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Bridging Theory in Practice

Bridging Theory in Practice. Transferring Technical Knowledge to Practical Applications. Chapter 10 Introduction to Switching Regulators. Objective of Chapter 10 is to answer the following questions: What is a switching power supply? What types of switchers are available?

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Bridging Theory in Practice

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  1. Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

  2. Chapter 10 Introduction to Switching Regulators Objective of Chapter 10 is to answer the following questions: What is a switching power supply? What types of switchers are available? Why is a switcher needed? How does a switcher operate in general? How does a buck converter operate? How to calculate power loss? How to select external components?

  3. Introduction to Switching Regulators Intended Audience: Electrical engineers with limited power supply background A simple, functional understanding of inductors and capacitors is assumed A simple, functional understanding of transistors is assumed Expected Time: Approximately 60 minutes

  4. Outline Switching Regulator Overview What is a Switching Regulator? Why is a switcher needed? What are the main differences between a switching and linear regulator? Buck, Boost, Buck-Boost (Inverting) Switching Regulator Operation How does a Switching Regulator Operate? Buck Converter Design Example Qualatative explanation Quantative explanation Volt-Second princple (CCM) Discountinous Conduction mode (DCM) Practical Guidelines How to select components (Transistor, Inductor, Diode, Capacitor)? Integration vs. Mixed What could go wrong? Troubleshooting suggestions. How to calculate total power loss (Switching + Conduction)? Stability Pointers Control Mechanisms Voltage Mode Control Current Mode Control Pulse Skipping Mode Soft Start

  5. What is a Switching Regulator? Converts an input voltage into desire output voltage. The power transistor operates as a switch, completely on or off. An energy storage part (inductor) is used in the architecture Switching Regulator

  6. Choosing Between Linear and Switching Regulators When possible, most designers would prefer to use a linear voltage regulator rather than a switching voltage regulator Linear regulators are usually lower in price Linear regulators are usually simpler to implement Linear regulators do not have associated noise/ripple problems apparent in switching regulators

  7. Choosing Between Linear and Switching Regulators When to use a switching regulator #1: When the minimum input voltage is at or below the desired output voltage Linear regulators cannot provide an output voltage greater than the input voltage VIN < VOUT

  8. Choosing Between Linear and Switching Regulators When to use a switching regulator #2: The heatsinking of a linear regulator is prohibitive in price or space

  9. Choosing Between Linear and Switching Regulators When to use a switching regulator #3: The efficiency of a linear regulator cannot maintain the junction temperature below the specified maximum The maximum junction temperature is usually 150C The efficiency of linear regulators often prohibit their use in high voltage, high current applications

  10. Why are switching regulators needed? The power dissipation is too high for a linear regulator The efficiency of a linear regulator cannot maintain the junction temperature below maximum (150 °C) The heat sinking of a linear regulator is prohibitive in price or space OutputPower Switching Regulator Linear Regulator Maximum Power Dissipation Linear Regulator

  11. Why are switching regulators needed? The desired output voltage is greater than the input voltage Linear regulators cannot provide an output voltage greater than the input voltage The desired output voltage is opposite polarity than the input voltage Linear regulators cannot invert an input voltage Power Supply 1.5 V Battery 5 V Required Power Supply 12 V Battery -12 V Required

  12. Types of Switching Regulators AC-DC, AC-AC, DC-AC, and DC-DC Converters DC-DC AC-AC DC-AC AC-DC 110 Vac 110 Vac 12 Vdc 12 Vdc t t t t 220 Vac 110 Vac 12 Vdc 5 Vdc t t t t

  13. Types of DC-DC Converters Step Down, Step Up and Inverting V V Vin = 12 V Step Down Buck Vout = 5 V t t V V Vout = 12 V Step Up Boost Vin = 5 V t t V V Inverting Buck-Boost Vin = 5V t Vout = -10 V t

  14. Basic Circuit Configuration Buck VIN > VOUT Boost VIN < VOUT Buck-Boost VIN < -VOUT < VIN VIN VIN VIN ISW ISW VGATE VGATE IL IL L VOUT VOUT VOUT VM L VM VM C C VGATE C IL ISW L • All topologies consists of the same basic components but are arranged differently

  15. Buck Configuration The input voltage is always greater than the output voltage VIN VOUT VIN ISW 20V 10V VGATE 7.5V 15V IL VOUT 5V 10V L VM 2.5V 5V C 0V 0V time time

  16. Boost Configuration The input voltage is always less than the output voltage VIN VOUT VIN 24V 20V 20V IL L 15V 15V VOUT 10V 10V VM C VGATE 5V 5V ISW 0V 0V time time

  17. Buck-Boost Configuration The input voltage is always not constrained by the output voltage VIN VOUT VIN time 0V ISW 20V VGATE VOUT 15V -5V VM -10V 10V C IL L -15V 5V -20V 0V time

  18. Other Switching Voltage Regulator Topologies SEPIC Push-Pull and Forward Converter Flyback Converter

  19. How a Switching Regulator Works VIN VOUT Switching Regulator 5V Filter Network Voltage OK time 50% Output Monitor VOUT Duty Cycle Controller

  20. How a Switching Regulator Works VIN VOUT Voltage Regulator 5V Filter Network Voltage OK time 50% Output Monitor VOUT Duty Cycle Controller

  21. How a Switching Regulator Works VIN VOUT Voltage Regulator 5V Filter Network Voltage OK time 50% Output Monitor VOUT Duty Cycle Controller

  22. How a Switching Regulator Works VIN – 1V VOUT Voltage Regulator 5V Filter Network Voltage Low time 60% Output Monitor VOUT Duty Cycle Controller

  23. How a Switching Regulator Works VIN – 1V VOUT Voltage Regulator 5V Filter Network Voltage Low time 60% Output Monitor VOUT Duty Cycle Controller

  24. How a Switching Regulator Works VIN VOUT Switching Regulator 5V Filter Network Voltage Ok time 50% Output Monitor VOUT Duty Cycle Controller

  25. 10.5 Switching Regulator Components

  26. Switching Power Supply Block Diagram Switching Power Supply VIN VOUT Switch Network Network PWM Controller Error Amplifier Bandgap Reference

  27. Bandgap Reference Voltage Need very small temperature coefficient Balances negative temperature coefficient of pn junction's VBE with positive temperature coefficient of thermal voltage, Vt = kT/q kT/q VBE -2mV/C T +0.085mV/C T VINPUT A0 Vt = kT / q VREF = VBE + A0Vt  + VBE -

  28. Bandgap Reference Voltage • Internally generated with tight tolerance, traditionally ~ 1.2V • VOUT is “built” from this voltage reference by “zener zapping” VINPUT VREF 1.24V R3 = + 3% R4 = - 2% R5 = - 1% R5 = -1% 1.22V R4 = -2% VREF TARGET 1.20V R3 R1 1.18V R3 = +3% R4 R5 R2 1.16V

  29. Error Amplifier The error amplifier determines if VOUT is valid VOUT is divided down and compared to the reference voltage VOUT R1 PWM Controller VREF R2

  30. PWM Controller In a switching voltage regulator, the pass transistor is used as a switch - it is either on or off The output voltage, however, is an analog value PWM controller senses error in VOUT via the error amplifier PWM controller updates the duty cycle of the of transistor adjusting the output voltage 0-100% Error Amplifier PWM Controller VOUT

  31. Switching TransistorBipolar and MOSFET Drain Collector Base Gate Emitter Source

  32. Switching Power Supply Block Diagram Switching Power Supply VIN VOUT Switch Network Network PWM Controller Error Amplifier Bandgap Reference

  33. External Network An external network (consisting of an inductor, capacitor, and diode) transforms the energy from the PWM controlled power switch into a desired output voltage Switch Network VIN VOUT VIN = 12 V VOUT = 5 V

  34. Switching Power Supply Block Diagram Switching Power Supply VIN VOUT Switch Network Network PWM Controller Error Amplifier Bandgap Reference

  35. Step Down Switching RegulatorSteady State Operation VIN VGATE VGATEgoes high VM~ VIN VL = VM – VOUT t ISW VM VGATE IL t -VF VOUT ISW VM + VL - t RLOAD - VF + IL COUT t VOUT t

  36. Step Down Switching RegulatorSteady State Operation VIN VGATE VL Constant t ISW VM ILandISWincrease VGATE IL t -VF VOUT ISW VM + VL - t RLOAD - VF + IL COUT is charged byIL and VOUTincreases COUT t VOUT t

  37. Step Down Switching RegulatorSteady State Operation VIN VGATE VGATE= 0V The pass transistor is turned off ISW= 0A t ISW VM VMgoes negative VL =VM–VOUT VGATE IL t -VF VOUT ISW VM + VL - t RLOAD - VF + ILcannot go to 0A instantly: IL COUT t VOUT t

  38. Step Down Switching RegulatorSteady State Operation VIN But,VMis clamped to -VF and IL decays through the diode VGATE t ISW VM VGATE IL t -VF VOUT ISW VM = -VF + VL - t RLOAD - VF + COUT stabilizes the output voltage so VOUTwill only slowly decay IL COUT t VOUT t

  39. Step Down Switching RegulatorSteady State Operation VIN The MOSFET is turned on and off to repeat the sequence VGATE t ISW VM VGATE IL t -VF VOUT ISW VM = -VF + VL - t RLOAD - VF + IL COUT t VOUT t

  40. Volt-Second Principle VIN ISW VGATE VGATE IL VOUT VM t + VL - RLOAD IL COUT t

  41. Voltage-Second Principle VL VIN - VOUT DT T (1-D)T t -VOUT • In steady state, the inductor current ripples about an average, IL,AVG: • Therefore, the total area (or volt-seconds) under the inductor voltage waveform is zero.

  42. Voltage-Second Principleand the DC Transfer Function From: we can calculate the transfer function of the step down switching voltage regulator

  43. VIN vs. VOUTand Duty Cycle, D • During steady state: VL,AVG = 0V VIN ISW VL VIN - VOUT SIN IL DT VOUT L T time (1-D)T + VL - -VOUT RLOAD SGND COUT

  44. VOUT Increases with DVOUT = DVIN VIN VGATE ISW t VGATE VL SIN IL VIN - VOUT VOUT VM + VL - t -VOUT RLOAD SGND COUT VOUT t

  45. VOUT Decreases with DVOUT = DVIN VIN VGATE ISW t VGATE VL SIN IL VIN - VOUT VOUT VM + VL - t -VOUT RLOAD SGND COUT VOUT t

  46. Duty Cycle and Switch Loss VIN • In practice, voltage drop across the top switch (VSIN) and the bottom switch (VSGND): ISW + VSIN - SIN IL VOUT L + VL - RLOAD - VSGND + SGND COUT

  47. Duty Cycle and Switch Loss VL VIN - VSIN - VOUT DT T time (1-D)T -VSGND - VOUT

  48. Ripple Current VIN • Recall, IL is the sum of the current flowing through SIN and SGND ISW IL ISW IGND SIN IL VOUT IL,AVG IGND RLOAD COUT SGND time

  49. Inductor Ripple Current IL (and IOUT) has both DC and AC components The maximum value of the deviation from the DC value is given by (I/2)+IL,AVG IL (I/2)+IL,AVG IL,AVG IOUT time

  50. Selection of I The amount of allowed inductor ripple current, I, is one of the key decisions made in designing a power supply It is an important factor in correctly sizing the other components in the power supply Typical values of I are 30% to 50% of IL,AVG Small values of I might be desired, but can result in more complex and expensive power supplies

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