DCDC Converters 101 Understanding Power Supply Basics and

DC/DC Converters 101 Understanding Power Supply Basics and Terminology Brian King

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? 1. Every electronic system uses power. 2. Your power source never matches your system needs. Power Source What you need Typically 5 V, 12 V or 24 V 6. 0 Vdc-16 Vdc 40 Vdc Surge 3. 0 Vdc-4. 2 Vdc DC/DC Supply gets you from here to there 1. 2 V Core @ 2 A 2. 5 V I/O @ 1. 2 A 3. 3 V 5 V +/-12 V 3

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…) 4

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 5

Dropout Voltage • Dropout (headroom): The minimum required voltage across an LDO to maintain regulation + Vdo - Example: – Vin = 3. 1 V to 4. 2 V – Vout = 2. 5 V @ 100 m. A – Need at least 600 m. V headroom 6

Linear Regulator vs LDO • Linear Regulator has Higher Dropout Voltage. – Transistor or Darlington pair pass element – LM 317 (1. 5 A linear regulator) • 1. 5 V to 2. 5 V dropout voltage • Good for larger Vin to Vout ratios, 12 V to 5 V output • CHEAP!!! • LDO = Low Dropout Regulator – Typically higher performance • PSRR, regulation tolerance, transient response, etc – MOSFET pass element – TPS 72501 (1 A LDO) • 170 m. V dropout voltage • Good for 3. 3 V to 3. 0 V output 7

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

Linear Regulator vs Switcher 2. 5 W LDO + ground plane as heat sink 6 W Switcher 9

Switcher ADVANTAGES: DC VIN DC VOUT § § § § High efficiency VOUT>=<VIN Wide input voltage range Low power dissipation (small heatsink) High Watt/cm 2 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 10

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

Synchronous vs Non Sync Non-Synchronous Buck Non-synchronous 1. Diode voltage drop is fairly constant with output current 2. Less efficient 3. Less expensive 4. Used with higher output voltages Synchronous Buck Synchronous 1. MOSFET has lower voltage drop 2. More efficient 3. Requires additional control circuitry 4. Costs more 12

Synchronous vs Non Sync Vin=5 V Vout=1 V Iout=1 A Rdson_sync=0. 12 ohm 1 V Output Synchronous Vf_diode=0. 5 V 1 V 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 13

Synchronous vs Non Sync Power FET Synchronous FET 14

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

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 16

Converter (Fully Integrated) TPS 62293 2. 3 V to 6 V input 1 A Output Current 2. 25 MHz Everything is integrated, minimum external components 17

Converter (Partially Integrated) TPS 54620 4. 5 V to 17 V input 6 A Output Current Internal FETs, external Soft. Start, Compensation, Frequency set… more flexibility Set frequency Compensation 18

Controller TPS 40303/4/5 3 V to 20 V input 10 A Output Current 300 k. Hz to 1. 2 MHz External FETs Compensation Softstart Current limit 19

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

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

Efficiency vs Vout Simplified power dissipation equations assuming no inductor current ripple 3. 3 V Output 1 V Output Power FET Conduction Losses Sync FET Conduction Losses Total FET Losses 0. 173 W 0. 136 W (does not include other circuit losses) 22

Efficiency vs Vout 3. 3 V Output 1 V Output 23

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 24

PWM vs PFM mode PWM mode 25

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 26

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

Startup - Sequencing • Ratiometric Sequencing • Simultaneous Sequencing 28

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 29