Demonstrating How to Analyze VRM Stability With NISM
- Kathleen Love
- 5 days ago
- 3 min read
In this video, we are performing a non-invasive stability measurement (NISM) on our PDN to check impedance. When looking at the measurement graph in the Bode Analyzer Suite, we can see that there is a sharp peak in the graph, indicating PDN instability. We are measuring the output of the voltage regulator module (VRM) and using the Bode Analyzer Suite to view the impedance with the VRM on. For our setup, we have the Omicron Labs Bode 500 as our VNA, a Picotest J2113 to break the ground loop and ensure a high accuracy measurement, a Picotest 2-port PDN probe to measure the shunt through impedance, and a Picotest S50 Demo Board. Setting up a non-invasive stability measurement is simple, and involves placing two cursors: one on impedance and one on the QTG. Once we have the cursors placed, the Bode analysis software gives us the phase-margin measurement, which in this case is 7 degrees, indicating high instability.
What is Non-Invasive Stability Measurement (NISM)?
Non-invasive stability measurement, or NISM, was developed by Steve Sandler of Picotest and is a method of determining control loop stability margins without requiring access to the feedback loop, which allows you to easily check control loop stability in scenarios where traditional measurements are challenging or unfeasible. For example, in many cases it is not possible to access the control loop, such as with POLs, fixed voltage regulators, voltage references, high BW op amps, or PMICs, which makes a Bode plot assessment impractical. In other cases, such as when the hardware is integrated, it might be unfeasible to break the control loop because doing so would require you to cut a PCB trace or lift components.
Enter NISM. This measurement is a mathematical solution based on minor loop gain theory and is calculated by converting output impedance to group delay. Then, the ‘Q’ is derived from the group delay and the stability margin from Q. NISM can be used to optimize not only control loop but also PDN design, as it can be applied to both active circuits and passive circuits. In this demo, NISM is performed using a VNA to measure the 2-port shunt through impedance.
How to Troubleshoot an Unstable VRM
High instability like we found with this NISM measurement in this demo can have serious repercussions for the success of your VRM. It is critical to check for and address this issue in the design phase to ensure that your DUT is stable and power efficient. The instability indicated by the high phase margin measurement means that this design will oscillate, which will create more noise and more jitter. This can also lead to electromagnetic compatibility or EMC issues, such as radiated emissions, from the power supply under test.
How to fix PDN instability depends on the type of VRM you are working with. One of the first things to try is to optimize the control loop by making changes to the bill of materials or settings, depending on features of VRM. However, sometimes you are limited on how much optimization you can get out of control loop, and you will need to make PDN changes by changing capacitor values. You can find the right ESR for your capacitor by using the equation C = L / R², which can be calculated based on your impedance measurement.
If you need more help troubleshooting and identifying the right design fixes for your PDN, we offer end-to-end power integrity simulation, modeling and measurement services. Reach out to us at info@signaledgesolutions.com to discuss your project.
For further reading on calculating VRM instability with NISM, check out the DesignCon presentation "Unmasking Voltage Regulator Instability: What Vendor Reference Designs Aren't Telling You" by Signal Edge Solutions CEO and Chief Technologist Benjamin Dannan and Will McCaffrey of Northrup Grumman.
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