The quickest and easiest way to improve the test efficiency of LED power supply is to choose an appropriate electronic load. If the knowledge of electronic loads is not familiar enough, or the proficiency is not enough to master, it may even cause a decline in the confidence of the test results, which will affect the quality of the product, and even cause accidents in serious cases. This article mainly describes the principle of electronic load CV, and introduces some misunderstandings in LED power supply testing.
The CV mode load of electronic load is the basis of LED power test. CV is a constant voltage, but the load is only a current-loading device and cannot provide a constant voltage itself. Therefore, the so-called CV is only through the voltage negative feedback circuit to servo the change of the output current of the LED power supply, so that the LED output capacitor The charge balance, and then achieve the purpose of constant voltage. Therefore, there are two core factors that determine the accuracy of CV:
The size of the output capacitor of the
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 change, causing oscillations. When the oscillations occur, the load input voltage changes sharply, and the LED output capacitor will perform frequent high-current charging and discharging. , the current ripple detected at this time will be much larger than the actual current ripple when the LED power supply works in a steady state.
When the load bandwidth is insufficient, if the output capacitor 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. The test of LED power supply has very strict requirements on the load bandwidth.
The manufacturer will not directly mark the bandwidth index of the load, and can only refer to another index: the rise time of the full-scale current. Obviously, the smaller the rise time of the full-scale current, the higher the bandwidth of the load. The higher the load bandwidth, the lower the requirement for the output capacitor of the LED power supply. Generally speaking, a load with a full-scale current rise time of 10uS can meet the test needs of most LED power supplies, but theoretically speaking, any load in CV mode In this case, when the LED output capacitance remains unchanged, the higher the load bandwidth, the smaller the amplitude of the oscillation, and the higher the confidence of the test results. 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 exceeds the range, the entire test result will no longer be reliable. This point 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 to improve the test efficiency, the data refresh frequency is often high, so the data jumps a lot, many users use this to determine whether the load is suitable for testing LED testing, in fact, this is a very serious misunderstanding, whether the data is stable or not, is actually very easy to achieve, only need to increase the time measurement of data filtering, very short low-end electronic load, because the measurement accuracy is low, Therefore, filtering with a large time scale has to be carried out, but it is a blessing in disguise, making the data seem more stable, but in fact this is an illusion. To achieve accurate measurement, the fundamental method can only be to increase the sampling rate. If the sampling rate is not increased, the confidence level of such measurement results will be very low, which may cause serious quality accidents.
Based on the above analysis, the LED power supply test has strict requirements on the load. The main points are as follows:
The full-scale current rise time is fundamental to ensure accurate loading, the lower the value, the better;
The data sampling rate is fundamental to ensure accurate measurement, the higher the value, the better;
The real-time display of Vpp is the basis for judging whether the measurement data is credible;
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 that claim to be special electronic loads for LED power supply testing, but they are actually general-purpose electronic loads that have been modified, and are generally converted from electronic loads that do not meet the test requirements in terms of bandwidth and sampling rate. It does not improve its own bandwidth, because the bandwidth technology is the core technology of the load, and it is also closely related to the cost, so it is difficult to improve. It is often improved through three ways to make the current data more stable, but also more unreliable.
The easiest 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 both the non-oscillating situation and the large-scale oscillation situation become oscillations with smaller amplitudes.
Increase 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 for testing the DC operating 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 to the ideal state, 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 cause many parasitic problems.
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