Master of Science Thesis in Electrical Engineering Department of Electrical Engineering, Linköping University, 2020 MOSFET packaging for low voltage DC/DC converter Comparing embedded PCB packaging to newly developed packaging Emil Dahl Master of Science Thesis in Electrical Engineering MOSFET packaging for low voltage DC/DC converter: Comparing embedded PCB packaging to newly developed packaging Emil Dahl LiTH-ISY-EX--20/5275--SE Supervisor: Tomas Uno Jonsson isy, Linköping University Magnus Karlsson Flex Power Modules Mikael Appelberg Flex Power Modules Examiner: Mark Vesterbacka isy, Linköping University Division of Integrated Circuits and Systems Department of Electrical Engineering Linköping University SE-581 83 Linköping, Sweden Copyright © 2020 Emil Dahl Abstract This thesis studies the options of using PCB embedding bare die power MOSFET and new packaging of MOSFET to increase the power density in a PCB. This is to decrease the winding losses in an isolated DC/DC converter which, according to "Flex Power Modules", can be done by improving the interleaving between the layers of the transformer and/or decreasing the AC loop. To test the MOSFET packaging two layout are made from a reference PCB, one using embedded MOS- FET and the other using the new packaging. The leakage induction and winding losses are simulated and if they are lower compared to the reference PCB proto- types are manufactured. The simulated result is that PCB embedded MOSFET decrease the leakage induction but the winding loss is higher. With the new packaging the leakage induction is higher and the winding loss has linear char- acteristics. Only the PCB with the new MOSFET packaging is made because the MOSFET die gate pad is too small for the PCB manufacturer to make a via connec- tion to it. The PCB is tested that it operates as a DC/DC converter with a 40-60 V input and a 12 V output. The PCB is put on a test board in a wind-tunnel to test its characteristics under different wind speeds, input voltage and loads. The result is that the PCB has a higher efficiency than the reference PCB but it has worse thermal resistance. Further development of the design needs to be made to improve the thermal resistance. Using new packaging is a way to continue the development of power converter with lower efficiency but embedding MOSFET needs a less complicated manufacturing process before there is any widespread usage. iii Acknowledgments I would like to thank my supervisors who have helped me during this thesis work. Linköping, April 2020 ED v Contents List of Figures ix List of Tables xi Notation xiii 1 Introduction 1 1.1 Motivation . 1 1.2 Purpose . 3 1.3 Problem definition . 3 1.4 Delimitation . 3 2 Related work 5 2.1 Embedded components . 5 2.1.1 Embedded MOSFETs . 6 2.1.2 Embedded MOSFETs and CTE . 8 2.2 Parasitic and packaging . 8 3 Theory 9 3.1 Transformer loss . 9 3.1.1 Leakage inductance . 9 3.1.2 Core loss . 10 3.1.3 Winding DC loss . 11 3.1.4 Skin depth . 11 3.1.5 Proximity effect . 12 3.2 Isolated DC/DC converter . 15 3.3 Thermal resistance . 16 4 Method 19 4.1 Layout . 19 4.2 Simulation . 22 4.2.1 DC simulation . 22 4.2.2 Electromagnetic simulation . 22 vii viii Contents 4.2.3 Small-signal simulation . 25 4.2.4 Thermal simulation . 25 4.3 Output induction simulation . 26 4.4 Measurement . 27 4.4.1 Electrical verification . 27 4.4.2 Quality verification . 28 5 Result 29 5.1 Layout . 29 5.2 DC-simulation . 30 5.3 Electromagnetic simulation . 30 5.3.1 Current distribution . 30 5.3.2 Winding loss . 31 5.3.3 Leakage inductance . 33 5.4 SPICE simulation . 34 5.5 Thermal simulation . 35 5.6 Inductor simulation . 36 5.7 Manufacturing . 38 5.8 Measurements . 38 5.8.1 Electrical verification . 39 5.8.2 Wind tunnel . 39 6 Discussion 41 6.1 Leakage inductance . 41 6.2 Winding loss . 42 6.3 Thermal simulation . 42 6.4 Current distribution . 43 6.5 Manufacturing of embedded PCB . 43 6.6 Measurements on new PCB . 43 7 Closing comment 45 A Thermal performance of new PCB 49 Bibliography 51 List of Figures 2.1 A MOSFET packaging with copper clip. 6 2.2 A manufacturing process for embedded components. 7 2.3 A manufacturing process for embedded components. 7 3.1 Transformer model with leakage inductance L1 and L2 . 10 3.2 Transformer . 12 3.3 Interleaved transformer. 14 3.4 MMF diagram for the interleaved transformer. 14 3.5 Two phase full bridge isolated DC/DC . 15 4.1 Schematic for the rectifier. 20 4.2 Stackup of the layout. 21 4.3 Schematic model for "Comsol Multiphysic" simulation for winding loss. 23 4.4 Schematic model for "Comsol" simulation for leakage inductance in phase A. 24 4.5 Schematic model for "Comsol" simulation for leakage inductance in phase B. 24 4.6 Schematic of the AC resistance in the winding. 25 4.7 H-B characteristic of the ferrite material used. 27 4.8 Wind tunnel used for stress test. 28 5.1 Primary side winding loss. 31 5.2 Secondary side winding loss. 32 5.3 Total winding loss. 32 5.4 Leakage inductance phase A. 33 5.5 Leakage inductance phase B. 33 5.6 Temperature distribution over the PCB. 36 5.7 Current density in the output inductor winding . 37 5.8 Magnetic flux density inside the core . 37 5.9 Picture of the manufactured PCB. 38 5.10 Efficiency of module at a temperature of 23 5 C. 39 ± ◦ 5.11 Efficiency of module in wind tunnel. 40 5.12 Thermal resistance of the module with heat sink and test board. 40 ix x LIST OF FIGURES A.1 Thermal resistance of the module with heat sink excluding pins. 49 A.2 Thermal resistance of the test board. 50 A.3 Thermal footprint of the module with test-board and heat-sink. 50 List of Tables 5.1 Drain current for MOSFETs (TXXX) at different frequencies. 30 5.2 Drain current for MOSFETs (TXXX) at different frequencies. 31 5.3 Power losses from SPICE simulation. 34 5.4 Simulated temperatures in reference PCB. 35 5.5 Simulated temperatures in embedded PCB. 35 xi Notation Abbreviations Abbreviation Meaning ac Alternating Current cte Coefficient of Thermal Expansion dc Direct Current dosa Distributed-power Open Standards Alliance emi Electromagnetic Interference fem Finite Element Method igbt Isolated Gate Bipolar Transistor mmf Magnetomotive Force mosfet Metal-Oxide Semiconductor Field Effect Transistor nda None Disclosure Agreement pcb Printed Circuit Board pth Plated Trough Hole rms Rote Mean Square smps Switching Mode Power Supply spice Simulation Program with Integrated Circuit Emphasis wbg Wide Band Gap xiii 1 Introduction With the increasing need for smaller feature size in electronics the interest for Printed Circuit Board, PCB, embedded components have increased [2]. An em- bedded component is a component that has been inserted between the metal lay- ers inside a PCB. This frees up space on the surface of a PCB and the embedded component has less parasitic compared to a corresponding surface mounted com- ponent. This thesis studies the option of embedding power Metal Oxide Semi- conductor Field Effect Transistors, MOSFETs, used as a rectifier in an isolated DC/DC converter and comparing it to using surface mounted MOSFETs with a package that uses a clip for connection. 1.1 Motivation An isolated Direct Current, DC, to DC converter with a 40 to 60 V input and a 12 V output was designed by "Flex Power Modules" on a PCB with the Distributed- power Open Standards Alliance, DOSA, quarter brick standard for isolated power modules [1]. The converter is galvanic isolated using a planar PCB transformer and the secondary side rectifier is an active H bridge rectifier where each group of transistors consist of four MOSFETs in parallel to decrease the drain source on resistance. Simulation of the transformer’s winding loss showed that it becomes smaller if the Alternating Current, AC, loop to the rectifier is shorten and the in- terleaving between the primary and secondary side are improved. To have a short AC loop the MOSFETs for the rectifier should be put as compact as possible. 1 2 1 Introduction The MOSFETs can be put more compactly if the number of MOSFETs in parallel decreases. But this has the disadvantage of increasing the on resistance. "Flex Power Modules" have recently got a new MOSFET that has a lower on resistance. Using this MOSFET in the rectifier the number of transistors in parallel can be decreased while not increasing the on resistance. One added benefit of using less MOSFETs in parallel is that the parasitic capacitance decreases which lowers the rise and fall time. What needs to be evaluated is how much this decreases the winding loss and if it has any added benefit in the converter. Using another kind of MOSFET packaging is not the only option for shortening the AC loop. In recent time the development of embedded components for power electronic have increased [2]. By paralleling embedded bare die MOSFETs in the PCB and surface mounted MOSFETs the second side rectifier can be stacked in different layer of the PCB which increases the high-power density and frees up surface space on the PCB. The issue with embedded component is that the manufacturing process is not as well developed compared to surface mounted components [9]. Embedded com- ponent increases the power density inside the PCB and the prepreg that is used to separate the PCB’s metal layers have a low thermal dissipation increasing the dif- ficult of extracting the heat from the MOSFETs which can lower their efficiency.