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EECS 473Advanced Embedded Systems
Lecture 11ish:
Power review & Switching power supplies
A number of slides taken from UT-Austinβs EE462L power electronics class.http://users.ece.utexas.edu/~kwasinski/EE462LS14.html -- very useful!
Random stuffs
β’ Projectβ Has everyone signed up for the Expo?
β’ Ford is the sponsor
β’ Office hours shift tomorrow (and break)β Friday 9-10:30am (instead of 10:30-12)β Will hold hours on Tuesday from 3-4:30
β’ different time than normal, usually 470 only this is 473 only
β Todayβs office hours are going to get slammed by 470 students (they have an exam today)
β’ HW1 is due on Wednesday @10pm
What are DC converters?
β’ DC converters convert one DC voltage level to another.
β Very commonly on PCBsβ’ Often have USB or battery power
β’ But might need 1.8V, 3.3V, 5V, 12V and -12V all on the same board.
β On-PCB converters allow us to do that
Images from http://itpedia.nyu.edu/wiki/File:V_reg_7805.jpg, http://www.electronics-lab.com/blog/wp-content/uploads/2007/10/p1000255.JPG
Different types of DC converters
Linear converters Switching converters
β’ Simpler to design
β’ Low-noise output for noise-sensitive applications
β’ Can only drop voltage
β And in fact must drop it by some minimum amount
β The larger the voltage drop the less power efficient the converter is
β’ Can be significantly more complex to designβ Worth avoiding for this class
unless you have to do it.
β’ Can drop voltage or increase voltageβ βbuckβ and βboostβ
respectively
β’ Generally very power efficientβ 75% to 98% is normal
Characteristics of DC Convertersβ’ To better understand how to pick a converter we will go over the
following characteristics seen in all DC converters
β Power wasted (as heat)
β Quiescent current, πΌπβ’ The leakage current that occurs regardless of operation.
β Power supply rejection ratio (PSRR)β’ The ability to reject output noise at different frequency
β External capacitors and equivalent series resistance(ESR)
β’ Output noise filter that helps keeping the signal clean
β’ These characteristics are what people generally look for when selecting converters, but theyβre not by any means the only characteristics that matter.
1. Power Wasted (as Heat)β’ Linear converters waste power = (Vinβ Vout)*Iload
β Exampleβ’ 12 V battery supplying 5V to each device
β Microcontroller that draws 5mA β Ultrasonic rangefinder that draws 50mA
β’ Use LM7805 (linear regulator) to drop 12V to 5Vβ’ Power wasted = (12V β 5V) * (0.050A + 0.005A) = 0.385W
β Which is actually more than the power consumed!β Is this acceptable?
Β» Hope so, because the alternative (switching converter) is a lot more difficult.
β’ Switchers generally waste a more-or-less fixed percentβ Say 15% or so, but as little as 3% is reasonable.
http://www.dimensionengineering.com/info/switching-regulators is the source for this example. They go into more detail on their site.
β’ In generalβ¦ β All have quiescent current
(πΌπ), which is different in each IC
β’ πΌπ is affected by the input and temperature the device is operating at.
β’ Will drain battery so choose carefully when picking converters!
β’ For this device, IQ is huge.β Designed to move 1A.
LM7805 π°πΈ during operation
2. Quiescent current, πΌπ
Diagrams from http://www.fairchildsemi.com/ds/LM/LM7805.pdf
3. Caps and ESR.
β’ Noise
β Capacitors!β’ Each converter requires at least a πΆππ’π‘ and sometimes a πΆππ to
reduce noise in the system
β Will be specified in datasheets
β Capacitors size generally needed from smallest to largest:
Β» General linear converters -> LDOs -> switching converters
min 22ππ min 22ππ
LDO LM2940Linear LM7805Diagrams from http://www.fairchildsemi.com/ds/LM/LM7805.pdf and http://www.ti.com/lit/ds/symlink/lm2940-n.pdf
β’ Capacitors arenβt the only thing that will determine stabilityβ Sometime an operation demands higher
πΌπ and leaves the safe-operating-area (SOA) causing instability as well
β’ So in addition to the capacitors, equivalent series resistance (ESR) comes into play
β’ Like everything else, the capacitorand its ESR will be specified in
each ICβs datasheet
β’ Thereβs also more than just ESR that affects the stability as well forvaried πΌπ and is discussed more inthe link below
http://www.bcae1.com/switchingpowersupplydesign/datasheets/ldoregulatorstabilityinfoslva115.pdf
3. Caps and ESR.
3. Caps and ESR
β’ So letβs take a look at an example of stability/instability with a changing πΌπ
β Note the amount of noise in the top waveform (ππ) as πΌπchanges with the presence of ESR
Load transient with π. πππ ceramic capacitor and ππ ESRLoad transient with π. πππ ceramic capacitor
4. Input Noiseβ’ PSRR indicates how well the supply deals
with input noise.β Recall we rely on the VRM (voltage
regulation module) to keep noise down atlow frequencies.
β We donβt want noise on the outputβ You can determine how well a
linear converter handle noiseby its PSRR
β’ PSRR is used to describe the amount of noise rejectedby a particular device
β What does PSRR mean for noise rejection?
β’ Take 40dB @100kHz and 1V input, so 1π β 10β40
20 = .01 = 10ππβ’ Meaning for every 1V there may be 10ππ superimposed on the output
β’ 70dB @ 10KHz is 1π β 10β70
20 ~ .00032 = 0.32 ππ, so 3% of the noise at 100KHz!
β PSSR performance is crucial for noise sensitive operation if youβve got a noisy source
β’ Commonly switching supply followed by an LDO.
Typical PSRR profile for an LDO, 40dB @ 100kHz
Graph from digikey http://www.digikey.com/us/en/techzone/power/resources/articles/hybrid-power-supplies-noise-free-voltages.html
Quick look at the options
β’ Linear converter
β LDO
β’ Switching converter
β Buck
β Boost
β Buck-Boost
Linear converter
β’ One can think of a linear converter as a βsmartβ voltage divider.β If we were using a very small
amount of current, that would work.
β But hugely wasteful.
β’ Instead, we want the topresistor to vary with the load.β As load draws more current,
R1 drops resistance to keepvoltage constant.
Figures on this slide and the next taken from http://cds.linear.com/docs/en/application-note/AN140fa.pdf, which is a great app-note.
Linear Convertersβ’ Soβ¦
In general linear converters:
β Act like a variable resistor
β Drop voltage by heat dissipation through the network of resistors
β Often have a fairly high minimum voltage drop.
β’ If you want to drop less, need a specific type of linear converters
β βlow-drop outβ or LDO
LM7805 Linear Voltage Regulator Schematic
All this fits in the IC!
Diagrams from http://www.fairchildsemi.com/ds/LM/LM7805.pdf
β’ What are low-dropout regulators(LDO)?
β LDOs are more complex linear regulators, using a transistor and error amplifier for negative feedback
β Larger capacitor is now neededβ’ Inherently, the capacitors will have equivalent series resistance that will also
contribute to noise reduction.
β Also implemented as ICs like the other linear regulators
LP5900
Generic LDO schematic
Linear Converters - LDO
Voltage converters/regulators:Review Linear
β’ Drops voltageβ Heat waste > (Vin-Vout)/Vin
β’ Thatβs a lot if we are dropping much voltage
β Has minimum dropβ Uses current even if no load (quiescent current)β Often needs an output (and maybe input) cap.
β’ LDO β low dropout is main variation. β Minimum drop is often very small (though can vary by
current drawβ¦)β Generally needs larger caps. β Can have larger quiescent current
β’ Depends on if βresistorβ is BJT or FET, FET more common these days.
Stability
β’ The opamp adjusts the effective resistance of the transistor to control the voltage.β Clearly weβve got a control
loop and there is delay in that loop.
β’ That means that if the load (or sourceβ¦) is varying at a certain frequencies, we could get positive feedback.
β’ The output (and sometimes input) cap can filter out those frequencies.
An MOSFET LDO.
http://www.ti.com/lit/an/snva020b/snva020b.pdf provides a very nice, detailed overview with Bode plots etc., though it is a bit dated.
Is stability a real issue for linear regulators?
β’ Well yes. β But these days, the requirements are pretty lax.
β’ The LT3007 wants a 2.2ΞΌF ceramic cap at the output and maybe a ~1 ΞΌF input cap.
β Needs moderately low ESR. β Data sheet expresses concerns about cheap ceramic caps.
β Older linear regulators had a lot more restrictionsβ’ Often required some additional ESR, but not too much.
β’ Older and cheaper devices might have some pretty significant (and sometimes costly) needs.β Just be aware that in the early 2000s this was a pretty
big issue.
http://cds.linear.com/docs/en/datasheet/3007f.pdf
Switching Converters
β’ Once you leave the realms of linear converters it gets more complex.
β Introducing common switching converters!β’ All include a diode, transistor, inductor and a capacitor
Table from http://www.nxp.com/documents/application_note/APPCHP2.pdf
Converters General Topology Application
Buck Drop voltage
Boost Increase voltage
Buck-boost(inverting)Increase or decrease voltage and inverse
polarity
β’ Common switching converters
β Converters now include a transistor and diode used forswitching and an inductor as energy storage.
β’ In general, a switching converters works by controlling the frequencyand duty cycle that thetransistor is operating at
β Similar to linear converters, most of the work is already done.
β Only have to pick IC with the right parameter and follow the datasheets given for appropriate inductors and capacitors.
Functionality β Switching Converters
Size of inductor in relation to the rest of the component
Example switching regulator from http://ycprojects.wordpress.com/
β’ Common switching convertersβ Aside from the noble goal of making circuit analysis
more complex () both the inductor and capacitor play important roles in switching converters.
β’ Capacitorβ Used to store energy due to the voltage applied thus maintaining
a constant voltage β Generally selected to limit ππ ripple to the correct specification
β’ Inductorsβ Similar to the capacitor, but an inductor is used to store energy
due to current flow. This in turn maintains a constant current or is used to limit the rate of change of current flow.
β This will also determine the peak to peak current in the circuit which affects the transistor, diode and the βmodeβ the converter will operate at.
Functionality β Switching Converters
β’ Common switching converters
β Letβs start with the buck converter
β’ Note the drive circuit, this is what the IC is
β’ For simplicityβs sake letβs disregard π πΏ and π πΆ , the internal resistance of the inductor and capacitor
β’ It looks pretty complex, so letβs try to understand why we need each component!
Functionality β Buck
25
Switching β lossless conversion of 39V to average 13V
If the duty cycle D of the switch is 0.33, then the average
voltage to the expensive car stereo is 39 β 0.33 = 13Vdc. This
is lossless conversion, but will it work?
Rstereo
+
39Vdc
β
Switch state, Stereo voltage
Closed, 39Vdc
Open, 0Vdc
Switch open
Stereo
voltage
39
0
Switch closed
DT
T
26
Convert 39Vdc to 13Vdc, cont.
Try adding a large C in parallel with the load to
control ripple. But if the C has 13Vdc, then
when the switch closes, the source current
spikes to a huge value and burns out the
switch.
Rstereo
+
39Vdc
βC
Try adding an L to prevent the huge
current spike. But now, if the L has
current when the switch attempts to
open, the inductorβs current momentum
and resulting Ldi/dt burns out the switch.
By adding a βfree wheelingβ diode, the
switch can open and the inductor current
can continue to flow. With high-
frequency switching, the load voltage
ripple can be reduced to a small value.
Rstereo
+
39Vdc
βC
L
Rstereo
+
39Vdc
βC
L
A DC-DC Buck Converter
lossless
β’ Common switching converters - buckβ During operation, the buck
converter functions differentlydepending on the switchβ’ On state
β Current flows through the transistor but reverse bias prevents current flow through the diode
β Inductor begins to charge and a smaller current becomes πΌππππ
β’ Off stateβ Switch opens and the inductor
starts discharging as the only power source
β’ This on-off state will determine the βmodeβ the converter operate in
Functionality β Buck
Buck: switch on
Buck: switch off
Figures from Wikipedia
Capacitors and Inductors
In capacitors:dt
tdvCti
)()(
Capacitors tend to keep the voltage constant (voltage βinertiaβ). An ideal
capacitor with infinite capacitance acts as a constant voltage source.
Thus, a capacitor cannot be connected in parallel with a voltage source
or a switch (otherwise KVL would be violated, i.e. there will be a
short-circuit)
The voltage cannot change instantaneously
In inductors:
Inductors tend to keep the current constant (current βinertiaβ). An ideal
inductor with infinite inductance acts as a constant current source.
Thus, an inductor cannot be connected in series with a current source
or a switch (otherwise KCL would be violated)
The current cannot change instantaneouslydt
tdiLtv
)()(
Functionality β Switching Converters
β’ Common switching convertersβ In switching converters, load current and voltage is
largely determined by the operation of transistor switching
β’ The duty cycle of the switching will determine the mode each converter operate at various loads
β Continuous ModeΒ» The inductor current will never fall to zero when the
switch is offΒ» Smaller current peak during operation
β Discontinuous ModeΒ» Inductor current will reach zero before the end of the
full duty cycleβ Each mode has its advantages and disadvantages depending
on the switching converters. Β» In generally itβs how they change the frequency
response.
D is the duty cycle
Figure from Wikipedia
Functionality β Buck
β Continuous mode β Discontinuous mode
Common switching converters
Using buck converter as an example, you can see the pulses with a certain πππ/πππΉπΉ. This is the duty cycle of the transistor switching
When the transistor is on it also creates a reverse bias voltage (ππΆβπ) across the diode.
Functionality β Buck
β Continuous mode β Discontinuous mode
Common switching converters
As the switch is on there is current flow πΌπ through the transistor
Current is a ramp because of the inductor
High πΌπ peak also creates more stress on the transistor
Functionality β Buck
β Continuous mode β Discontinuous mode
Common switching converters
During the off time there is now forward bias current flowing through the diode from inductor discharging
Inductor is discharging so the current starts to ramp down
Functionality β Buck
β Continuous mode β Discontinuous mode
Common switching converters
The output now has a ripple to it and not desirable for controlling the converter
Instead of πΌπ = πΌπΏ , Io = ILavg, so when the feedback happens the control can
read a constant value
From the feedback the switching duty cycle is changed to keep the converter in a particular state.
Functionality β Buck
β Continuous mode β Discontinuous mode
Common switching converters
For buck converters we generally want to operate at continuous mode because ππonly depends on the duty cycle which makes it easier to control
There are many published paper that cover the subject, a good one ishttp://www.ti.com/lit/an/slva057/slva057.pdf
35
Examine the inductor current
Switch closed,
Switch open,
L
VV
dt
diVVv outinL
outinL
,
L
V
dt
diVv outL
outL
,
sec/ AL
VV outin
DT (1 β D)T
T
Imax
Imin
Iavg = Iout
From geometry, Iavg = Iout is halfway
between Imax and Iminsec/ A
L
Vout
ΞI
iL
Periodic β finishes
a period where it
started
36
Effect of raising and lowering Iout while
holding Vin, Vout, f, and L constant
iL
ΞI
ΞIRaise Iout
ΞI
Lower Iout
β’ ΞI is unchanged
β’ Lowering Iout moves the circuit toward discontinuous
operation
37
Effect of raising and lowering f while
holding Vin, Vout, Iout, and L constant
iL
Raise f
Lower f
β’ Slopes of iL are unchanged
β’ Lowering f increases ΞI and moves the circuit toward
discontinuous operation
38
iL
Effect of raising and lowering L while
holding Vin, Vout, Iout and f constant
Raise L
Lower L
β’ Lowering L increases ΞI and moves the circuit toward
discontinuous operation
Functionality β Switching Converters(again)
β’ Common switching convertersβ In switching converters, load current and voltage is
largely determined by the operation of transistor switching
β’ The duty cycle of the switching will determine the mode each converter operate at various loads
β Continuous ModeΒ» The inductor current will never fall to zero when the
switch is offΒ» Smaller current peak during operation
β Discontinuous ModeΒ» Inductor current will reach zero before the end of the
full duty cycleβ Each mode has its advantages and disadvantages depending
on the switching converters. Β» In generally itβs how they change the frequency
response.
D is the duty cycle
Figure from Wikipedia
Net effectβavoiding discontinuous mode
β’ Lower load current, lower inductance and lower switching frequency move us toward discontinuous mode.
There are pros and consto both modes, but onereal issue is that switchingbetween them can cause ringing on the inductorβs output.(See UT slides for why, has to do with parasiticcap in transistor and diode.
42
Vin
+Vout
βC
iC
IoutiinBuck converter iL
L
+ vL β
Boost converter
Vin
+Vout
βC
iC
IoutiiniL
L
+ vL β
43
Boost converter
This is a much more unforgiving circuit than the buck converter
Vin
+Vout
βC
iC
IoutiiniL
L
+ vL β
iD
β’ If the MOSFET gate driver sticks in the βonβ position, then there
is a short circuit through the MOSFET β blow MOSFET!
β’ If the load is disconnected during operation, so that Iout = 0, then
L continues to push power to the right and very quickly charges
C up to a high value (250V) β blow diode and MOSFET!
β’ Before applying power be sure that a load is solidly connected
!
β’ On stateβ Current flows through the
transistor(least resistance)making the diode reverse bias and no πΌπ·
β Inductor is charged in the process
β’ Off stateβ The energy discharged by the
inductor is superimposed to theinput, generating a higher output
Functionality β Boost
Generic boost schematic
Boost: on state
Boost: off stateSchematics from http://en.wikipedia.org/wiki/Boost_converter
D
VV in
out
1
Functionality β Switching Converters
β Continuous mode β Discontinuous mode
Common switching converters - Boost
From the timing diagram you can see this is very similar to the buck timing diagram with a difference in that ππ only affects ππ directly when the switch is off
Similar to buck, in continuous mode the output is control by the duty cycle.
But unfortunately, boost favor discontinuous mode, in continuous mode there is a complex second order characteristic in noise. Discontinuous mode removes the inductance noise, producing simpler response to compensate and control
Schematics from http://en.wikipedia.org/wiki/Boost_converter
β’ Common switching converters
β Buck-boost(inverting)β’ The baby of buck and boostβ¦
β’ On stateβ Diode is reverse biased and the
inductor is charged but input and output is isolated from each other
Β» Creates more stress on the diode!
β ππ is now the charged ππ from off state but different polarity
β’ Off stateβ Inductor voltage reverses, passing
energy to the capacitor and load through forward biased diode
β’ Due to the characteristics of boost, buck-boost suffers the same problem. Therefore a discontinuous mode is favored
Functionality β Switching Converters
Generic buck- boost schematic
Buck-boost: on state
Buck-boost: off state
Schematics from http://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter
β’ To give you a better idea of how duty cycle affect the output of each converter in continuous mode refer to the table below
β Each duty cycle equation is equal to the ratio of πππ’π‘
πππ
β In discontinuous mode these equations are no longer true, but we will not discuss them here and they can also be found online
β More information on each of the switching converter can be found at http://www.nxp.com/documents/application_note/APPCHP2.pdfhttp://ecee.colorado.edu/copec/book/slides/Ch5slide.pdfhttp://www.ti.com/lit/an/slva057/slva057.pdf
Functionality β Switching Converters
Converters Duty cycle
Buck1
π·
Boost1
1 β π·
Buck-boost(inverting)π·
1 β π·
Picking converters
β’ Hopefully at this point you can start to see how much more complicated a switching converter is relative to a linear converter
β Every change made to the capacitor, inductor, transistor or diode will have an significant effect in not only the efficiency of each converter but also the life and way they operate
β Fortunately it wonβt be the end of the world if you decide to use switching converters
β’ Using mic2168a, a controller for buck converter, as example. We can see not only do they provide information on the controller but also the external components needed for proper functionality (9pages!).
β ON semiconductor also provide a detailed published paper on both linear and switching regulator, covering the theory and design consideration needed.
β’ For those who arenβt as interested in all the technical details behind all this can refer to TIβs power management portal.
β’
β’ You can just enter the parameters and TI will come up with recommendation
β’ Other companies also offer similar things so youβre not stuck with only TIβ Try Micrel, Freescale or other brands as needed.
Picking converters
Efficiency of switchers
β’ Itβs going to vary a lot.
β Depends on the converter,the output current, etc.
β Graph on the right is of a TPS5420 under reasonableconditions.
β’ Typically has only 18ΞΌA current whenyou shut it down (needs a control pin)
TPS5420