Remaining useful life prediction of IGBTs in MRI gradient amplifiers

Transcription

1 Remaining useful life prediction of IGBTs in MRI gradient amplifiers Reference: Date: Author(s): Distribution: P Martijn Patelski PE Event 2018 attendees a passion for technology Template PN: Template date: Slide of 22

2 Content Introduction to the NG2200-XP Gradient Amplifier System topology Remaining useful life prediction Results and Accuracy Recommendations & ongoing improvements Conclusion Slide 2 of 22

3 Introduction Prodrive Technologies designs and manufactures a wide range of electrical products for Automotive, Industrial, Energy & Infrastructure and Medical markets. Prodrive NG2200-XP is a high tech Gradient Amplifier(GA) for MRI scanners - error of <0.1%, extremely linear output - 3-axes system, each containing 2100V / 1200Apk / ±360ARMS end-stages - Each end stage contains several H-bridges - 2.5MVA peak apparent power - Insulated Gate Bipolar Transistors (IGBT) used as power switches Slide 3 of 22

4 System Topology H-bridge topology with several parallel IGBT and freewheeling diode dies Several H-bridges in series in each axis - Large part of BOM Most power is dissipated in the silicon of the power switches - Average 10kW per axis - Very local heating Temperatures vary Slide 4 of 22

5 Why lifetime prediction? Customers want to know how long amplifier will last. Customers design their own current profiles using a tool which includes the thermoelectric - Use of amplifier differs greatly between (research) hospitals - Few hours up to 24h per day - Different types of scans - restricted only by protections such as - RMS & peak output current - Maximum junction temperature of IGBT / Diode Certain sequences are very intensive for lifetime - Unexpected failures result in expensive downtime Possible solution: GA calculates remaining useful lifetime based on electrical and thermal measurements in real-time. - No extra hardware required Slide 5 of 22

6 IGBT Failure Modes Temperature gradients and mismatch in thermal expansion coefficient (CTE) results in of the interconnections between material. Typical failure modes are - Bond wire lift-off - Heel cracking of the wire bond - Fatigue of the solder connections Different conditions cause different failures Simplify: Regard all failure modes as one Slide 6 of 22

7 Overview of lifetime prediction Slide 7 of 22

8 Overview of lifetime prediction Slide 8 of 22

9 Overview of lifetime prediction Slide 9 of 22

10 Overview of lifetime prediction Slide 10 of 22

11 Overview of lifetime prediction LC Slide 11 of 22

12 : Junction-Heatsink To calculate Junction temperature the manufacturer Junction-Case thermal can be used TIM (Case-Heatsink) thermal impedance still unknown T j t = Z th,jh P j t + T h Slide 12 of 22

13 : Junction-Heatsink Relationship between voltage drop and temperature is measured at 1A. Slide 13 of 22

14 : Junction-Heatsink Relationship between voltage drop and temperature is measured at 1A. Can be used to measure junction temperatures Slide 14 of 22

15 : Junction-Heatsink Power and temperatures are known Z th,jh t = T j t T j ( ) N P j Z th,jh (t) = r i (1 e t τ i ) i=1 Slide 15 of 22

16 : Heatsink-NTC Only interested in dies which fail first only interested in hottest dies Create simple from NTC to temperature to hottest part of case: Z th,h NTC (t) = r (1 e t τ) R=0.017 K/W τ=9s C=517.2 J/K Slide 16 of 22

17 Rainflow counting principle not error free - Simplification of reality: up to 32% error Rainflow best performing counting - Up to 15% error compared to FEM simulation Slide 17 of 22

18 data IGBT module N f = α ΔT n j e k b T jm Where α, n and Ea are found experimentally. Kb is the Boltzmann s constant and Ea represents the activation energy of the deformation process. is simplified: E a - Heating time (ton) effect unknown for this particular IGBT - Short heating time stresses bond wires - Long heating time stresses solder layers - In reality both cycles occur - More testing required Slide 18 of 22

19 Palmgren-Miner rule & lifetime consumption All (half) cycles add up to the total, inversely proportional to Nf Palmgren-Miner rule: LC = T uptime i n i N f (i) Industry standard method to combine contributions Simple method with acceptable accuracy Disadvantages: No (simple) way to relate predicted lifetime to probabilities Does not take into account that fatigue under different circumstances could be independent - SEMIKRON research indicates it might even be completely independent under certain circumstances - More testing is required Slide 19 of 22

20 Recommendations & ongoing improvements Additional temperature sensors - Eliminate 5% NTC error In progress: Improve reliability data - Power cycle tests - Compare simulation results with actual failure data Vce/Vf measurement at known temperature and current - Eliminate and reliability error - Failure occurs at 5% voltage increase (~30mV) Slide 20 of 22

21 Conclusion Accurate is now available Thermo electrical is verified and implemented - Also important for other implementations such as maximum junction temperature protection Relatively easy to implement in FPGA Much uncertainty in reliability data (extrapolation of data) Online condition monitoring useful addition Tracking of thermal cycles will give valuable Prodrive information for product development Slide 21 of 22

22 a passion for technology Prodrive Technologies T E contact@prodrive-technologies.com I Slide 22 of 22