The CV model load is the basic of the LED power supply test. CV is constant voltage, but the load is only the current load equipment, itself does not provide constant voltage, therefore, the so-called CV is just through the voltage negative feedback circuit, to servo the change of LED power supply output current, to make the LED output capacitor charge balance, thus achieve the purpose of constant voltage. Therefore, there are two factors to affect the CV accuracy: Load bandwidth and The output capacitance value of LED power supply.
When LED power supply output current ripple frequency is very high, it can’t servo current change if the load bandwidth is insufficient, and caused a shock, when shocks occur, rapid changes in load input voltage can make the LED output capacitance happen large current charge and discharge frequently, at this moment, the detection of ripple current will be far greater than the LED power supply’s actual current ripple.
When load bandwidth is not enough, and if the LED power supply output capacitance is large enough, then the oscillation amplitude can be controlled within the acceptable range, but, LED power supply’s price is very competitive, and general the insufficiency of the output capacitance, therefore, to test the LED power supply, the load bandwidth is very demanding.
The bandwidth index of the load, the manufacturer will not directly shows, only can reference another target: Full scale current rise time, apparently, the shorter the current rise time, shows that the higher load bandwidth; Load bandwidth more higher, the lower requirements of LED power supply output capacitance. In general, 10uS full scale current rise time load can meet the most of LED power supply test requests, But in theory, any load in CV mode has the possibility of concussion, under the condition of LED output capacitance invariable, the higher load bandwidth, and the smaller oscillation amplitude, the higher test results, therefore, users in the use of electronic load testing, must pay close attention to the change of the load input voltage ripple Vpp, once its out of range, the test results was no longer trusted, this point is very important.
In CV mode, the voltage is constant, and current ripple is usually very large, load to improve the efficiency of test, the data refresh rate is often higher, thus data fluctuations is very large, many users use this to judge whether the load suitable for LED test, in fact this is a very serious mistake, stability data or not, in fact, it is very easy to implement, only need to increase the time of data filtering measure can be achieved a short low-end electronic load, because of low measurement precision, so have to make long time scale of the filter, but rather a blessing in disguise, make the data appears to be more stable, it is an illusion. To achieve accurate measurement, fundamental method is to improve the sampling rate only, do not increase the rate of sampling, the measured results of confidence level is very low, could cause serious quality accidents.