Misunderstanding Analysis of Electronic Load in LED Measurement

The CV mode load of electronic load is the basis of LED power supply test. CV is a constant voltage, but the load is only a current loaded device, which cannot provide a constant voltage by itself. Therefore, the so-called CV only serves the change of the output current of the LED power supply through the voltage negative feedback circuit, so as to balance the charge on the LED output capacitor, so as to achieve the purpose of constant voltage. Therefore, there are two core factors determining CV accuracy:

Load bandwidth

Output capacitance of LED power supply

When the ripple frequency of the output current of the LED power supply is very high, if the load bandwidth is insufficient, it will not be able to servo the current change, resulting in oscillation. When the oscillation occurs, the load input voltage changes sharply, and the LED output capacitor will be charged and discharged frequently with large current. At this time, the detected electric ripple will be much larger than the actual current ripple when the LED power supply works stably.

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. Unfortunately, the price competition of the LED power supply is very fierce, and the output capacitance capacity is generally insufficient. Therefore, testing the LED power supply requires very strict load bandwidth.

The manufacturer will not directly mark the bandwidth index of the load, but can only refer to another index: the rise time of full-scale current. Obviously, the smaller the rise time of full-scale current, the higher the bandwidth of the load. The higher the load bandwidth, the lower the requirements for the output capacitance of the LED power supply. Generally speaking, the load with 10us full-scale current rise time can meet the test needs of most LED power supplies. However, theoretically, any load can vibrate in CV mode. In this case, when the LED output capacitance remains unchanged, the higher the load bandwidth, the smaller the oscillation amplitude, The higher the confidence of the test results, therefore, when using the electronic load for test, the user must pay close attention to the change of load input voltage ripple VPP. Once it exceeds the range, the whole test results will no longer be credible. This is very important and the user must keep in mind.

In CV mode, the voltage is constant, and the current ripple is usually very large. In order to improve the test efficiency, the data refresh frequency is often high, so the data jump is very large. Many users use this to determine whether the load is suitable for LED test. In fact, this is a very serious misunderstanding. Whether the data is stable or not is very easy to realize, It only needs to increase the time measurement of data filtering. For a very short low-end electronic load, because the measurement accuracy is low, it has to filter on a large time scale, but it is a blessing in disguise, making the data seem more stable. In fact, this is an illusion. In order to achieve accurate measurement, the fundamental method can only be to improve the sampling rate without increasing the sampling rate. In this way, the confidence of measurement results is very low, which may lead to serious quality accidents.

Based on the above analysis, LED power supply test has strict requirements for load, mainly including the following points:

The rise time of full-scale current is the fundamental to ensure accurate load. The lower the value, the better;

The data sampling rate is the fundamental to ensure accurate measurement. The higher the value, the better;

VPP real-time display is the basis for judging whether the measured data is credible;

The filter speed adjustment function is a small means to obtain stable current data;

Finally, please note that there are some loads on the market, which are called special electronic loads for LED power supply test. In fact, they are transformed from general electronic loads, and they are generally refitted from electronic loads whose bandwidth and sampling rate do not meet the test requirements. They do not improve their bandwidth, because bandwidth technology is the core technology of the load and is closely related to the cost, It is difficult to improve. It is often improved through three ways to make the current data more stable, but also more unreliable.

The simplest way is to increase the filtering strength 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 to strongly filter the voltage feedback signal to reduce the amplitude of current oscillation. This method goes the opposite way, further reducing the load bandwidth, so that the situation of no oscillation and large oscillation become smaller oscillation.

By increasing the capacitance inside the load, this method can suppress the occurrence or amplitude of oscillation, but the measured current ripple will be seriously smaller than the actual ripple, but it is very helpful to test the DC working point. However, because the rated working voltage of the load is generally high, the price and size of high-voltage capacitor are very serious problems, so it is difficult to increase to the ideal situation, and it is often used in combination with the second method. Another problem is that in this case, relatively cheap high-voltage electrolytic capacitors are often used, which will bring many parasitic problems.

This article comprehensively introduces the principle of electronic load, especially the misunderstanding of electronic load in the process of LED measurement. Moreover, some feasible solutions are proposed in this paper in order to obtain more stable current data. I hope you can gain something after reading this article.

Source; Power network