3.3 DC-DC converter efficiency measurement
The use of DC-DC converters has become almost universal in modern electronics. One important factor that needs to be evaluated, especially when the power comes from a battery, is the conversion efficiency.
I decided to evaluate a MAX 17503 step-down DC-DC converter evaluation board efficiency at an 8 Ω load. The SMU was connected to the DC input of the converter, and a Keithley DMM6500 to the output. By dividing the output power (which ideally should be constant) by the input power, the efficiency at different input voltages can be computed.
I used Kickstart to perform a simultaneous voltage sweep with the SMU and voltage measurement with the DMM. After setting each instrument parameter I pressed the top left "play" button to initiate simultaneous measurements.
Since I set the measure count of the DMM to infinite I had to manually stop the measurements once the SMU finished (I am not aware of a way to tell KickStart to stop the DMM once the SMU finishes).
I then saved the data of each instrument to a separate file and used Python to integrate the data (DCDCConverter.py). During the measurement I set the sweep delay to 1 s to make the integration of the data easier, but I found that the timestamps of the measurements were highly synchronized.
The converter reached the target voltage at around 6 V, and reached its maximum efficiency at around 7.5 V. It can operate up to 60 V at the input, but with the load that I used I was not able to source above 20 V (as for voltages above 20 V the SMU limits the maximum current to 100 mA). Alternatively I could have used a smaller load and performed a 0 to 60 V sweep, but the 8 Ω load was already small for a converter that supports up to 2.5 A.
A better way to characterize the efficiency of course would be to vary the input voltage and the output load, but this would require a programmable voltage source and a programmable current sink.