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DC/DC Converters 101

DC/DC Converters 101. Understanding Power Supply Basics and Terminology. Agenda. Lecture Overview Linear Regulators Switching Power Supplies Topologies Synchronous vs. Non-synchronous Controller vs. Converter Selecting the Best Power Solution. Why should I care about power?.

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DC/DC Converters 101

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  1. DC/DC Converters 101 Understanding Power Supply Basics and Terminology

  2. Agenda • Lecture • Overview • Linear Regulators • Switching Power Supplies • Topologies • Synchronous vs. Non-synchronous • Controller vs. Converter • Selecting the Best Power Solution

  3. Why should I care about power? 1. Every electronic system uses power. 2. Your power source never matches your system needs. Power Source DC/DC Supply gets you from here to there What you need Typically 5V,12V or 24V 6.0Vdc-16Vdc 40Vdc Surge 1.2V Core @ 2A 2.5V I/O @ 1.2A 3.3V 5V +/-12V 3.0Vdc-4.2Vdc

  4. Linear Regulators vs. Switching Supplies • Linear Regulator • Pass element operates in the linear region • Down conversion only • Switching Power Supply • Pass elements switch, turning fully on/off each cycle • Filtering includes an inductor • Multiple topologies (Buck, Boost, Buck-boost…)

  5. Linear Regulator ADVANTAGES: • Low O/P ripple & noise • Fast transient response • Low cost (for low power, at least) • Easy to design • No EMI to worry about DISADVANTAGES: • Low efficiency at VIN>>VOUT • High dissipation (needs large heat-sink) • VOUT<VIN – always! APPLICATIONS: • Extremely low ripple & noise apps • Low input to output voltage difference • Tight regulation • Fast transient response

  6. Dropout Voltage Example: • Vin = 3.1V to 4.2V • Vout = 2.5V @ 100mA • Need at least 600mV headroom • Dropout (headroom): The minimum required voltage across an LDO to maintain regulation + Vdo -

  7. Linear Regulator vs LDO • Linear Regulator has Higher Dropout Voltage. • Transistor or Darlington pair pass element • LM317 (1.5A linear regulator) • 1.5V to 2.5V dropout voltage • Good for larger Vin to Vout ratios, 12V to 5V output • CHEAP!!! • LDO = Low Dropout Regulator • Typically higher performance • PSRR, regulation tolerance, transient response, etc • MOSFET pass element • TPS72501 (1A LDO) • 170mV dropout voltage • Good for 3.3V to 3.0V output

  8. Linear Regulator Power Dissipation Input Current = Output Current Power Loss = Iout * (Vin – Vout) • Power loss is usually a limiting factor!

  9. Linear Regulator vs Switcher 2.5W LDO + ground plane as heat sink 6W Switcher

  10. VOUT VIN Switcher ADVANTAGES: • High efficiency • VOUT>=<VIN • Wide input voltage range • Low power dissipation (small heatsink) • High Watt/cm2 • Isolation possible (with transformer) • Multiple O/Ps possible (with transformer) DISADVANTAGES: • EMI • Slower transient response • More difficult to design • Higher output ripple & noise APPLICATIONS: • High efficiency power supplies • High ambient temperatures • Large input to output voltage difference • Space constraints • High output power DC DC

  11. VOUT VIN VIN VOUT VIN VOUT Basic Topologies Buck Boost Buck/Boost

  12. Synchronous vs Non Sync Non-Synchronous Buck • Non-synchronous • Diode voltage drop is fairly constant with output current • Less efficient • Less expensive • Used with higher output voltages • Synchronous • MOSFET has lower voltage drop • More efficient • Requires additional control circuitry • Costs more Synchronous Buck

  13. Synchronous vs Non Sync Vin=5V Vout=1V Rdson_sync=0.12ohm Vf_diode=0.5V Iout=1A 1V Output Synchronous 1V Output Non-Synchronous Sync vs Non-sync is less of an issue with higher Vout Higher duty cycles = less power dissipation in Sync FET or Catch Diode

  14. Synchronous vs Non Sync Power FET Synchronous FET

  15. Synchronous vs Non Sync Integrated Power FETs Rectifier Diodes Integrated Power FET and synchronous FET

  16. Controller vs Converter • Controller • Discrete MOSFETs • Provides the “brains” to control the power stage • More complicated to design • Full control over FET selection, switching frequency, overcurrent, compensation, softstart • Can tailor the power supply to meet your specific needs • Converter (Fully integrated) • Integrated switches • “plug and play” design • Limited range of output filter components • Limited control over functionality • Converter (Partially integrated) • May offer full or partial feature set , internal or external compensation • Internal Power FET, external sync-FET or catch diode • Limited control over frequency, overcurrent, softstart, etc • Allows wider range of output filter components

  17. Converter (Fully Integrated) TPS62293 2.3V to 6V input 1A Output Current 2.25MHz Everything is integrated, minimum external components

  18. Converter (Partially Integrated) TPS54620 4.5V to 17V input 6A Output Current Internal FETs, external SoftStart, Compensation, Frequency set… more flexibility Set frequency Compensation

  19. Controller TPS40303/4/5 3V to 20V input 10A Output Current 300kHz to 1.2MHz External FETs Compensation Softstart Current limit

  20. Size vs. Cost vs. Efficiency Cost Efficiency Synchronous Non-synchronous Linear Regulator Power Density Cost Converter (Fully Integrated) Converter (Partially Integrated) Controller

  21. Efficiency vs Vout • Efficiency depends on output voltage? • Why isn’t MY supply 95% efficient? The datasheet says:

  22. Efficiency vs Vout Simplified power dissipation equations assuming no inductor current ripple 3.3V Output 1V Output Power FET Conduction Losses Sync FET Conduction Losses Total FET Losses (does not include other circuit losses) 0.136 W 0.173 W

  23. Efficiency vs Vout 3.3V Output 1V Output

  24. PWM vs PFM • Pulse Width Modulation • Constant frequency • Low output voltage ripple • Used with high output currents • Pulse Frequency Modulation • Varying frequency with Vin and load • Very high efficiency at very light loads • Higher output voltage ripple • Potential operation in audio range

  25. PWM vs PFM PFM mode PWM mode

  26. Startup - Softstart • Slowly turning on the power supply • Controlled rise of output voltage • Minimizes inrush currents • Minimizes system level voltage drops • Pulling high currents out of input bus • High impedance batteries • Internal vs SS capacitor • Larger SS capacitor = longer softstart time

  27. Startup - Sequencing • Sequencing • Controlling the order that different power supplies are turned on • Important for uP loads • Minimizing overall inrush current Sequential sequencing

  28. Startup - Sequencing • Ratiometric Sequencing • Simultaneous Sequencing

  29. Easy Answers – Power Quick Search • Provides a list of possible linear regulators, controllers and converters based on inputs • Great starting point for selecting a device

  30. Easy Answers – Power Quick Search

  31. More Answers – Browse The Product Tree

  32. Easy (Simulated) Answers – WEBench • Provides a complete design based on inputs • Best for customers with little or no power background

  33. Easy (Real) Answers – TI Designs/PowerLAB • Searches reference designs based on input

  34. THANKS!! Questions??? ufseniordesignanaloghelp@list.ti.com

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