2 Legend: Figures licensing [PD] Public domain[CC] Creative Commons[GFDL] GNU Free Doc. License[GPL] GNU Public License[O] Other. See source for licensing.
3 Why this workshop? Electronics need a power source! Various wall wart supplies (left). A computer power supply (right). Fig. [PD] Marc-Alexandre Chan. Original.
4 Why this workshop? Power supplies: You use them all the time… But do you know how they work?7805, LM317:magic linear voltage regulatorComputer power supply, laptop/phone charger:magic switch-mode power supplyIdeal voltage source:magic (Really. It doesn’t exist; we just imitate it to the extent necessary.)
5 What will we learn? Basic types of power supplies & converters Choose the best kind for your projectDesign & build your own!Get a glimpse of safety & advanced concepts
6 Outline AC-DC converter: Using a power outlet for electronics DC-DC: Linear voltage regulatorsSwitched-mode converters: buck & boostSafety & advanced topologiesApplications to real-world power supplies: What’s a serious power supply unit look like?
7 1 AC-to-DC conversion or using a power outlet for electronics
8 1 AC-DC Converter Wall outlet AC power Electronics DC power How to make this work?0V →0V →Top: 110V 60Hz AC voltage from a wall outlet; Bottom: 12V DC from a car battery (bottom).   [PD] JAK83 https://commons.wikimedia.org/wiki/File:NEMA_5-15_Outlet_120V-15A.jpg  [PD] Marc-Alexandre Chan. Original.Two NEMA 5-15 sockets 
9 AC-DC converter. [PD] Marc-Alexandre Chan. Solution: Convert AC to DC (rectification)Parts: Transformer, rectifier, filterAC-DC converter. [PD] Marc-Alexandre Chan.
10 1 AC-DC: Transformer “Transform” AC voltage Step-up → voltage ↑Step-down → voltage ↓Does not work with DCTurns ratio 𝑁 𝑆 : 𝑁 𝑃 (of wires)Ideal: 𝑉 𝑠 = 𝑁 𝑆 𝑁 𝑃 𝑉 𝑃Real: losses + frequency* Transforms AC voltage: increase or decrease it* Works ONLY with AC, relies on changing voltage + current (derivatives).* Side with more windings = higher voltage* Example: 1000 turns to 100 turns, 10V primary gives 1V secondary* Or 100 turns to 1000 turns, 10V primary gives 100V secondary* Reality: losses and frequency dependence (specific design)A small mains transformer (top) ;Basic transformer showing wire windings and electrical-to-magnetic energy conversion . [CC] ZngZng https://commons.wikimedia.org/wiki/File:SmallTransformer.JPG [CC] BillC https://commons.wikimedia.org/wiki/File:Transformer3d_col3.svg
11 1 AC-DC Converter: Transformer What is the role of the transformer?120VAC (169V amplitude) too high for electronics!Step-down transformer: reduce 𝑉 to usable valueExample: 120VAC to e.g. 9VACReminder: VAC is the RMS voltage = 𝑉 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 2So 9VAC is ≈12.7V amplitude
12 1 AC-DC Converter: Rectifier Input is AC: sine wave goes positive & negativeTo rectify = to make one-sidedOutput is only positive (sometimes only negative)Output is not DC yet… still “bouncing!”Most commonly uses diodes… let’s review!Voltage plots of (blue) an AC waveform and(orange) the rectified waveform[PD] Marc-Alexandre Chan. Original.
13 1 AC-DC Converter: Diodes One-way current gateIdeal: a one-way “wire”Non-ideal:Constant voltage drop VD when passing forward currentReverse leakage: small reverse currents possible (μA)Different diode types: silicon, germanium, Schottky…Better non-ideal model: exponential𝑉 𝐷 = 𝐼 𝑆 𝑒 𝑉 𝐷 𝑉 𝑇 −1Voltage increases a bit when current increasesDiode schematic symbol. Current can flow A→K, but not K→A.𝑉 𝑇 ≈26𝑚𝑉 at 300K𝑉 𝑇 = 𝑘𝑇 𝑞 ,k = Boltzmann const,q = electron charge
14 1 AC-DC Converter: Diodes AnodeCathodeDiodes, oriented for current flow left to right. A line (black or silver) marks the cathode. a) DO-35 glass package . b) DO-41 package . c) Schematic symbol. [CC] Vonvon. https://commons.wikimedia.org/wiki/File:Diode_1n4148.jpg  [CC] Vonvon. https://commons.wikimedia.org/wiki/File:Diode_1n4001.jpg
15 1 AC-DC Converter: Rectifier Half-wave rectifier: one diodePasses current one way only = no more negative voltage“Blocks” half the sine wave: less available power!DemoTODO: Add sine graphHalf-wave rectifier a) circuit; b) simulation plot of the output voltage. [PD] Marc-Alexandre Chan. Original.
16 1 AC-DC Converter: Rectifier Full-wave rectifier: four diodesDiodes force current into the + node and out of the – node for the sine wave (positive and negative)Absolute value functionDiode directionsFull-wave rectifier a) circuit; b) simulation plot of the output voltage. [PD] Marc-Alexandre Chan. Original.
17 1 AC-DC Converter: Filter Rectifier output is all positive … but “bumpy”!Use a filterBig capacitor to store chargeHelps V stay up by providing I in “valleys” of the “bumps”Full-wave rectifier with filter a) circuit; b) simulation plot of the output voltage. [PD] Marc-Alexandre Chan. Original.
18 1 AC-DC Converter: Ripple Even with filter, some “bumps”: called rippleDepends on capacitor size + load currentNeed impractically big cap for high current + low rippleDesign: Given 𝑉 𝑚𝑎𝑥 ,𝑓, choose 𝑉 𝑟 , 𝑅 𝐿(𝑚𝑖𝑛) , and find 𝐶 neededAverage (DC)* Ripple because the capacitor has to discharge in order to compensate for the big “bumps” V drops* For very small ripple (< 100s mV) and high currents (> 100mA), cap becomes huge* Real power supplies: 470μF to 4700μF or so (or higher → very high current)* Big & expensive: for small electronics, can we do better? (will see later)Ripple𝑉 𝑟 = 𝑉 𝑚𝑎𝑥 1− 𝑒 − 1 2𝑓 ∗ 1 𝑅 𝐿 𝐶 for R load𝑉 𝑟 = 𝐼 2𝑓𝐶 for constant I load𝑓= AC frequency (60Hz here)
21 1 DESIGN Rectifier: Specifications Let’s choose 𝑉 𝑖𝑛 =9𝑉 (AC rms)Because that’s the transformer we have for now!𝑉 𝑜𝑢𝑡 ≤9 2 =12.7𝑉 DC after rectificationDepends on output capacitor / amount of ripplePeak of output voltage “bumps” is 12.7V, average is less𝑉 𝑟𝑖𝑝𝑝𝑙𝑒,𝑚𝑎𝑥 =1.0𝑉This is big, but we’re learning regulators next!𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥 =100𝑚𝐴We will also look at 𝐼 𝑜𝑢𝑡 =25𝑚𝐴 to see what happens.Transformer: generally good to choose peak voltage close to highest target voltage, but still higher (diode drop + regulator drop…)
22 1 DESIGN Rectifier: Diode Diode choice: 1N4001 rectifier diode (1N4002 to 1N4007)𝑉 𝐷 =0.75𝑉 at 100mA (from datasheet I-V curve)Max reverse voltage, forward current, temperature…Schottky performs better but less efficient (reverse leakage).Output capacitorRipple equation (const current): 𝑉 𝑟 = 𝐼 2𝑓𝐶𝑉 𝑜𝑢𝑡,𝑚𝑎𝑥 ≈ 𝑉 𝑖𝑛,𝑝𝑒𝑎𝑘 −2 𝑉 𝐷 =12.7−1.5=11.2𝑉 (ripple peak)𝐶= 𝐼 2𝑓 𝑉 𝑟 =833𝜇𝐹 → let’s go for 1000μFFor 25mA, 𝐶=208𝜇𝐹 → go for 220μFLoad: Use resistor to simulate circuit to power.At 11.2V, 100mA → ≈100Ω, 25mA → ≈470Ω (closest common Rs)* Diode:* Explain that 1N4002…7 can block a bigger reverse voltage without breaking down
23 1 DESIGN Rectifier: Diode * Can see reverse voltage (peak, RMS), max DC and peak current, forward voltage, reverse leakage…1N4001 datasheet excerpt: absolute maximum values.[O] Diodes, Inc. Fair use (educational).
24 1 DESIGN Rectifier: Diode * Voltages changes depending on current.* 100mA = 0.75V* 1.0A = 0.9V.* Graphs can be more informative than tables (tables give you “typical” or min-max values for some conditions)1N4001 datasheet excerpt: Voltage drop vs. forward current (axes usually switched).[O] Diodes, Inc. Fair use (educational).
25 1 DESIGN Rectifier: Thermal design Heat flow circuit modelVoltage = temperature(oC)Current = heat output (W)Thermal capacitance & resistanceWorst case (long-term):DC steady-stateSo ignore capacitorsGet 𝜃 𝐽𝐴 and max temp. from datasheet𝑇 𝑗 = 𝑃 𝑗 ∗ 𝜃 𝐽𝐴 + 𝑇 𝑎𝑚𝑏= 𝑉 𝐷 𝐼 𝐷(𝑚𝑎𝑥) 𝜃 𝐽𝐴 + 𝑇 𝑎𝑚𝑏=0.75𝑉∗0.1𝐴∗100 °𝐶 𝑊 +27°𝐶𝑇 𝑗 =34.5°𝐶Max temp is 150°𝐶→ more than OK* Look at temperature of components (approximate / very rough)* Make sure no fire / damaged parts / magic smoke* Decide if we need heatsinks / bigger heatsinks* Does not predict temperature of too-thin wires or bad connections - breadboard* Heat output = P = VI of resistive elements (V ACROSS the resistor, not of the power supply or one of the nodes!)* Thermal capacitance & resistance: use example of two metal blocks connected by a metal rod* Worst case (long-term):* DC steady-state* Ignore capacitors at DC* Ground = ambient temperature
26 1 DESIGN Rectifier: Thermal design Meant to be a quick & dirty analysisWorst case scenario: is it well below the max temp?Do we need to add heatsinking or cooling?Forced air (e.g. fan) = lower temperaturesShould be measured in lab if a concernMore info:
29 But first… what’s this breadboard thing? Breadboard: Make connections without a mess!No solder, no PCB design, no chemicalsGood for quick low-frequency prototypingLots of capacitance everywhere!Next slide: pattern of connected holes (green)Power rails may or may not be connected: look at line!Tip: insert leads straight downward with even, steady pressure, one at a time. Otherwise your leads will bend or break!
30 But first… what’s this breadboard thing? Breadboard: Make connections without a mess!No solder, no PCB design, no chemicalsGood for quick low-frequency prototypingLots of capacitance everywhere!Next slide: pattern of connected holes (green)Power rails may or may not be connected: look at line!Tip: insert leads straight downward with even, steady pressure, one at a time. Otherwise your leads will bend or break!Fig. 5 Breadboard connections (green). Power rails (top and bottom) are not connected on this breadboard, but may be on other models; the red and blue lines usually shows this. (Source: Nevit Dilmen, Wikimedia Commons)
31 The line marks the direction (symbol and real parts shown). Diode directionBlack line on glass bodySilver line on black bodyThe line marks the direction (symbol and real parts shown).Figures shown previously.
32 “Can” capacitors: polarity! Some capacitors are polarized. We will only use these ones; pay attention to the “minus” mark. These capacitors can explode if backwards or overvolted.[PD] Elcap. https://commons.wikimedia.org/wiki/File:Polarity-wet-Al- Elcaps.jpg
33 Barrel connectorXConnect the two circled terminals. Polarity does not matter, as the source is isolated AC. Careful not to connect two pins into connected holes on the breadboard—short circuit risk![O] CUI, Inc. Educational use.
34 IMPORTANT: Safety tips! Never work on a circuit while plugged in!Unplug before touching/moving components.Risk of accidental short circuit, shock at high voltage, etc.Work with one hand in your pocket/lap.Low voltage → low risk. But good habit for all power electronics.Will save your life with low-impedance high-V supplies.Be careful about short circuiting supplies.Supplies can provide a LOT of current: smoke, fire, etc.This goes for whole circuit, but supplies most dangerous.Only connect oscilloscope ground to circuit ground.Details aside, this avoids mistakes that can damage oscilloscope.
35 1 MAKE: AC-DC ConverterThe resistor simulates the circuit you want to powerTry different 𝑅 𝐿 and 𝐶 (R↓ or I↑ or C↓ → ripple ↑)C = 1000μF or 220μF 𝑅 𝐿 = 470Ω or 100ΩSee the Handout
36 Break time! Want a snack? Wash your hands. Lead-based solder at the lab.
37 Linear DC-DC Regulator 2Linear DC-DC RegulatorImage: LM2940L-5.0 LDO die photograph. Source: [CC] ZeptoBars, https://commons.wikimedia.org/wiki/File:LM2940L-HD.jpg
38 2 Motivation Electronics want a DC voltage AC-DC converter has ripple noiseCauses signal noise, errors in digital data, IC resettingSmall ripple → huge, expensive capacitorOther sources of problemsAC voltage maybe not constant (noise, surge/sag, etc.)An existing DC supply is at wrong voltage or too noisyCan we solve this any other way?
39 2 Regulators Voltage regulator: Keeps output voltage constant Uses a control system to read & adjust outputInput-output assumptions don’t affect accuracyELEC372 + ELEC level controls coursesReacts to change in 𝑉 𝑖𝑛 (line regulation)Reacts to change in 𝐼 𝑜𝑢𝑡 (= 𝑅 𝐿 ) (load regulation)
40 2 Linear Voltage Regulator Strategy: Transistor as Variable ResistorAdjust resistance to keep output voltage constantOutput voltage function of 𝑉 𝑖𝑛 and 𝐼 𝑜𝑢𝑡Can only reduce voltageA 7805 (top) and LM137 TO-220 package. This package is often used for three-pin voltage regulators and high-current transistors. [CC] John Dalton https://commons.wikimedia.org/wiki/File:TO- 220_Package_Four_Different_Projections.jpg
41 2 Linear Voltage Regulator AdvantagesCheap, simpleVery good regulation (low output noise)Disadvantages:Inefficient (<60%): wastes extra energy as heat!They get hot! Proportional to voltage drop and current:𝑃 𝑑𝑖𝑠𝑠 = 𝑉 𝑑𝑟𝑜𝑝 𝐼 𝑜𝑢𝑡
43 2 DESIGN: Linear Regulator Design ProcedureChoose V 𝑖𝑛,𝑚𝑖𝑛 , 𝑉 𝑜𝑢𝑡 , 𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥Choose a suitable linear regulator chipChoose capacitorsThermal analysis: avoid going poof!Done!
44 2 DESIGN Linear: 2 Specifications Assume we need to power something with 5V𝑉 𝑜𝑢𝑡 =5𝑉Assume we have a ≈10 to 13VDC supply available𝑉 𝑖𝑛,𝑚𝑖𝑛 =10𝑉Assume our circuits will not use more than 80mATo be safe, let’s add a 25% overhead𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥 =100𝑚𝐴From datasheet: max current 1A → 100mA is OKBut check temperature later too!
45 2 DESIGN Linear: 2 Choose a Chip Common/”classic” chips78xx family (e.g = 5V)LM317 (adjustable output voltage)Let’s choose the 7805 because fixed 5V is OKMinimum voltage drop (“dropout”)𝑉 𝑖𝑛 − 𝑉 𝑜𝑢𝑡 has to be bigger than minimum dropoute.g. 7805: 𝑉 𝑑𝑟𝑜𝑝,𝑚𝑖𝑛 = 2V, 𝑉 𝑜𝑢𝑡 = 5V, so 𝑉 𝑖𝑛,𝑚𝑖𝑛 = 7VWe said 𝑉 𝑖𝑛,𝑚𝑖𝑛 for our supply is 10V, so this is OK
46 Aside: On new regulator chips Newer chips: low-dropout regulators (LDO)Dropout less than 500mV possibleSmaller 𝑉 𝑖𝑛 − 𝑉 𝑜𝑢𝑡 (actual one, not the minimum) = more efficient & less heatLook at this if we need 𝑉 𝑜𝑢𝑡 closer to 𝑉 𝑖𝑛
47 2 DESIGN Linear: 2 Choose a Chip LM317:Output V is set by R1, R2 voltage dividerC1 filters any input noiseC2 filters noise caused by load (when circuit current changes over time)* Why capacitors? They don’t seem useful at DC, right?* C1/Cin filters input noise* C2/Cout filters noise on the output caused by the load* Load regulation: the chip is good at dealing with slow (DC) changes* Load regulation: fast/sudden changes are better dealt with by capacitor* Load current can change over time: for example, digital circuits switching* For filtering RF noise, can put another small Cout (depending on frequency) in parallel* Remember that caps have series resistance + inductance: they have a max freq (and get worse at higher freq)* So Cparallel = C1+C2 is wrong for real caps: they will each act like good capacitors at diff frequenciesTypical circuit applications for a) 78xx ; b) LM317 . [O] LM7805 Datasheet (Fairchild Semiconductor). Educational use. [O] LM317 Datasheet (Texas Instruments). Educational use.
48 2 DESIGN Linear: 3 Capacitors Input capacitorWith AC-DC converter, filter cap is OKExcept if AC-DC and linear regulator far away (a few cm)Generally, with “good” DC supply, 0.1–10.0μFfilter noise due to wire impedance/RF noiseStart with a value, and increase if neededAlso good to use datasheet or application note example circuits (see last slide)
49 2 DESIGN Linear: 3 Capacitors Output capacitor: general rule μFChip’s datasheet is a good place to start: 0.1μFFilters noise and current transientsIf sudden Δ𝐼 → helps in time before chip can reactBig cap = slow but filters big Δ𝐼; small = fast, filt small Δ𝐼Simulation DEMO laterChange capacitor when needed (check w/scope)Big changes AND fast? High-frequency noise?Two different caps in parallel! Guideline: 2 decades apart(Capacitors have seris inductance, so not C eq = 𝐶 1 + 𝐶 2 )
51 2 DESIGN Linear: 3 Thermal If 𝑇 𝑗 too high?Add heatsink?Add fan?Heatsink: similar model𝜃 𝐽𝐶 : from chip datasheet𝜃 𝐶𝐻 : from thermal paste datasheet (if used) or heatsink datasheet or best guess𝜃 𝐻𝑆 : from heatsink datasheet or best guess* Go through each element one by one and explain it and where to find it (don’t really read the text on the side)
53 But first: 7805 PinoutPinout for 78xx family in TO-220 package. Pins are numbered 1-3 from left to right in this diagram.[O] LM78xx datasheet, Fairchild Semiconductor. Educational use. https://www.fairchildsemi.com/datasheets/lm/LM7805.pdf
55 2 DEMO: Linear regulator Fig. 19 Linear voltage regulator simulation circuit. Rfb and Cfb simulate a non-ideal amplifier response. At the beginning of the simulation, the power turns on and starts powering the 500Ω load; then, at 10μs, the load current suddenly increases (≈10Ω).
56 2 DEMO: Linear regulator Fig. 20 Linear voltage regulator simulated output voltage. This shows the turn-on response and load regulation (load increase at 10μs).
57 2 DEMO: Linear regulator Line regulationInput voltage down to 7V → output still 5V.Minimum dropoutBelow 7V input (2V dropout), loss of good line regulation.Load regulationWith different currents, voltage stays close to target.
58 (Reminder: wash hands.) Break time!Stretch your legs.Sit back and relax.Ask questions.(Reminder: wash hands.)
59 3 Switching regulators Fundamentals: Buck & Boost converter 2015-11-21 Image: XT PC Power Supply. Source: [CC] Hans Haaser, https://commons.wikimedia.org/wiki/File:XT-PC-Power-Supply-PSU-PCB-IMG_0435.JPG
60 3 Switching regulators Linear voltage regulators are inefficient Reduce voltage by dissipating extra power as heatWhat if we switch power on & off then filter it?Switches are theoretically 100% efficientReminder: 𝑃 𝑠𝑤 = 𝑉 𝑠𝑤 𝐼 𝑠𝑤When switch off, 𝐼 𝑠𝑤 =0 (open circuit)When switch on, 𝑉 𝑠𝑤 =0 (short circuit)Reality: leakage, series R, switching loss, controller needs power…
61 3 Switching regulators Example: Computer power supply unit 300 to 1000WA similar linear supply is:Very hot, very heavyHuge heatsink, xformer, fansAnd you thought your gaming rig heats up the room?A computer power supply. [PD] Marc-Alexandre Chan. Original.
62 3 Switching regulators Example: phone charger Not old DC wall warts Tiny and lightweightLinear would be bigger, heavier and need cooling!Not old DC wall wartsHeavy, blocky thingTransformer, rectifier, filterNot a switching supplyVarious DC wall warts. Top left is a mains-transformer-based supply. The remainder are switch-mode supplies. [PD] Marc-Alexandre Chan. Original.
63 3 Switching regulators Advantages Disadvantages Small, lightweight and efficient (≥90% possible)Skip the transformer: use rectified 120VAC directly (not here, too dangerous w/o training!)Need isolation? High frequency = smaller transformer!DisadvantagesMore complex to designRequires a minimum output loadSwitching noise on the output (bad for precision circuits!)EMI emissions
64 3 Switching: Buck converter Simplest switching DC-DC converterDecreases DC voltagePrinciple: turn input voltage on and off quicklyOutput voltage = take the average output over timeUse PWM or PFM to change the average output voltageUse a control system to get a desired output voltageUse filtering to take the average (LC filter)
65 3 Switching: Buck converter 10% average50% average90% averagePWM waveform (blue) and average voltage (orange), various duty cycles. If, for example, the input is 12V, the bottom waveform has a 12*0.9 = 10.8V average.[O] Nathaly Arraiz Matute. Microprocessors Tutorial 2: Arduino Robotics [slideshow]. IEEE Concordia, 10 October 2013.
66 3 Switching: Buck converter VPWMVOUTVINA theoretical buck converter circuit. The switch is turned on and off electronically by a controller. VPWM is filtered by the inductor, which stores energy when switch is on and releases it as current when switch is off. The diode + inductor ensure that VPWM is connected to ground when the switch opens.[PD] https://en.wikipedia.org/wiki/File:Buck_circuit_diagram.svg
67 3 Switching: Buck converter Minimum load current is requiredInductor must have “continuous current” (= never 0)Otherwise you don’t get PWM waveform! Hard to predict!How to lower the minimum current?Higher frequency, same inductorLess switch off timeBut more switching losses → lower efficiencySame frequency, bigger inductorInductor holds more energy to release during switch off timeBut bigger, heavier & more expensive
68 3 Switching: Buck converter If less than minimum load currentGood controller: works but less effective/efficientBad/no controller: 𝑉 𝑜𝑢𝑡 can go up to 𝑉 𝑖𝑛 & cause damage!Reason #104 not to buy cheap knockoff phone chargersHow to ensure voltage is regulated?Put a permanent resistor: always minimum currentOr put a “smart” load that turns on only when neededWastes power! (→ Why to unplug your phone charger!)
69 3 Switching: Transistors But wait! How do we make an electronic switch?TransistorsMany types: BJTs, MOSFETs, IGBTs…Idea: 3 terminals. Current or voltage on one terminal opens/closes the “switch” between two other terminals.Details outside scope of this workshopSee ELEC311 and ELEC433Aside For AC-DC and AC-AC: Thyristors, TRIACs
70 3 Switching: Transistors Transistor schematic symbols. Left to right (top, bottom): BJTs, MOSFETs, MOSFETs (CMOS symbols), IGBT.[PD] Marc-Alexandre Chan. Original.
72 3 DESIGN: Buck converter Design procedureChoose a chipFollow the chip’s design guide (datasheet)Done!
73 3 DESIGN: Buck converter MC34063A: Old but reliableBuck, boost, invertingCheap, easy to use, comes in DIP packageLess efficient, low frequenciesTI’s “Simple Switcher” series:Five-pin chip, more expensive, efficient enoughNeed a quick drop-in switching converter? Go for these!Advanced/modern chips (often surface-mount)Current limit, shutdown input, status outputs…Pre-built modules: 100% plug-and-play
74 3 DESIGN: Buck converter Show the parallel to the theoretical one we showed before.- Explain R_SC is to detect the current, and limit the maximum current if it reaches a limit (for safety of the inductor).Circuit schematic for a buck (step-down) converter from TI’s MC34063A datasheet.[O] Texas Instruments. Educational use.
75 3 DESIGN: Buck converter Specifications to choose for a step-down converter from the TI MC34063A datasheet.[O] Texas Instruments. Educational use.
76 3 DESIGN: Buck converter Design guide for a step down (buck) converter from the MC34063A datasheet. Follow this line by line![O] Texas Instruments. Educational use.
77 3 DESIGN: Buck converter Follow the guide in the datasheet!Just follow the steps, and proceed, my dear friend!Let’s start with specifications…NameValue𝑉 𝐶𝐸(𝑠𝑎𝑡)1.0V𝑉 𝐹0.4𝑉 𝑖𝑛(𝑚𝑖𝑛)10.0𝑉 𝑖𝑛 𝑚𝑎𝑥13.0𝑉 𝑜𝑢𝑡5.0NameValue𝐼 𝑜𝑢𝑡 𝑚𝑖𝑛10.0mA𝐼 𝑜𝑢𝑡(𝑚𝑎𝑥)100𝑓 𝑚𝑖𝑛kHz𝑉 𝑟𝑖𝑝𝑝𝑙𝑒10mVVCE(sat): Datasheet. Voltage when switch “on”. For buck, we use Darlington (for boost, non-Darlington).VF: Diode datasheet. Diode forward voltage drop. We will use the SB130T. At 100mA, 0.22 to 0.42V; at 500mA, 0.32 to 0.48V. Let’s say 0.4V is about right.Vin(min,max): self-explanatory, expected input voltages. The chip has a maximum input voltage of 40V (beyond which you may damage it!)Vout: desired output voltage. Must be lower than input voltage.Iout(min,max): desired minimum and maximum output current (at DC a.k.a. AVERAGE). The chip’s switch has a maximum output current of 1.5A: note that this is the peak instantaneous current of the switch, which is twice Iout: or in other words, Iout < Imax/2.Fmin: Minimum frequency. Higher = smaller inductor required or lower minimum current, but more switching losses.Vripple: maximum allowable ripple, peak-to-peak
80 3 DESIGN: Buck converter 𝑉 𝑜𝑢𝑡 = 𝑅 2 𝑅 1Therefore 𝑅 2 𝑅 1 =3Choose kΩ rangeClosest common values: 𝑅 1 =10𝑘Ω, 𝑅 2 =3.3𝑘ΩNeeded because the controller tries to keep the “comparator input” pin at 1.25VTherefore use resistive voltage divider: feedbackGet any desired output voltage between 1.25V and 𝑉 𝑖𝑛
81 3 DESIGN: Buck converter WAIT! 𝐿 𝑚𝑖𝑛 used 𝐼 𝑜𝑢𝑡 𝑚𝑎𝑥 .But we said smaller minimum current ↔ bigger inductor…Also, 𝐿 𝑚𝑖𝑛 maybe bigger for 𝑉 𝑖𝑛 𝑚𝑎𝑥 - try this too!Datasheet mistakes?Recalculate and use biggest 𝐿 𝑚𝑖𝑛 …𝐿 𝑚𝑖𝑛 = 𝜇𝐻. Crisis averted. (Not for me… oops.)Let’s choose 𝐿=2.2𝑚𝐻 (common value + bigger)
82 3 DESIGN: Buck converter Oh, by the way…You need an input capacitor.Try 100μF.Maybe a smaller cap in parallel (fast switching).If you connect to rectifier with a few centimeters of wire distance, put it beside the MC34063.If you’re right beside the rectifier, skip it.
83 3 DESIGN: Buck converter We’re done!That was long… but not hard, right?
84 Aside: Capacitors “I have a good quality DC power supply.” “Do I need the input filter cap?”Yes. Always have a filter (decoupling) cap near the input of every chip and converter (not just for power chips).Short distances (few centimetres) → smaller capLong dist. → bigger cap or different parallel ones
85 Aside: Capacitors Here’s what happens with a buck converter Lab PSU, 12VDC~40cm of 18AWG wireAn hour wasted!Figure is of VDD at input of chipTop: No input cap, 4.8V to 17.8V ringingBottom: with 1μF, almost flat[PD] Marc-Alexandre Chan. Original.
87 But first: MC34063A pinout For all DIP chips: Pins numbered counterclockwiseDimple shows chip orientationAND/ORDot shows pin 1MC34063A DIP pinout. Pay attention to the pin numbers: your schematic has different positions.
88 3 MAKE: Buck converter!Buck converter circuit. Try both 47Ω and 470Ω loads, and scope the E_SW pin (switch/PWM).
89 Break time! Are we there yet? Almost, honey. Look outside. We’re in the mountains now!
90 3 Switching: Boost converter What if you need to increase voltage?Boost converterUses energy storage of inductorBuck: only as a filterBoost: uses it to increase voltage!Not “free” power𝑃= 𝑉 i𝑛 𝐼 𝑖𝑛 = 𝑉 𝑜𝑢𝑡 𝐼 𝑜𝑢𝑡 − 𝑃 𝑙𝑜𝑠𝑠If 𝑉 𝑜𝑢𝑡 > 𝑉 𝑖𝑛 , then 𝐼 𝑜𝑢𝑡 < 𝐼 𝑖𝑛 .
91 3 Switching: Boost converter Example: Minty-Boost kit (see soldering tutorial)USB Charger using two AA batteries (2V to 3.2V total)Uses a boost converter to produce 5V outputExample: Joule Thief𝑉 𝑜𝑢𝑡Fig. 33 a) Minty-Boost ; b) Joule Thief  [CC] Adafruit  [CC] Electron9 File:Joule_thief.png
93 3 Switching: Boost converter Current in inductor must be continuousMore important than buck converter!Means it can never go down to 0AConsequence again: Minimum output currentTo select inductor, given a minimum operational load:𝐿 𝑚𝑖𝑛 = ( 𝑉 𝑜𝑢𝑡 − 𝑉 𝑖𝑛 )(1−𝐷) 𝑓 𝑠𝑤 𝐼 𝐿,𝑚𝑖𝑛To determine the theoretical output voltage:𝑉 𝑜𝑢𝑡 𝑉 𝑖𝑛 = 1 1−𝐷If not continuous: lower voltage than expected!
94 3 Switching: Boost converter If discontinuous (current = 0 at some point)Inductor “used up” all its stored energySo the “flyback” doesn’t last entire switch-off timeVoltage is lower than expected!Complicated equation!𝑉 𝑜𝑢𝑡 𝑉 𝑖𝑛 =1+ 𝑉 𝑖𝑛 𝐷 2 2𝐿 𝐼 𝑙𝑜𝑎𝑑 𝑓 𝑠𝑤
95 3 Switching: Switched capacitor a.k.a. Charge Pump, e.g. MAX1595Can be used to increase or invert voltagePrinciple: capacitor stores chargeCharge cap, ↑voltage on “low” side, “high” side is ↑ too!Advantages: Caps cheaper/smaller than inductors, simple circuit, efficient, low EMIDisadvantage: Low currents onlyMore difficult/expensive to make high-current designs
96 5 What’s next? Advanced circuits, safety, and serious supplies
97 5 More: Performance & safety Transients: transition when something changesWhen turning onWhen output current changesWhen input voltage changesTransients can go above desired output voltageDepends on quality of controller + your design…May damage circuits!Reason #76 not to buy cheap knockoff phone chargers…Serious projects: analyze these!
98 5 More: Isolation and safety Various safety devicesUsage details outside of scope of workshopCritical to good & safe designs!Fuse: physically open-circuits if current too highCircuit breaker: like fuse but resettablePTC: automatic resetting “fuse”Higher temperature = becomes very high resistanceMOV: metal oxide varistorShort-circuits if voltage above a certain value
99 5 More: Isolation and safety Safety design features: choice of topologyIsolated switching power supplyOutput “floating” relative to input voltage: no common groundLess dangerous to interconnect different devices/equipmentProtect circuit from surges/spikes (depends on design)Simplest isolator: a transformer (unless the two sides shorted!)Optocoupling (signal isolation)Prevent surges/spikes/failures damaging controller circuitsMaintain isolation for an isolated switching supply’s feedbackCurrent limiting/short-circuit protection
100 5 More: Switching regulators Better/safer topologies of switching regulatorsIsolated switched-mode power suppliesFlyback, RCC, Half-Forward, Forward, Resonant Forward, Push- Pull, Half-Bridge, Full-Bridge, Isolated ĆukForward converter used in phone chargersUses transformers (high frequency = small, lightweight, cheap)Also more non-isolated SMPSSplit-pi, Ćuk, SEPIC, ZetaWall-plug power supplies use these better topologies!Buck/boost only used when you already have a good power supply and just need different voltages internally.
101 5 More: AC-DC Converters Advanced rectifierFull-wave rectifier using thyristors (SCRs)Controllable diodes (control turn-on)AC-DC with output voltage controlCan be used instead of a transformerNeeds better filtering, more complicated (control circuit)Animation of changing output voltage with thyristors: https://commons.wikimedia.org/wiki/File:Regulated_rectifier.gif
102 5 More: AC-AC Converters AC-AC regulator using a TRIACTRIAC: Bi-directional thyristorAC-AC voltage control (not frequency)Can “cut off” part of AC wave (voltage ↓)DC-AC inverter: transistorsHigh-frequency PWM sine wave + filterControl AC voltage and frequency“H-bridge” circuit can be used for thisSee ELEC433TRIAC regulator: synchronous or mains motor control; light dimmerInverter: induction motor controlTRIAC symbol. [PD] No author. https://commons.wikimedia.org/wiki/File:Triac.svg
103 5 More: Constant current supplies We’ve only talked about constant-voltage…Constant-current supplies exist too!e.g. for driving high-power/lighting LEDsSimple up to 1A: LM317-based circuit[CC] SilverSrv. https://commons.wikimedia.org/wiki/File: LM317_1A_ConstCurrent.svg
104 6 Real-world PSUs Once you’ve got this advanced stuff down Image: Computer ATX power supply. Source: [PD] mboverlord. https://en.wikipedia.org/wiki/File:PSU-gold-full_set.jpg
105 6 Applications to Real-World PSUs Combine these converters and regulators togetherOften need several voltages: e.g. 3.3V, 5V, 12VMay require low noise output (SMPS too noisy?)
106 6 Applications to Real-World PSUs Switch-mode power supply (no safety/isolation)Rectifier+Filter12V Buck Regulator5V LinearRegulator7V Buck RegulatorInverting–5V LinearRegulator3.3V LinearRegulatorConceptual block diagram of a switch-mode power supply featuring a high-current 12V rail and ±5V and 3.3V low-current low-noise rails.[PD] Marc-Alexandre Chan. Original.
107 7. Applications to Real-World PSUs a) ATX power supply. b) Typical block diagram of an ATX power supply.[PD] mboverlord. https://en.wikipedia.org/wiki/File:PSU-gold-full_set.jpg[O] ON Semiconductor. Educational use.
108 What next? Designing your own power supply Overvoltage protection, current limiting, short circuit protection, reliability/resistance to failure, isolation, load and line regulation, transient response, cooling, etc.Understanding specifications for choosing a supplyLoad and line reg., transient response, min/max load, cooling requirements, protection measures etc.Understanding the closed-loop controllerControl systems (ELEC372 and related 400-level courses)Electronics (for analog controller: ELEC311, ELEC312)Real-time systems (if digital/software: ELEC483)
109 Thank for you participating in this workshop! To contact the presenter: Marc-Alexandre Chan Info & downloads:Questions? Project ideas? Want to try something out in the lab? us! Or come visit us!
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