EECS 373

Power and batteries

Today

• Power issues– Circuits review– Power integrity

• Batteries– What is a battery? – What characteristics do we care about?– Define some terms.– Look in depth at a few battery types

Power issues

• There are a lot of electrical issues to deal with when working with high-speed PCBs.– Supplying power, storing energy and dissipating heat

• Power supplies, batteries, and heat sinks.

– Power Integrity (PI)• We need to be sure that we keep the power and ground at

approximately constant values.

– Signal Integrity (SI)• We need to make sure data on the wires gets there.

– Electro-magnetic interference/compatibility (EMI/EMC)• We need to watch out for generating radio-frequency noise

– The FCC is a bit picky about this.

• We don’t want RF noise to interfere with us.

EE issues overview

Take

473

Power!• Electric power is the rate at which electric energy is

transferred by an electric circuit. The SI unit of power is the Watt. (Wikipedia)

• Power (as opposed to energy), in-and-of itself is important in embedded system design.– For example there may be a cap on power draw from a given

set of batteries.• That is, they can’t supply energy at more than a given rate.

– Melting issues are power issues• Admittedly over time.

What do we mean by Power?

• Max Power: Artificial code generating max CPU activity• Worst-case App Trace: Practical applications worst-case• Thermal Power: Running average of worst-case app power

over a time period corresponding to thermal time constant• Average Power: Long-term average of typical apps (minutes)• Transient Power: Variability in power consumption for supply net

Understanding Power and Energy

Power vs. Energy

• Power consumption in Watts– Determines battery life in hours– Sets packaging limits

• Energy efficiency in Joules– Rate at which power is consumed over time– Energy = power * delay (Joules = Watts * seconds)– Lower energy number means less power to

perform a computation at same frequency

Understanding Power and Energy

Power vs. EnergyUnderstanding Power and Energy

Background issue #1: Inductance

• An inductor “resists the change in the flow of electrons”

• The light bulb is a resistor. The wire in the coil has much lower resistance (it's just wire)– so what you would expect when

you turn on the switch is for the bulb to glow very dimly.

• What happens instead is that when you close the switch, the bulb burns brightly and then gets dimmer. – And when you open the switch,

the bulb burns very brightly and then quickly goes out.

http://electronics.howstuffworks.com/inductor1.htm

EECS 215/Physics 240 “review”

Background issue #2: Capacitance

• A capacitor resists the change of voltage– When you first connect

the battery, bulb lights up and then dims

– If you then remove the battery and replace with a wire the bulb will light again and then go out.

http://electronics.howstuffworks.com/capacitor1.htm

EECS 215/Physics 240 “review”

Background issue #3: Impendence

• Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, in other words: how much the circuit impedes the flow of current. – It is like resistance, but it also takes into account the

effects of capacitance and inductance. I– Impedance is measured in ohms.– Impedance is more complex than resistance because

the effects of capacitance and inductance vary with the frequency of the current passing through the circuit and this means impedance varies with frequency!

• The effect of resistance is constant regardless of frequency.

http://www.kpsec.freeuk.com/imped.htm

EECS 215/Physics 240 “review”

1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+090.001

0.010

0.100

1.000

Pure induc-tor

Cap/resistor

Pure cap.

Frequency

Imp

edan

ce

A look at impedance(with capacitors, inductors and resistors vs. frequency)

Notice the log scales!

EECS 215/Physics 240 “review”

Today

• Power issues– Circuits review– Power integrity

• Batteries– What is a battery? – What characteristics do we care about?– Define some terms.– Look in depth at a few battery types

Power Integrity

• In order to get digital electronics to work correctly, they need a minimum voltage differential. – If we get below that, the devices might

• Be slow (and thus not meet setup times)• Lose state• Reset or halt• Just plain not work.

• Even a very (very) short “power droop” can cause the chip to die.– In my experience, this is a really common problem.

• Keeping power/ground constant and noise/droop free is “Power Integrity”

Power Integrity

So?

• We need the Vcc/Ground differential to be fairly constant.– But rapid changes in the

amount of current needed will cause the voltage to spike or droop due to inductance.

• We basically want a “no-pass” filter.– That is we don’t want to

see any signal on the Vcc/Ground lines.

– The obvious thing?• “Add a capacitor”

– That should keep the voltage constant, right?

• The problem is we need to worry about a lot of frequencies AND capacitors aren’t ideal.

Power Integrity

Lots of frequencies

• Even fairly slow devices these days are capable of switching at very high frequencies.– Basically we get drivers

that have rise and fall times capable of going 1GHz or so.

• This means we generally have to worry about frequencies from DC all the way to 1GHz.– Because our chip may be

varying its draw at rates up to that fast.

Power Integrity

Non-ideal devices.

• ESR is Effective Series Resistance• ESL is Effective Series Inductance• Ceff is the effective capacitance.

– How does quantity effect these values?

• Obviously impendence will be varying by frequency.

Power Integrity

Other things can add to ESR/ESL

• Generally a bad solder job can make ESR/ESL worse.

• Packaging has an impact– wires have inductance so surface-mount packages

preferred• Pads can have an impact

Power Integrity

Given the previous table..

1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+090.001

0.010

0.100

1.000

Decoupling Impedance vs Frequency

Z(pup)

Z(tant)

Z(1uF)

Z(0.1uF)

Z(0.01uF)

Z(pcb)

ZT

Z(LICA)

Frequency

Imp

edan

ce

Power Integrity

Removing the PCB…

1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+090.001

0.010

0.100

1.000

Decoupling Impedance vs Frequency

Z(pup)

Z(tant)

Z(1uF)

Z(0.1uF)

Z(0.01uF)

Z(pcb)

ZT

Z(LICA)

Frequency

Imp

edan

ce

Power Integrity

What is the PCB part?Power Integrity

But wait…• VRM

– Voltage regulator module

• bulk bypass (tantalum) and decoupling capacitors (ceramic).– These capacitors supply

instantaneous current (at different frequencies) to the drivers until the VRM can respond.

• However sets of different capacitors cause problems!

http://www.pcbdesign007.com/pages/columns.cgi?artcatid=0&clmid=65&artid=85396&pg=3&_pf_=1

Power Integrity

Power Integrity (PI) summary

• Power integrity is about keeping the Vcc/ground difference constant.– This is hard because the devices that sink power do so

in “pulses” due to their own clocks– Need caps to keep value constant

• But parasitic ESR/ESL cause problems• So lots of them==good

– Reduce ESR/ESL– Increase capacitance.

– But anti-resonance can cause problems!• Need Spice or other tools to model.

Power Integrity

Today

• Power issues– Circuits review– Power integrity

• Batteries– What is a battery? – What characteristics do we care about?– Define some terms.– Look in depth at a few battery types

Large parts of this section on batteries come from Alexander Cheng, Bob Bergen & Chris Burright

Background: What is a battery?

• Voltaic Cellso Two "half cells" connected in series by a conductive

electrolyte containing anions and cations.o One half cell contains the anode, which anions from the

electrolyte migrate to. The other the cathode, which cations migrate to.

• Redox Reaction o Anions at anode are oxidized

removes electronso Cations at cathode are reduced

adds electrons

• Creates an electrical current as electrons move. Image from wikipedia

3

What do we care about?

• When picking batteries there are a number of characteristics to be aware of including:– Voltage– Max current– Energy– Results of mechanical failure– Energy loss while idle

• You have a lot of options because– Many different battery types (Alkaline, LiPo, etc.) – Different topologies (ways to connect the cells together)

Lots of terms• Capacity

o The amount of electric charge it can store,

typically measured in mAh

• Charge Densityo Charge/Volume,

measured in mWh/cm^3 or mWh/kg

• Charge Limito The maximum voltage

the battery can produce under ideal conditions

• Primary Cellso Non-rechargeable

(disposable) batteries• Secondary Cells

o Rechargeable batteries

• Lifetimeo Primary Cells - "self

discharge", how long the battery lasts when not in use.

o Secondary Cells - recharge limits

• Cycle Lifeo The number of charge cycles

until battery can no longer reach 80% maximum charge

Let’s look at “capacity”

• Generally measured in mAh*, this tells us how much energy we can expect to get out of the device before it runs down.– The problem is, we get

less total energy the more quickly we drain the battery.

• Called “Peukert Effect”o Actual capacity is

dependent on the current draw.o The faster you draw

the current, the less you have total.

o Often irrelevant if just driving a microcontroller, but if have motors etc. it can be a big deal.

* While this unit isn’t really a measure of energy, it would be if voltage were fixed (which it more-or-less is). It is actually a measure of charge.

Peukert Effect

Image from http://www.vonwentzel.net/Battery/00.Glossary/

Alkaline Battery

• Primary Batteryo Disposable

• Most common "off the shelf" battery• Accounts for over 80% of manufactured batteries in the U.S.• Over 10 billion individual units produced worldwide

Image from Wikipedia

Lithium-Ion Polymer Battery

• Common abreviations:o Li-poly, Li-Pol, Li-Po, LIP, PLI or LiP

• Secondary cell batteries• Typically contain multiple cells in parallel

o Used to increase discharge current capacityo Can cause charging difficulties

Cells must be balanced for safe charging

Lithium-Ion Polymer - Chemistry

• Sony's original lithium-ion battery used coke for the anodeo Coke was a by-product of the coal industry

• Modern lithium-ions began using graphite for the anode in about 1997o Provides a flatter discharge curve

• Material combinations have been tested for the anodeo Tradeoffs are application dependent

Lead Acid Battery• Invented in 1859 by Gaston Plante• Oldest rechargeable battery type• Low energy to weight ratio• Low energy to volume ratio• Can supply high surge currents and

hence high power to weight ratio• The U.S. produces nearly 99 million

wet-cell lead-acid batteries each year

Lead Acid - TypesLead Acid Battery Constructs:• Flooded Cell (Wet Cell)• Valve Regulated Lead Acid (VRLA)

o Gelo Absorbed Glass Mat (AGM)

Lead Acid Battery Types:• Starting Battery• Deep Cycle Battery• Marine Cycle

Comparisons

Electrical Properties - Capacity• Alkaline

      

• Li-Po o Typically 1100-1500 mAh per cello Like most Li-Ion a single battery contains multiple cells

• Lead-Acido Varies by size and typeo Car batteries are usually 50 Ah

Electrical Properties - Current

• Alkaline o Dependent on the size of the batteryo Rule of thumb:

AA - 700mA max, 50mA typical• Li-Po

o Can drive large currents Batteries rated for 1000mAh at 100mA draw can

typically supply up to 1.5A, 15x their rated current This applies no matter the capacity or current draw

ratingso Connected in parallel to increase current rates

• Lead-Acido  Can produce up to 500 amps if shorted

Electrical Properties - Charge Density

• Alkalineo Much higher than other "off the shelf" battery typeso Common cells typically 110 Wh/kg

• Li-Poo 100-180 Wh/kg

• Lead-Acido 30-50 Wh/kg

Cost• Alkaline

o Very low cost to produce $0.19/Wh

o Most of the cost is placed on the consumer

• Li-Poo Varies with chemical composition

~$0.47/Who Cheaper than traditional Li-Ion

• Lead Acido $0.20/Wh

Relatively cheap for high voltage applications Expensive for a full battery

Hazards - Leaks

• Alkalineo Cells may rupture and leak potassium hydroxide

This will corrode the battery and the device May cause respiratory, eye, and skin irritation

• Li-Poo Unlikely to leak because of solid internals

• Lead Acido Cells may rupture or be punctured

Wet cells will leak strong sulfuric acid

Hazards - Explosions/Fires• Alkaline

o Unlikely to explode or catch fire• Li-Po

o May explode or catch fire if mishandled Charging/Discharging too quickly builds heat Charged damaged cells are prone to explosions/serious fire

(http://www.youtube.com/watch?v=QjkW3KUz5uo)• Lead Acid

o Electrolysis in flooded cells occurs when overcharge Produces hydrogen and oxygen gases which may explode if

ignitedo VRLA does not contain liquid electrolytes

lithium-ion fire (http://www.gazettetimes.com/news/local/article_803a17e6-afd8-11e0-bedd-001cc4c03286.html )

Hazards - Environmental Concerns

• Alkalineo Ends up in landfills after one useo Potassium hydroxide can corrode objects it touches

• Li-Poo No major recycling programs in place currentlyo Polymer requires strong chemicals and a lot of energy to

produce• Lead Acid

o Lead is a toxic metalo 97% of the lead is recycled

Alkaline Battery Review

• Proso Disposableo Cheap to produce, easy to obtaino Maintenance-free

• Conso Non-rechargeableo Moderate charge densityo Relatively low current drain limitso Must be justifiable to the user

• Applicationso Household and mobile electronicso Children's Toys o Must be low current to justify disposable costso Low up-front costs

Lithium-Ion Polymer - Review

• Pros:o High energy densityo Relatively low self-discharge o Low maintenance

No periodic discharge is needed No memory

• Cons:o Requires protection circuit to limit voltage and currento Subject to aging, even if not in useo Transportation regulations for shipping in large quantitieso Fire!

• Applications o Lightweight portable electronic devices

Cell phones, GPS, laptops, etc.o Radio controlled model planes/cars

Lead Acid - Review• Pros

o Relatively cheapo Long lifespano Able to provide extreme currents (500A+)

• Conso Heavyo Large physical sizeo Some models require periodic maintenance

• Applicationso Vehicle batterieso Energy storage

Off-the-grid systems Back up power supply Renewable energy systems

Solar, wind, etc.o Long term remote energy supply

Example Situations

• Battery powered flashlighto Must be compact and lightweighto Needs to be cheap up fronto Rarely usedo Battery needs to have a long shelf life

• MP3 Playero Must be compact and lightweighto Expensive product can incorporate a higher battery costo Must be rechargeableo Should recharge quickly o Needs to have large energy capacityo Must last 500+ recharge cycles without maintenance

Review power issues

• Review– Basic power issues– Power Integrity

Power review

Review: Basic power issues

• Electric power is the rate at which electric energy is transferred by an electric circuit.– We often look at average

power on different time scales depending on what we are wanting to know.

– Need to remember that lower power isn’t always the same as lower energy

• especially if the lower-power solution takes significantly longer

Power review

Review: Power integrity (1/2)• Processors and other ICs have

varying current demands– Sometimes at frequencies much

greater than the device itself runs at

• Why?

– So the power/ground inputs need to be able to deal with that.

• Basically we want those wires to be ideal and just supply how ever much or little current we need.

– If the current can’t be supplied correctly, we’ll get voltage droops.

• How much power noise can we accept?– Depends on the part (read the

spec). • If it can run from 3.5V to 5.5V we

just need to insure it stays in that range.

– So we need to make sure that given the current, we don’t end up out of the voltage range.

• Basically need to insure that we don’t drop too much voltage over the wires that are supplying the power!

Power review

Review: Power integrity (2/2)

* http://www.n4iqt.com/BillRiley/multi/esr-and-bypass-caps.pdf provides a very nice overview of the topic and how to address it.

• So we need the impedance of the wires to be low.

– Because the ICs operate at a wide variety of frequencies, we need to consider all of them.

– The wires themselves have a lot of inductance, so a lot of impedance at high frequencies.

• Need to counter this by adding capacitors.

• Problem is that the caps have parasitic inductance and resistance.

– So they don’t help as well as you’d like– But more in parallel is good.– Each cap will help with different frequency

ranges.

• We also can get a small but low-parasitic cap out of the power/ground plane.

• Finally we should consider anti-resonance*.

Power review

Review

• Why was 0.01 chosen as the target impedance?

• Answer: – If you can’t have more

than a .1V ripple and you are pulling 10 Amps you need your impedance to be below .01 Ohms

• (V=IR so R=V/I)

Power review