Reference: P1806141249 Remaining useful life prediction of IGBTs Date: 2018-06-14 in MRI gradient amplifiers Author(s): Martijn Patelski Distribution: PE Event 2018 attendees Reference: P1806141249R01 Template PN: 6001-1246-5511 a passion for technology Template date: 06-07-2017 Slide 1 of 22
Content • Introduction to the NG2200-XP Gradient Amplifier • System topology • Remaining useful life prediction • Results and Accuracy • Recommendations & ongoing improvements • Conclusion Reference: P1806141249R01 Slide 2 of 22
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 - Current error of <0.1%, extremely linear output 3-axes system, each containing 2100V / 1200A pk / ± 360A RMS end-stages - - Each end stage contains several H-bridges - 2.5MVA peak apparent power - Insulated Gate Bipolar Transistors (IGBT) used as power switches Reference: P1806141249R01 Slide 3 of 22
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 Reference: P1806141249R01 Slide 4 of 22
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 model - Use of amplifier differs greatly between (research) hospitals - Few hours up to 24h per day - Different types of scans - Profiles 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 Reference: P1806141249R01 Slide 5 of 22
IGBT Failure Modes • Temperature gradients and mismatch in thermal expansion coefficient (CTE) results in degradation 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 Reference: P1806141249R01 Slide 6 of 22
Overview of lifetime prediction Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 7 of 22
Overview of lifetime prediction Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 8 of 22
Overview of lifetime prediction Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 9 of 22
Overview of lifetime prediction Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 10 of 22
Overview of lifetime prediction LC Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 11 of 22
Thermal Model: Junction-Heatsink • To calculate Junction temperature the manufacturer Junction-Case thermal model can be used • TIM (Case-Heatsink) thermal impedance still unknown 𝑈 𝑘 𝑢 = 𝑎 𝑢ℎ,𝐾𝐼 𝑄 𝑘 𝑢 + 𝑈 ℎ Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 12 of 22
Thermal Model: Junction-Heatsink • Relationship between voltage drop and temperature is measured at 1A. Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 13 of 22
Thermal Model: Junction-Heatsink • Relationship between voltage drop and temperature is measured at 1A. • Can be used to measure junction temperatures Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 14 of 22
Thermal Model: Junction-Heatsink • Power and temperatures are known 𝑎 𝑢ℎ,𝐾𝐼 𝑢 = 𝑈 𝑘 𝑢 − 𝑈 𝑘 (∞) 𝑄 𝑘 𝑂 − 𝑢 𝑎 𝑢ℎ,𝐾𝐼 (𝑢) = 𝑠 𝑗 (1 − 𝑓 𝜐 𝑗 ) 𝑗=1 Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 15 of 22
Thermal Model: Heatsink-NTC • Only interested in dies which fail first only interested in hottest dies • Create simple model from NTC to temperature to hottest part of case: 𝑎 𝑢ℎ,𝐼−𝑂𝑈𝐷 (𝑢) = 𝑠 (1 − 𝑓 − 𝑢 𝜐 ) R=0.017 K/W τ=9s C=517.2 J/K Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 16 of 22
Rainflow Cycle Counting • Cycle counting principle not error free - Simplification of reality: up to 32% error • Rainflow best performing counting model - Up to 15% error compared to FEM simulation Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 17 of 22
Reliability data IGBT module 𝐹 𝑏 −𝑜 ∗ 𝑂 𝑔 = 𝛽 ∗ 𝛦𝑈 𝑘 𝑓 𝑙 𝑐 ∗𝑈 𝑘𝑛 • Where α, n and Ea are found experimentally. Kb is the Boltzmann’s constant and Ea represents the activation energy of the deformation process. • Model is simplified: - 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 Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 18 of 22
Palmgren-Miner rule & lifetime consumption • All (half) cycles add up to the total degradation, inversely proportional to Nf • Palmgren-Miner rule: 𝑜 𝑗 𝑀𝐷 = 𝑈 𝑣𝑞𝑢𝑗𝑛𝑓 𝑂 𝑔 (𝑗) 𝑗 • Industry standard method to combine degradation 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 Cycle Accumulate Current Dissipation Thermal Counting Reliability degradation Lifetime Profiles model Model (Rainflow Model (Palmgren- Consumption Reference: P1806141249R01 method) Miner) Slide 19 of 22
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 model and reliability error - Failure occurs at 5% voltage increase (~30mV) Reference: P1806141249R01 Slide 20 of 22
Conclusion • Accurate model is now available • Thermo electrical model 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 Reference: P1806141249R01 Slide 21 of 22
a passion for technology Prodrive Technologies T +31 40 2676200 E contact@prodrive-technologies.com I www.prodrive-technologies.com Reference: P1806141249R01 Slide 22 of 22
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