Building a power supply that can handle various loads without oscillating can be a challenge. Computational models and computer simulations can help get your design headed in the right direction, but physical testing is essential to proving the performance of your design.
One method of quickly determining stability is to use a load step response.
In this test, a DC electronic load is used to provide a current load that steps from a low current draw to a higher value in a short period of time. By directly measuring the voltage and current output of the supply with the stepped load, we can visually observe the recovery of the power supply feedback loop and make changes to the design to optimize the response.
For this note, we are going to perform identical tests on two supplies and compare the output voltage and current waveforms: One has been tuned so that the output quickly recovers with minimal overshoot and ringing. The other supply is not tuned and subsequently oscillates. We will also discuss some measurement techniques to help get the right data as quickly as possible.
We also have a video to accompany this note:
Power Supply Design: Load Step Response with a SIGLENT DC Electronic Load
The Equipment:
- A DC Electronic Load: The SIGLENT SDL1020X-E is a 200 W load with dynamic testing capabilities to perform the load step. It also features remote sense capabilities to compensate for the voltage drop across the load leads. High currents can provide a substantial voltage drop across the leads and will add unwanted error.
- An oscilloscope: The SIGLENT SDS2354X Plus scope has a large display, easy-to-use interface, and features that make capturing these waveforms very easy.
- A power supply: The SIGLENT SPD1168X single output supply delivers power to our power supply board
- A current probe: The SIGLENT CP4070 features a 150 kHz bandwidth that will minimize most switching noise from the measurement
- Power supplies to test: The Analog Devices LTM4646 series of uModule Regulators. This module features two 10A DC-DC converters. One has been “detuned” to show some common problems associated with power supply design. The other supply has been left in it’s tuned state as a comparison to the detuned supply.
