Parasitic Component Analysis and Acoustic Noise Evaluation in Voltage Regulator Modules (Vrms)
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Parasitic Component Analysis and Acoustic noise evaluation in Voltage Regulator Modules (VRMs) A THESIS SUBMITTED TO THE FACULTY OF UNIVERSITY OF MINNESOTA BY PRANIT JANNAWAR IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Ned Mohan, Adviser June, 2015 © Pranit Jannawar 2015 Acknowledgements I would like to thank my adviser Prof. Ned Mohan for his support and advice. I appreciate his patience in guiding me throughout the period. I have learnt what kind of effort is put into the research and what kind of attitude is required to address a technical issue. I would like to mention the efforts of my fellow mates in the lab who helped me obtaining components and hardware in timely manner. Finally, I express my gratitude to my parents for their emotional support and encouragement. i Abstract Stable and regulated supply voltage has been an important topic of discussion in a consumer electronic products. Every chip and device requires specified voltages with small margins. Voltage regulated modulators (VRMs) have always been proven to provide a solution to this problem. Building a power stage using point of load method helps overcome variability issue in the signal but introduces a converter with lot of supporting components. Multiple combination of capacitors and resistors of different material composition are required to obtain an optimal design for the product. PCB parasitic comes into play and due to high current slew rate physical stress builds up in the caps which tend to come out in form of acoustic noise. This thesis deals with such issues which can improve the product quality but keeping the cost low by utilizing and analyzing components the proper way. ii Table of Contents List of figures…………………………………………………………………………..iv 1. Introduction 1 1.1 Concept of VRMs…………………………………………………………..1 1.2 Problem statement…………………………………………………………..3 1.3 Organization of thesis……………………………………………………....4 2. Functioning of VRM 5 2.1 Power modes……………………………………………………………….5 2.2 Types of IC controllers…………………………………………………….6 2.3 Voltage feedback and compensation loop…………………………………8 3. Literature Review 12 3.1 Impedance Measurement…………………………………………………..13 3.2 Voltage Injection Method …………………………………………………14 3.3 Feed forward Capacitor……………………………………………………17 3.4 Output capacitor selection…………………………………………………18 4. Simulation and Hardware setup 20 5. Results 23 5.1 Parasitic Impedance……………………………………………………….23 5.2 MLCC audible noise issue………………………………………………...31 6. Conclusion and Future Work 36 7. References 37 8. Appendix 39 iii List of figures Figure 1. Basic linear regulator…………………………………………………………………2 Figure 2. Example of discrete converter [24]…………………………………………………...6 Figure 3. Example of Fully Integrated converter [3]……………………………………………7 Figure 4. Block diagram of closed loop converter………………………………………………8 Figure 5. Closed loop system of a synchronous buck converter………………………..............10 Figure 6. Type 3 compensation network………………………………………………………..11 Figure 7. Input and output signals through the coaxial cable…………………………………...13 Figure 8. Modification to circuit for voltage injection method ………………………………...15 Figure 9. Hardware setup for voltage injection method………………………………………...16 Figure 10. Placement for feed-forward capacitor……………………………………………….17 Figure 11. Dynamics inside MLCC [23]………………………………………………………..19 Figure 12. Circuit diagram for TPS40055 [4]…………………………………………………...20 Figure 13. Tps40055 evaluation module.......................................................................................22 Figure 14. PSpice circuit simulation for TPS40055……………………………………………..23 Figure 15. Standard model for output LC filter………………………………………………….24 Figure 16. Bode plot of output filter of standard model…………………………………………25 Figure 17. Practical model for output LC filter………………………………………………….26 Figure 18. Bode plot of output filter of practical model…………………………………………27 Figure 19. Network analyzer output for un-optimized Cff………………………………………28 Figure 20. Network analyzer output for optimized Cff………………………………………….29 Figure 21. Improvement by using optimized Cff………………………………………………..30 Figure 22. Capacitor acoustic noise w.r.t. solder amount………………………………………..31 Figure 23. High magnified picture of cross section of capacitor plates [23]…………………….32 Figure 24. Capacitor acoustic noise w.r.t. orientation of plates………………………………….33 iv 1. Introduction In today’s world of rapidly evolving consumer electronic industry, there is a constant demand of low voltage, small form factor and high efficiency power supply units (PSUs) which supply power at required specifications to millions of devices across the globe. Including small gadgets like smartphones which consume few watts to power hungry enterprise servers which require few kilo-watts, everything needs their own customized DC/DC power supply units. With changing technology and miniaturization of devices, different components on the same platform require different power levels. This can be achieved by stepping down voltage at different levels so that every components has its desired voltage. For example inside a computer, microprocessor and DIMMs requires different voltages and same is the case with PCIe module and USB ports. In the past, this was done using a single power cabinet at one place and wires were used to connect individual components. This method did work when voltage levels were in few volts like ±12, ±5 etc. But now, when microprocessor takes 0.85V and other ICs on board need 1.8V, long wires can no longer be used as they introduce parasitic inductance and resistance which causes lot of noise and stability issues. 1.1 Concept of VRMs To address this problem, Point of Load (POL) the concept was introduced which requires the conversion of power to a different level as close as possible to the load. This helps in supplying high peak current demand, low noise margins and high power densities 1 resulting in highly efficient converters. These DC/DC converters are called Voltage Regulator Modules or VRMs. A Voltage Regulator Module (VRM) is a circuit that is soldered or connected to the baseboard which is designed to supply required power and voltage to different ICs and microprocessors. Every VRM on the board is custom designed for the specification mentioned by the vendor. It is a power electronics circuit, essentially a synchronous buck converter which regulates the voltage with very tight precision. A VRM is a closed loop amplifier, it converts one level of electrical energy to a different level with a regulated current or voltage specifications. Regulation is performed by tapping the output voltage and comparing it to a reference voltage which is then amplifies error signal and used to control a PWM that helps to maintain a voltage level desired by the load. Vin Vout Vref R1 R2 Figure 1. Basic linear regulator There are basically two types of VRMs: Linear Regulators and Switching Regulators. [1] Former employs an active switch like a BJT or MOSFET controlled by a 2 high gain differential amplifier. It compares the output voltage with a precise reference voltage and adjusts the switch to maintain a constant voltage. The latter converts dc input voltage to switched voltage applied to a switch. The output voltage is fed back to a circuit that controls the duty cycle which maintains the output voltage regardless of load variations and supply voltage. Linear regulators are normally used where converter efficiency is not a concern. It is easy to design, has small form factor, cheap, less noisy but high in heat dissipation. On the other hand when high efficiency converters are required switching regulators are preferred. Although it has high noise, difficult to design and has number of parts, its low heat dissipation and more control on regulation gives it an upper hand to be used in number of applications. 1.2 Problem statement The line of regulators that is discussed in this thesis is IC based DC/DC buck converter which finds its application in almost every consumer electronic device. TVs, gaming consoles, graphic cards, motherboards, tablets etc. every gadget available around today has one of these converters which helps them to perform better. Small size, affordable, durable and highly efficient which gives means longer battery life. This converter gets attention from CPU and GPU design as they require low voltage, high current with very low ripple output voltage for their low power configuration which takes up around 35-45 Watts or enterprise server CPU and GPU which demands 150-170 Watts. The closer one gets to the load, the sensitive the circuit becomes which includes effect from parasitic impedance such as stability and bandwidth issues. During transients, these 3 impedances like on board capacitance and interconnects play an important role in determining output waveform of the converter. Along with this issue there is a problem of audible noise from the components itself. Because of piezoelectric effects of MLCCs and inductor coils, the component expands and contracts in its own space and vibrates in multiple frequencies. The one which falls under 20Hz-20 KHz becomes significantly audible and becomes a consumer product problem. This thesis addresses some of such issues and finds ways to minimize its effect. 1.3 Organization of thesis With the introduction of the concept in chapter 1, and describing the functioning of VRMs in chapter 2, chapter 3 deals with the literature review that introduces new concepts in VR design. Chapter 4 contains the hardware implementation of the tests and experiments suggested.