Tektronix test technology overcomes bottlenecks in LED lighting applications

Recently, the 2nd Asia-Pacific LED Technology Forum Summit was held in Shenzhen and Ningbo. The important position of Shenzhen and South China in the LED lighting industry in China naturally does not need to be said, but the LED supporting industry in Ningbo and the entire Hangzhou Bay area is unexpectedly rising rapidly, and many LED general lighting, landscape lighting, signal lights, R&D and manufacturing companies and supporting manufacturers of advertising display cards, automotive lighting, and backlight display. For the LED industry insiders in these two regions, this conference invited technical experts from many internationally renowned companies to focus on sharing innovative technologies including LED driver and power solutions, circuit protection for LED lighting and LED test solutions. Application development philosophy. Unlike many presentations on LED driver ICs and component applications, Tektronix's "Integrating Your LED Test Technology Solution" presentation is particularly striking, and the company's experts explain the LED lighting application design and test challenges in a simple and straightforward manner. The corresponding solution has received enthusiastic response from the audience.

In early 2009, the Ministry of Science and Technology of China launched the “Ten Cities and Ten Thousands” semiconductor lighting application demonstration city program, which led to the spread of LED heat in China. According to statistics, China's LED industry will reach 150 billion yuan in the next two years. Therefore, we can see that the traditional lighting giants such as Op, NVC, TCL, Buddha Photo, and Schattt have entered the LED field with high-profile, and almost all local lighting companies are eager to focus on this field... a “all-people LED movement” It is in full swing in China.

Three major obstacles hindering the bright future of LED lighting applications

Although there are constantly people in the industry who throw out all kinds of hidden concerns in the LED industry, they still can't stop the enthusiasm of the majority of enterprises who want to eat this "hot potato". Enthusiasm needs rational support. The majority of developers must find ways to overcome various bottlenecks in the actual development and design process. In particular, the test fever Tecktech and many other experts mentioned the fever (life), cost, and standards are in line with these three problem.

Chen Wenquan, Customer Manager of Distribution Products Department of Tektronix Technology South China pointed out: "The heat generation of LED single tube is obvious, and the heat dissipation problem directly affects its substitution in the field of lighting. At present, the cost of LED is still very high, especially the large investment in the early stage has affected it. Expanding to a wider range of areas; today there are no clear standards in the country and it is expected to be formed during the year. At this stage, it can be designed with reference to some stricter industry standards."

Figure 1: Chen Wenquan, Account Manager of Distribution Products Division of Tektronix Technology South China, explains the LED lighting application design and testing challenges and corresponding solutions in a simple way.

The other two component giants Murata and Tyco Electronics experts mentioned in the speech that for LED lighting applications, heat is an unavoidable problem, because theory and time have proved that LED performance and life is with the LED pn The junction operating temperature is closely related. Overcurrent, overvoltage and overheating can significantly reduce the LED's luminescence performance and lifetime. Before developing an over-temperature protection scheme for these situations, it is necessary to refer to the corresponding standards and practical applications in order to obtain the most cost-effective results. For road traffic signal applications, it is possible to immediately protect or send an alarm notification when the specified temperature is reached, so as not to affect the minimum light intensity level required by the relevant standards, resulting in traffic safety hazards; for road or home lighting, it may be possible to start at a certain temperature. Protection, first reduce the current, and immediately protect after reaching the temperature protection Curie point.

Whether it is self-protection or protection, the switching power supply circuit that drives the LEDs is the first line of defense to ensure safe and reliable operation of the system. It also improves the overall energy efficiency of the LED lighting system, reduces its overall cost, and achieves corresponding The key to control functions such as dimming is to help select and/or determine the design of active switching devices and corresponding power circuits through appropriate test solutions.

From the perspective of the development of practical application circuits such as LED driver circuits and protection circuits, overcoming the above three major problems of LEDs is inseparable from the accurate, reliable and low-cost implementation of various electrical performance test and measurement, especially the electrical performance test of switching power supplies. And more focused on current testing.

Test and measurement considerations for bottlenecks

So what are the key electrical properties for LED lighting application development that are worthy of special consideration for engineers? Tektronix Chen Wenquan said that LED current ripple is the first to bear the brunt, and it is related to the cost of switching power supply and the balance of luminous flux balance.

He analyzed that ripple current affects LED performance by increasing power consumption, which can result in increased junction temperature and a significant impact on LED lifetime. According to experience, for every 10 °C reduction in junction temperature, the lifetime of semiconductors is extended by a factor of two. In addition, the relative light output (light flux) as a function of the operating current is closely related to the diode current, so dimming can be performed by changing the forward current. When the current is low, if the diode current is doubled, the light output will also be doubled; but when the current is high, a 100% increase in current can only increase the light output by 80%. The LED is driven by a switching power supply that produces a large ripple current. In fact, the cost of the switching power supply is determined to some extent by the amount of current allowed. The higher the ripple current, the lower the power supply cost, but the light output is affected.

Chen Wenquan further pointed out that the switching rate (di/dt, dv/dt), switching loss measurement, and safe working area (SOA) of switching power supply equipment are all considerations for test and measurement. These indicators test the efficiency of switching power supplies. Transistor switching circuits consume the most energy during the conversion process. Commonly used measurements include closed-point loss, open-point loss, power loss, dynamic open-point resistance, and safe operating area (instantaneous power).

Furthermore, modulation analysis of switching power supplies that implement LED dimming is also essential. There are two main ways to achieve LED dimming: one is to reduce the current of the LED; the other is to quickly switch the LED, and the dimming by the pulse width modulation (PWM) of the current waveform is more accurate. Obviously, the latter has become the mainstream of the industry. In lighting and display applications, PWM needs a frequency higher than 100Hz, so that the naked eye does not feel flicker, 10% of the pulse width is in the range of ms, and the bandwidth of the power supply is required to be greater than 10kHz. Moreover, the control loop is always active and achieves extremely fast transient response.

In addition, Chen Wenquan pointed out that automatic line measurement, including power quality and harmonic analysis, is also an indispensable measurement indicator for LED lighting applications, so that the corresponding standard requirements can be met at the factory. These include real power, reactive power, apparent power, power factor, crest factor, current harmonic measurement, and THD.

The right medicine: find the most suitable test and measurement program and oscilloscope

In the speech, Chen Wenquan shared with the engineers who attended the meeting the things to be aware of when testing the above series of electrical performance indicators and the choice of the corresponding oscilloscope and accessories.

For ripple measurements, the choice of current probe is important. The current ripple index of the LED single tube is 1m A or even hundreds of uA. If the dynamic range of the probe is not reached, the tiny current can be measured by adding the winding. The TCP0030 AC/DC current probe, directly connected to the Tektronix DPO4000/7000 oscilloscope, provides a precise and simple current ripple measurement solution and supports higher voltages. The TACPA300+TCP312 (amplifier) ​​can be connected to any brand of oscilloscope to form a power test solution.

Figure 2: Tektronix engineers are using the MSO4000 Series oscilloscopes and signal boards to demonstrate accurate capture of power glitches.

For active switching device loss measurements, such as the MPO/DPO4000 oscilloscope with DPO4PWR power analysis application module software, it is very convenient to automatically calculate open point loss, closed point loss and conduction loss. However, Chen Wenquan reminded that the timing between the voltage waveform and the current waveform must be accurate, and the offset between the voltage probe and the current probe can be eliminated by means of a TekVPI probe and an offset correction kit. In addition, to handle the switching signal frequency component oscilloscope to provide sufficient bandwidth and rise time, and to have a fast sampling rate to capture the jump, in addition to the need to provide deep record length for long-term acquisition.

“The rise time of the switching signal may be quite fast. For accurate measurement, the rise time of the measurement system (oscilloscope plus probe) should be 5 times faster.” Chen Wenquan emphasized, “For power-on modulation analysis, the record length is important, the switch The pulse width and amplitude of the unit's control signal voltage and current can be recorded very completely.” In fact, Tektronix DPO/MSO4000 and 3000 Series oscilloscopes have record lengths of up to 10M and 5M points, respectively, with sampling rates up to 5GS/ s.

In addition, in the case of today's very long record lengths, conventional oscilloscope knobs have been out of date and time consuming. In addition, switching power supply and energy loss measurement are more to see transient changes, if you still use manual or computer programming to calculate, it will be very troublesome. Therefore, he especially recommends the unique Wave Inspector search and navigation tools provided by Tektronix oscilloscopes. The front panel control function and forced outer loop feedback can greatly improve the operation convenience, and can support capturing load change events, tracking transient power values, and transients. The power point corresponds to the corresponding time domain waveform analysis and the waveform details are amplified.

For power quality measurements, Chen Wenquan's measurement recommendations are: 1. Verify the interaction of the power supply with its service environment; 2. Measure the voltage and current directly on the input power line; 3. Require a high voltage probe, usually a differential probe. As for the relevant standards that must be met, such as EN61000-3-2, MIL-STD-1399, etc., Tektronix oscilloscopes provide options for these standards, and users can easily complete the conformance test to see if it can pass.

As mentioned earlier, China's LED lighting industry is currently in a state of lack of standards, and relevant organizations are stepping up to develop such industry standards. In this case, developers can choose some internationally accepted industry-recognized standards for reference, such as the example mentioned by Tektronix experts in the speech - the US standard DMX512 (RS-485 compatible) in the field of LED stage lighting, US military standards MIL-STD-883G, ESD-STM5.2-1999, etc. for circuit protection and component testing. "The manufacturers that require these standards to conduct relevant test and measurement are our actual customers and are worth learning from." Chen Wenquan said, "From the MSO/DPO4000 and DPO3000 series oscilloscopes, to the power analysis bundle solution, that is, the power measurement module software, probes. , calibration tools, Tektronix has a complete set of power measurement tools, and constantly updated, follow-up standards, can meet the test and measurement needs of LED lighting application development, help overcome the application challenges , which in turn reduces overall development costs."

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