviation cannot be corrected accurately, the measurement result of oscilloscope is unreliable. Tektronix's oscilloscope power measurement scheme not only has a comprehensive switching loss test function, but also has a powerful probe calibration function to completely eliminate the time delay error. The unique waveform search function makes it easier to find the time when the maximum loss occurs when the power supply or load changes.

Specific to the model, DPO / mso4000b and DPO / MSO3000 Series Oscilloscopes are good choices for the R & D process test with high requirements; For the design optimization and parameter test application of factory and production process, DPO / MSO2000 series can fully meet the requirements and further reduce the cost. These oscilloscopes have a long storage length per channel, which can capture the whole cycle of driving power measurement with high sampling rate. Waveform navigation is convenient for power system troubleshooting, advanced mathematical calculation function, and users can customize comprehensive mathematical expressions and automatically calculate power and other test results, so as to greatly improve the test efficiency.

Table 1: selection of three grades of oscilloscopes that can improve the test efficiency of LED driving power supply and increase the test accuracy.

In addition, for the excitation source, Tektronix can currently provide PWS4000 / PWS2000 series programmable DC desktop power supply series to provide accurate and high-resolution constant current and constant voltage output for driving LED.

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

Real energy-saving household LED lighting products will eventually have dimming function. At present, it is also the demand of high-end markets such as Europe and America, and will be gradually popularized in other countries and markets in the future. The latest technology of LED dimming is PWM. By generating PWM signals with different pulse widths, the ratio of on and off of power devices is controlled, so as to achieve the purpose of adjusting 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 deviation, avoid the energy loss caused by resistance dimming, and maintain the high efficiency of LED lighting.

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

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