Brian King

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?

3

1. Every electronic system uses power.

2. Your power source never matches your system needs.

3.0Vdc-4.2Vdc

Power Source

What you need

1.2V Core @ 2A2.5V I/O @ 1.2A3.3V5V+/-12V

DC/DC Supplygets you fromhere to there6.0Vdc-16Vdc

40Vdc Surge

Typically5V,12V or 24V

Linear Regulators vs. Switching Supplies

4

• 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…)

Pass Element(s)

Filtering FilteringINPUT OUTPUT

Linear Regulator

5

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

Dropout Voltage

6

• Dropout (headroom): The minimum required voltage across an LDO to maintain regulation

Example:– Vin = 3.1V to 4.2V– Vout = 2.5V @ 100mA– Need at least 600mV headroom

+ Vdo -

Linear Regulator vs LDO

7

• 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

Input Current = Output Current

Power Loss = Iout * (Vin – Vout)

• Power loss is usually a limiting factor!

Linear Regulator Power Dissipation

8

in

out

inin

outout

in

out

V

V

IV

IV

P

PEfficiency

Linear Regulator vs Switcher

9

2.5W LDO + ground plane as heat sink

6W Switcher

Switcher

10

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

VIN

VOUT

DC DC

Basic Topologies

11

INOUT

INOUT

VDV

,VV

D1

VDV

,V,V

INOUT

INOUT

Buck

VINVOUT

VIN VOUT

D1

VV

,VV

INOUT

INOUT

Boost

Buck/Boost

VIN VOUT

Synchronous vs Non Sync

12

Non-Synchronous Buck

Synchronous Buck

Non-synchronous1. Diode voltage drop is fairly

constant with output current2. Less efficient3. Less expensive4. Used with higher output

voltages

Synchronous5. MOSFET has lower voltage

drop6. More efficient7. Requires additional control

circuitry8. Costs more

Q1L

C0D1

Q1L

C0Q2

Synchronous vs Non Sync

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1V Output Non-Synchronous1V Output Synchronous

Vin=5VVout=1V Rdson_sync=0.12ohm Vf_diode=0.5VIout=1A

PFET_SYNC Iout 1 D 2 Rdson

PFET_SYNC 1A 0.8 2 0.12

PFET_SYNC 0.096W

88%

Pdiode Idiode_avg Vdiode

Pdiode 1 D( ) Iout 0.5 V

Pdiode 0.4W

69.4%

Sync vs Non-sync is less of an issue with higher VoutHigher duty cycles = less power dissipation in Sync FET or Catch Diode

Synchronous vs Non Sync

14

Power FET

Synchronous FET

Synchronous vs Non Sync

15

Rectifier Diodes

Integrated Power FETs

Integrated Power FETand synchronous FET

Controller vs Converter

16

• 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

Converter (Fully Integrated)

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TPS622932.3V to 6V input1A Output Current2.25MHz

Everything is integrated, minimum external components

Converter (Partially Integrated)

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TPS546204.5V to 17V input6A Output Current

Internal FETs, external SoftStart, Compensation, Frequency set… more flexibility

Set frequency

Compensation

Controller

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TPS40303/4/53V to 20V input10A Output Current300kHz to 1.2MHz

External FETs

Current limitSoftstart

Compensation

Size vs. Cost vs. Efficiency

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Cost

SynchronousNon-synchronousLinear Regulator

Efficiency

Cost

Converter (Fully Integrated)Converter (Partially Integrated)Controller

PowerDensity

Efficiency vs Vout

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• Efficiency depends on output voltage?

• Why isn’t MY supply 95% efficient?

The datasheet says:

Efficiency vs Vout

22

3.3V Output 1V Output

Power FET Conduction Losses

Sync FET Conduction Losses

Total FET Losses(does not include other circuit losses)

0.173 W 0.136 W

Simplified power dissipation equations assuming no inductor current ripple

Efficiency vs Vout

23

3.3V Output 1V Output

TPS62400 Efficiency vs Vout (Vin=5V,Iout=300mA)

89.5

90

90.5

91

91.5

92

92.5

93

93.5

94

94.5

1.5 2 2.5 3 3.5

Vout (V)

Eff

icie

nc

y (

%)

PWM vs PFM

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• 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

PWM vs PFM

25

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000

VOUT2 = 1.8 V

VIN = 2.7 V

VIN = 2.7 VVIN = 5.0 V

VIN = 5.0 V

VIN = 3.6 V

VIN = 3.6 VPower-Save Mode (PSM)

Forced PW M Mode

Load Current, I (m A)OUT

Eff

icie

ncy

(%

)

PFM mode

PWM mode

Startup - Softstart

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– 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

Startup - Sequencing

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• Sequencing– Controlling the order that different power supplies

are turned on– Important for uP loads– Minimizing overall inrush current

Sequential sequencing

Startup - Sequencing

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• Ratiometric Sequencing

• Simultaneous Sequencing

Easy Answers – Power Quick Search

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• Provides a list of possible linear regulators, controllers and converters based on inputs

• Great starting point for selecting a device

Easy Answers – Power Quick Search

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More Answers – Browse The Product Tree

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Easy (Simulated) Answers – WEBench

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• Provides a complete design based on inputs• Best for customers with little or no power background

Easy (Real) Answers – TI Designs/PowerLAB

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• Searches reference designs based on input

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THANKS!!

Questions???

ufseniordesignanaloghelp@list.ti.com