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Lecture 23
• Switch Mode Power Supplies
• Pulse Width Modulation
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What is PWM
Why PWM?
When to PWM?
CPU controlled PWM SMPS
• Design idea: Using the ATMEGA128 PWM
to make an efficient SMPS.
Simple DC-DC Converter Topologies
Buck Converter or
Step Down Converter
Boost Converter or
Step Up converter
Buck-Boost
Converter
Linear Voltage Regulator Topology
LM2937 Linear Low Drop Out Regulator (LDO) - will regulate
until Vout - Vin = 0.5V, at I Out = 500mA.
7805 regulator - Vout - Vin = 2V, at I Out = 1A .
Why Pulse Width Modulation?
• Doesn’t the design complexity of a SMPS
counter any benefits in power efficiency?
• What about all that electromagnetic noise?
• Don’t you need to follow very careful
circuit layout to achieve a good low noise
design?
• Do switch mode converters fail more than
linear converters?
Higher Efficiency
• A linear voltage regulator may typically
achieve an efficiency of 50% to 65%.
• A Switch-Mode Power Supply (SMPS)
regulator typically achieves an efficiency of
75% to 95%.
SMPS benefits
– Very wide input voltage range.
• For example: most personal computer power supplies
are SMPSs - accepting AC input 90V to 250V.
– Lower Quiescent Current than linear regulators
– Less heat than an equivalent linear regulator.
• Much Lower Green House Gas emissions
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Overall Smaller geometry components are used
Lighter Weight
Lower running cost - Lower total cost of ownership (TCO).
Battery operated devices - longer lifetime.
SMPS disadvantages
– Significant Output Ripple
• May need a post filter to decrease ripple
• May need a secondary linear low drop out regulator to
ensure damaging voltage transients keep away from
voltage sensitive elements - electronics.
• An SMPS May add too much cost.
– How much is too much?
SMPS Break even point determination
• For example if a small SMPS adds a cost of approx $15,
and mains Electricity is charged at 15c/KWH.
– $15 equates to 100KWH. If that energy is saved in 3 Years - a fair
payback point from most accountant’s perspective -
• Calculate the Average power for 100KWH over 3 years?
– 100KWH = the power saved improving efficiency from 50% to 95%
– Implies device requires 222KWH in 3 Years.
– How many hours in 3 Years? 26280 hours.
– Average Power? 222000WH / 26280hours = 8 Watts.
– So if your device consumes 8 Watts or more, 24 Hours a day,
seven days a week, 365 days per year, over three or more
years, an SMPS should be considered on economic grounds.
• Also consider what is the total product lifetime savings?
– Can that be a sales feature?
When to use a SMPS
– When your device is using an average of 8 watts or
more. If lower power, still consider the lifetime
savings. Plan PS design time into the budget.
– Where energy cost is forecast to significantly over
15 cents per KWH within a 3 year period an SMPS
should be a consideration.
– Where there is only battery, solar or wind powered
electricity supply (I.e. energy costs more than
15c/KWH)
– When your client is Green / environmentally
responsible / sensitive or too much time allocated.
When not to use a SMPS
• An SMPS May be totally un-neccessary – For power requirements - micro amps to pico amps - an ultra
low power linear regulator would be a better choice.
• In some battery powered systems, no regulation may be
acceptable if the working range of the semiconductors is
chosen carefully. (Eg 2xAA / CR2032 battery systems)
• Particularly cost sensitive clients may not appreciate
their time spent on something they see as costly - use
your judgement - the world has limited resources
• Initial Prototyping of RF, Analog or Microprocessor
systems. The switching noise could make it difficult or
impossible to debug the complete system.
• Project deadline has passed.
Why Pulse Width Modulation?
• What about all that electromagnetic noise
– Yes switching noise needs to be managed
– Using toroidal or shielded inductors may help
limit electromagnetic / inductive radiation.
– Using Low ESR capacitors limits the noise
radiated from the SMPS input and output wires.
– Careful Circuit layout can result in excellent
line and load regulation.
ATmega128 SMPS Design
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1. Determine Design Requirements:
SOURCE: VIn = 10V
LOAD: VOut = 5V at 1A (Maximum)
Output ripple < 200mV
Switching frequency approx 50kHz
• Standards bodies state - need to test harmonic
radiation to 60 x 50kHz = 3MHz.
• Efficiency target 85%
PWM SMPS using ATMEGA128
ATmega128 SMPS Design
• Tentatively choose:
• Timer 1
– Fast Pulse Width Modulation mode
– top = ICR1
– pwm level = OCR1A
• We can vary the system’s frequency of
operation by altering the value in ICR1
• We can also vary the PWM level by altering
the value in OCR1A : 0 <= OCR1A <= ICR1
PWM SMPS using ATMEGA128
ATmega128 SMPS Design
• Control pins:
– PWM_CTL = OC1A = Port B, Pin 5
• Measured Values:
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ADC0 = I Out x 0.02
ADC1 = V Out / 5
ADC2 = V In / 11
ADC3 = I In x 0.8
= PORTF Pin 0
= PORTF Pin 1
= PORTF Pin 2
= PORTF Pin 3
– (if R5 = 200K, and R2 =0.02 Ohm).
• Efficiency = P Out / P In
PWM SMPS using ATMEGA128
SMPS design using ATMEGA128
• Capacitor Theory : I = C.dv/dt
– => (I / C).dt = dv
• Design for 1A out. Switch induced
Capacitor Ripple = 18mV
• Extra ripple due to Capacitor’s Effective
Series Resistance (ESR)
SMPS design using ATMEGA128
• Inductor Theory : V = L.di/dt
• Design V Out = 5V at I Out = 1A.
• Vin = 10V; We can expect I In about 0.6A.
• From National Semiconductor AN1197
• Calculate L >= 100uH,
– Choose Pulse Engineering Toroidal Inductor PE92108K: 100uH, 3.6A (because we had them on hand).
SMPS design using ATMEGA128
• So the circuit works by initially switching
on transistor Q2 (via node PWM_CTL)
which in turn switches on Q1.
• Current flows through inductor L1, and
charges capacitor C2, and provides the load
on JP3 with current.
• Energy is stored in L1’s magnetic core as
current starts to rise. If the core saturates,
current plateaus, and no more energy is
stored.
SMPS design using ATMEGA128
• After a period, determined by ATMEGA128
counter 1 - ICR1, the voltage on
PWM_CTL, Q1 is switched off, Charge is
bled off Q2’s Gate & it also switches off.
• When Q1 & Q2 are switched off current
continues to flow through L1, and through
the schottky diode D1, until the energy from
L1 is returned to the output load and/or
Capacitor C2.
A PWM SMPS using ATMEGA128
• We need to frequently measure the output
voltage, compare that with the desired output
voltage, and adjust the PWM level to suit.
• How could we improve the feedback
responsiveness of the circuit?
• How can we guarantee stability?
• Load Regulation?
• Line Regulation?
More SMPS Resources
• TI, National Semiconductor, On-Semi, Linear Tech
• National Semiconductor Application Notes:
– AN 1149 - Layout Guidelines for Switching Power
Supplies
– AN 1197 - Selecting Inductors for Buck
Regulators
• Reference Design Software
– Power Supply design, layout & simulation tools
• http://www.national.com/appinfo/power/webench.html
• Switchers Made Simple, Web Sim, Web Therm
Acknowledgements
– These notes were produced by Paul Main,
• www.sea.vg Oct 2007
• Simple DC-DC converter technologies illustration– The IEEE Electrical Engineering Handbook,
Richard Dorf et al,
• Fig 46 - Timer1/Counter1 Block Diagram
– Atmel ATMEGA128 full data
• Atmel website www.atmel.com and
• http://www.sea.vg/mic/2007/Atmel/Atmega128ManualDoc2467.pdf