To improve the test efficiency of LED power supply, the quickest and easiest way is to choose the right electronic load. If the knowledge of the electronic load is not familiar enough, or the proficiency is not enough to master, it may even cause the confidence of the test results to decline, thereby affecting the quality of the product and seriously causing accidents. This article mainly describes the principle of electronic load CV, and introduces some misunderstandings of LED power supply testing.
The electronic load is loaded in CV mode, which is the basis of LED power supply testing. CV is a constant voltage, but the load is only a current-pulling device, and it cannot provide a constant voltage by itself. Therefore, the so-called CV only serves to change the output current of the LED power supply through the voltage negative feedback circuit, so that the LED output capacitor The balance of the charge, and then achieve the purpose of constant voltage. Therefore, there are two core factors that determine the accuracy of CV:
* Load bandwidth
* The size of LED power output capacitor
When the ripple frequency of the output current of the LED power supply is very high, if the load bandwidth is insufficient, the servo current cannot be changed, which causes oscillation. When the oscillation occurs, the load input voltage changes sharply, and the LED output capacitor performs frequent large current charging and discharging. At this time, the detected current ripple will be much greater than the actual current ripple when the LED power supply is operating in steady state.
When the load bandwidth is insufficient, if the output capacitance of the LED power supply is large enough, the oscillation amplitude can also be controlled within an acceptable range, but unfortunately, the price competition of the LED power supply is very fierce, and the output capacitor capacity is generally insufficient. Therefore, LED power supply testing is very demanding on the load bandwidth.
The bandwidth index of the load is not directly marked by the manufacturer, and can only refer to another index: full-scale current rise time. Obviously, the smaller the full-scale current rise time, the higher the load bandwidth. The higher the load bandwidth, the lower the requirement for the output capacitance of the LED power supply. Generally speaking, a load of 10uS full-scale current rise time can meet the test needs of most LED power supplies, but in theory, any load is in CV mode Under the circumstances, when the LED output capacitance is unchanged, the higher the load bandwidth, the smaller the amplitude of the oscillation, and the higher the confidence of the test result. Therefore, users are using electronic loads When testing, you must pay close attention to the change of the load input voltage ripple Vpp. Once it exceeds the range, the entire test result is no longer reliable. This is very important and the user must keep in mind.
In CV mode, the constant is the voltage, and the current ripple is usually very large, and the load is often used to improve the test efficiency, the data refresh frequency is often higher, so the data jumps are very large, many users use this to determine whether the load is suitable for LED testing is actually a very serious misunderstanding. The stability of the data is actually very easy to achieve. It only needs to increase the time measurement of data filtering. It is very short and low-end electronic load, because the measurement accuracy is low, Therefore, it is necessary to perform filtering on a large time scale, but it is a blessing due to misfortune, making the data seem more stable. In fact, this is an illusion. To achieve accurate measurement, the fundamental method can only be to increase the sampling rate, without increasing the sampling rate. Such a measurement result has a very low confidence level and may cause serious quality accidents.
Based on the above analysis, the LED power supply test has strict requirements on the load, and the main points are as follows:
* Full-scale current rise time is the basis to ensure accurate loading, the lower the value, the better;
* The data sampling rate is the basis to ensure accurate measurement, the higher the value, the better;
* Vpp real-time display is the basis for judging whether the measurement data is credible;
* Filter speed adjustment function is a small means to obtain stable current data;
Finally, I would like to draw your attention to the fact that there are some loads on the market, known as special electronic loads for LED power testing. In fact, they are modified from general-purpose electronic loads, and are generally modified from electronic loads whose bandwidth and sampling rate do not meet the test requirements. It does not increase its own bandwidth, because bandwidth technology is the core technology of the load and is also closely related to cost. It is difficult to improve. It often improves through three ways to make the current data more stable, but it is also more untrustworthy.
The simplest way is to increase the filtering intensity and force the data to be stable. Simple use of this method can easily lead to misjudgment and quality accidents.
Adjust the voltage feedback loop and perform strong filtering on the voltage feedback signal to reduce the amplitude of current oscillation. This method works in the opposite direction, further reducing the load bandwidth, so that the cases of non-oscillation and the situation of large oscillations become smaller oscillations.
Increase the capacitance inside the load, this method can suppress the occurrence or amplitude of oscillation, but the measured current ripple will be significantly smaller than the actual ripple, but it is very helpful for testing the DC operating point. But because the rated operating voltage of the load is generally higher, the price and size of the high-voltage capacitors are very serious problems, so it is difficult to increase to the ideal condition, and often used in combination with the second method. Another problem is that, in this situation, they often use relatively inexpensive high-voltage electrolytic capacitors, which will bring a lot of parasitic problems.
This article fully introduces the principle of electronic load, especially the misunderstanding of electronic load in the LED measurement process. Not only that, but in this paper, some feasible solutions are also proposed in order to obtain more stable current data. I hope you will gain something after reading this article.
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