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LED driver final testing
The most important thing to test is that your current limiting logic works. (Don't worry, our final demonstration environment contains also fuses, but it is not nice to blow too many of them.) Hence, you should test very well that your LED driver feedback loop works well.
Testing is first done with a dummy load using only the shunt resistor as load. This is known to be a much lower resistance as the shunt resistor and led load in series, so tests should be performed using two shunt resistors in series as well. If time permits, testing can be done using two shunt resistors in parallel creating a yet lower resistance. This way the system's reaction to load variance is tested.
After passing all dummy load tests the system will be demonstrated to succesfully lighting a power rgb led. Normally these leds are driven using specialized ic products and the workshop session demonstrates that modern micro controllers have enough
The software should have internal limits on PWM frequency and duty cycle values. A too long pulse will fry the inductor. This can result from too low frequency or too high duty cycle compared to frequency. Too high output current will fry the load or load protecting fuse. This results from too high duty cycle.
The duty cycle is checked on an oscilloscope for each channel separately. The oscilloscope can be used as a rough measurement tool and for this purpose its resolution is enough.
Voltage over the inductor is also checked to see what goes on in the inductor.
Current through the inductor is measured as voltage over the shunt resistor This gives the final test on forward control, now we should have a clear view on how the system performs.
Limits for pulse length, voltage and current are:
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The software should check that the buck output current is always under maximum instantenous value and that the average of output current is always below maximum average value. Should the output current fall outside these bounds the software should either correct the fault immediately or stop commutating the buck regulator.
The output current is measured as voltage over the shunt resistor Rshunt. Measurement should be done using both a multimeter and an oscilloscope. The peak value can be only read using an oscilloscope while the average value may be read using a true rms multimeter.
Limits for current are:
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These tests should be monitored varying the inputs throughout their full dynamics and generally having the test system perform as wide variations and expected failure modes the system can be estimated to have. This varies a lot on the input types and processing of an individual system and have to be thought out in the group.
The control input of the system should be commanded to push too much current and tested to see if it either refuses to go over the limit (negative feedback) or stops commutating. Both habits are considered successfully passing the test. Any means of exceeding the limits is considered failing the test.