power supply and thermal issues
TRANSCRIPT
8/3/2019 Power Supply and Thermal Issues
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F. Z. Peng: Slide 1Feb. 15, 2006
How to Select and Use Power Supplies anddc/dc Converters for Your Applications
Fang Z. PengDept. of Electrical and Computer Engineering
Michigan State University
Phone: 517-336-4687, Fax: 517-353-1980
Email: [email protected]
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F. Z. Peng: Slide 2Feb. 15, 2006
Contents
Introduction to Power Supplies and dc/dc
Converters
Types & Technologies of Power Suppliesand dc/dc Converters
Circuit Selection and Design
Circuit Performance and Protection Features
Thermal Requirements and Design Issues
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F. Z. Peng: Slide 3Feb. 15, 2006
Introduction to Power Supplies and dc/dcConverters
Available/Raw Power Sources ± AC or DC (frequency)
± Un-regulated (changes with load, prime source, etc.)
±
Voltage (different level, polarity, isolation) ± Non-protected (against over load, fault, temp., etc.)
Load Demand ± Different AC or DC (frequency)
±Regulated (against load, prime source, etc.)
± Voltage (different level, polarity, isolation)
± Protected (against over load, fault, temp., etc.)
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F. Z. Peng: Slide 4Feb. 15, 2006
Power Supply
Introduction to Power Supplies and dc/dcConverters ±cont.
Power & Electronic
Circuits
Raw power inDesired power out
(V, I, P, F)
To loads:
Electronic cktsMotor
Computer
Equipment
Control
BatteryFuel Cell
AC Outlet
Solar
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F. Z. Peng: Slide 6Feb. 15, 2006
AC-DC Power Supplies-Circuit Selection and Design
Using Linear Regulators
Using LDO Regulator
http://www.national.com/pf/LM/LM78M05.html
Step-down Xfmer
Regulator 120 V AC
For low power (several watts or below) applications. Low efficiency, large size and weight (bulky step-down line
transformer)
Low cost
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F. Z. Peng: Slide 7Feb. 15, 2006
AC-DC Power Supplies-Circuit Selection and Design
Using Switching-Mode
High efficiency
Small size and light weight
For high power (density) applications
http://www.linear.com/index.jsp
http://www.linear.com/3770
http://www.electronicproducts.com/
TI Power Supply Technologies Poster
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F. Z. Peng: Slide 8Feb. 15, 2006
Selecting the Right dc/dc Converter
The Need for dc/dc Converters ± E.g., a single AA alkaline battery produces 1.5 V when fully charged and its
voltage drops to as low as 0.9 V when becoming depleted.
Dc/dc Converter Types
± Buck
± Boost
± Buck-Boost
Dc/dc Converter Technologies
± Linear Regulators
±Switching Regulators
± Charge Pumps
The MCP170 3 LDO is one type of dc/dc linear regulator
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F. Z. Peng: Slide 9Feb. 15, 2006
Selecting the Right dc/dc Converter ±cont.
Dc/dc converter technology comparison
Parameter Linear regulator
Switchingregulator
Charge pump
Efficiency Low High Medium
EMI Noise Low High Medium
Output current Low to medium Low to High Low
Boost (step-up) No Yes Yes
Buck (step-down)
Yes Yes Yes
Solution size small Large Medium
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F. Z. Peng: Slide 10Feb. 15, 2006
Selecting the Right dc/dc Converter ±cont.
VBAT = 3.7 V nom, BIN_BB = 1.2 V
Load Current = 600 m APower delivered to load = 600 m A * 1.2 V = 720 mW Power converted to heat =
720 mW * ((3.7/1.2) 1) = 1,500 mW T otal power consumed =
720 mW + 1,500 mW = 2,200 mW
32% goes to work, 68% goes to heating user hand and ear w hen using a Linear Regulator for amobile device
VBAT = 3.7 V nom; BIN_BB = 1.2 V Load Current = 600 m AConverter efficiency = 90%
Power delivered to load = 600 m A * 1.2 V = 720 mW Power converted to heat =720 mW * ((1/0.9) 1)=80 mW T otal power consumed = 720 mW + 80 mW = 800 mW
90% goes to work, 10% goes to heating user hand and ear W hen using a Switc h- mode regulator for a mobile device.
Linear regulators:Inexpensivesmall footprint low part count low noise
high ripple rejection
Switching regulators:a bigger footprint higher part count,more cost prone to conducted and radiated EMI.
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F. Z. Peng: Slide 11Feb. 15, 2006
Specs, Performance and Protection
Voltage ripple (+-50 mV, or 5%)
Isolation (e.g., 1,500 V ac for 1 min.)
Load regulation (e.g., 3%)
Dynamic response (transients, wake-up time, etc.)
Short circuit protection
OC protection
OV protection
OT protection
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F. Z. Peng: Slide 12Feb. 15, 2006
Power Losses and Thermal Design
For example, a 7815 linear regulator with input voltage of 20 Vand output current of 1 A. The power loss is (20-15)Vx(1 A)=5 W.
From the chip to the ambient, (Ti can be calculated according tothe thermal circuit using Ohm¶s law (R=V/I), where R is thethermal resistance, V is the temperature and I is the power dissipation.
case ambient Thcase ambient
dissipation
out dissipation in out out
op
T T R
P
P P P P P
L
!
! !
W here:T case is case T emp.T ambient is ambient T emp.P
dissipationis power loss
P in is input power P out is output power Lop is efficiency under
given operating conditions
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F. Z. Peng: Slide 13Feb. 15, 2006
Power Losses and Thermal Design--A more detailed thermal circuit
W : Device power loss
Tj : Junction temperature of device Tc : Device case temperature
Tf : Temperature of heatsink Ta : Ambient temperature
Rth(j-c) : Thermal resistance between junction andcase, specified in datasheet
Rth(c-f) : Contact thermal resistance between caseand heatsink, specified in datasheet
Rth(f-a) : Thermal resistance between heatsink andambient air, specified by the heatsink manufacturer
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F. Z. Peng: Slide 14Feb. 15, 2006
Power Losses and Thermal Design
Tc=W×{Rth(c-f) + Rth(f-a)}+Ta
Tj=W×Rth(j-c)+Tc
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