LED | PartQuest™ Explore

This example illustrates the challenging problem of operating a DC/DC converter at the end of a long cable or transmission line. In this case, a set of three LEDs are co-located with a Step-Down (Buck) switching converter, Together the LEDs consume 25W at 6VDC, and they are supplied by an 18V battery at the other end of an 800 meter cable. The simulation results indicate that the circuit operates properly with only one LED turned-on, but becomes unstable with the other two LEDs are switched on at 1 ms.The transmission line model is specified to use 16 coupled RLC lumped segments, each representing 50 meters of cable length. The conductor DC resistance is set to 1 mOhm/meter, corresponding to 5 AWG wire. This should result in a relatively small voltage drop at the expected 1.5A current needed to support the load. But the effect of the transmission line is more than the DC resistance. The effective impedance and propagation delay, combined with the effective negative impedance of the power converter (i.e. constant power load), result in an unstable power system interaction.You can try using a reduced cable length (e.g. 400 meters, 8 AWG = 2.1 mOhm/meter, or 100 meters, 14 AWG = 8.3 mOhm/meter) by making a copy of this design and re-running the simulation. You will see that the circuit performance is greatly improved with a shorter cable.The concept of Impedance Stability Ratio, and the ability to measure it using SystemVision's TDFS (Time Domain Frequency Sweep) method, will be demonstrated in a related design example and discussed in Part 3 of the TDFS Blog Series, both coming soon!

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IR LED test circuit

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