The quickest and easiest way to improve the efficiency of LED power testing is to choose the right electronic load. If the knowledge of the electronic load is not familiar enough, or the proficiency is not enough, the confidence of the test result will decline, which will affect the quality of the product and cause serious accidents. This article mainly describes the principle of electronic load CV, and introduces some misunderstandings of LED power test.
The CV mode of the electronic load is loaded and is the basis for LED power testing. CV is a constant voltage, but the load is only a current-carrying device, and it cannot supply a constant voltage. Therefore, the so-called CV is only a voltage negative feedback circuit to servo the output current of the LED power supply, so that the LED output capacitor The charge balance, in order to achieve a constant voltage. Therefore, there are two core factors that determine the accuracy of CV:
Load bandwidthLED power output capacitor size
When the ripple frequency of the LED power supply output current is high, if the load bandwidth is insufficient, the servo current change cannot be caused, and the oscillation is caused. When the oscillation occurs, the load input voltage changes abruptly, and the LED output capacitor performs frequent high-current charge and discharge. At this time, the detected current ripple will be much larger than the actual current ripple when the LED power supply is in steady state operation.
When the load bandwidth is insufficient, if the output capacitance of the LED power supply is large enough, the amplitude of the oscillation can 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, so The LED power supply is tested and the load bandwidth requirements are very demanding.
The load bandwidth indicator is not directly indicated by the manufacturer. It can only refer to another indicator: the 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. In general, the load of 10uS full-scale current rise time can meet the testing needs of most LED power supplies, but in theory, any load is in CV mode. Underneath, there is the possibility of oscillation. In this case, when the LED output capacitance is constant, the higher the load bandwidth, the smaller the oscillation amplitude, and the higher the confidence of the test result. Therefore, the user is using the electronic load. When testing, you must pay close attention to the change of the load input voltage ripple Vpp. Once it is out of range, the whole test result is no longer credible. This is very important, and users must keep in mind.
In CV mode, the constant voltage is, and the current ripple is usually very large, while the load is to improve the test efficiency, the data refresh frequency is often higher, so the data jitter is very large, many users use this to determine whether the load is suitable. LED test, in fact, this is a very serious misunderstanding, the stability of the data is actually very easy to achieve, only need to increase the time measurement of data filtering can be achieved, very short low-end electronic load, because of low measurement accuracy, Therefore, it has to be filtered on a large time scale, but it is a blessing in disguise, making the data seem more stable. In fact, this is an illusion. To achieve accurate measurements, the fundamental method can only be to increase the sampling rate without increasing the sampling rate. Such measurement results have very low confidence 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:
The full-scale current rise time is the basis for ensuring accurate loading. The lower the value, the better;
The data sampling rate is the basis for ensuring accurate measurement. The higher the value, the better;
Vpp real-time display is the basis for judging whether the measurement data is reliable;
The 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, which are known as the special electronic load for LED power test. In fact, it is a general-purpose electronic load changeover, and it is generally converted from an electronic load whose bandwidth and sampling rate do not meet the test requirements. It does not improve its own bandwidth. Because bandwidth technology is the core technology of load, it is closely related to cost, and it is difficult to improve. It is often improved through three ways to make current data more stable, but it is also less credible.
The easiest way is to increase the filtering strength and enforce data stability. Simply using this method can easily lead to misjudgment and lead to quality accidents.
Adjust the voltage feedback loop to strongly filter the voltage feedback signal to reduce the current oscillation amplitude. This method goes the other way, further reducing the load bandwidth, so that the situation of non-convex and large shocks becomes a smaller amplitude.
Increase the capacitance inside the load. This method can suppress the occurrence or amplitude of the oscillation, but the measured current ripple will be slightly smaller than the actual ripple, but it is very helpful for testing the DC working point. However, because the rated operating voltage of the load is generally high, the price and size of the high-voltage capacitor are very serious problems, so it is difficult to increase the ideal condition, and often combined with the second method. Another problem is that in this situation, it often uses relatively inexpensive high-voltage electrolytic capacitors, which brings a lot of parasitic problems.
This article comprehensively introduces the principle of electronic load, especially the misunderstanding of the electronic load in the LED measurement process. Not only that, but some feasible solutions have been proposed in this paper in order to obtain more stable current data. I hope everyone can gain something after reading this article.
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