Power Integrity Interview Questions
Power integrity is where power and signal integrity meet: decoupling, plane impedance, ground bounce, and the parts that keep a board alive when the outside world misbehaves.
Before you start
What you should already be comfortable with:
- Impedance of a real capacitor vs frequency: ESR, ESL and self-resonance
- L·di/dt and why inductance, not resistance, sets fast noise
- Plane capacitance between closely spaced copper planes
- Roughly how fast a switching regulator's loop can respond
What interviewers are really testing
- Designing decoupling from a target impedance instead of "one cap per pin"
- Knowing which part of the power path covers which frequency band
- Ground bounce and return paths, with numbers
- Keeping a board alive against the outside world — vehicle transients, hot-plug inrush
Start with these two — free full answers
How do you decide how many decoupling capacitors a processor needs, and how do you verify the PDN?
Think first: Before you read on: how much ripple can the rail take, and what current step does the load make? Those two numbers give you a target before you choose a single capacitor.
What is ground bounce and how do you reduce it?
Think first: Before you read on: what physical quantity sits between the chip's internal ground and the board ground, and which equation turns a change in current into a voltage?
Study order
Work through the stages in order. ● screening · ●● onsite technical · ●●● design / senior. Pages marked "Going deeper" add explanation that is not in the book; the two marked "Free answer" show the full answer; every page tells you where the full answer is in the book.
- PDN basics
- PI-03 — Why use thick, wide traces? ● Going deeper
- PI-02 — Ground bounce ● Free answer
- PI-01 — How many decoupling caps? ●● Free answer
- Protection
- PI-05 — Inrush when hot-plugging ●● Going deeper
- PI-04 — Automotive-grade robustness ●● Going deeper
Related questions in other chapters
- AN-15 — Frequency response of L, C and LC tanks ●
- PC-01 — Choosing a capacitor type ●
- PC-03 — Ferrite bead selection ●●
- PW-20 — eFuse vs PTC fuse; inrush ●●
- EMC-07 — Loop area and radiated emission ●●
- PCB-01 — Stack-up for a mixed-signal board ●●● Free answer
The book
The book has all 5 questions in this chapter with the full answer: how to explain it on a whiteboard, the numbers worth quoting, and the follow-ups that come next.
More topics
PCB Design Interview Questions · Power Electronics Interview Questions · Signal Integrity Interview Questions · Analog & Op-Amp Interview Questions · Embedded Hardware Interview Questions