1. Introduction to Power Supply Design
Power your creations!
Introduction to Power Supply Design
IEEE Concordia Student Branch
Marc-Alexandre Chan, Amit Desai
21 November 2015
Image: Seva Kovtoun, © 2015 IEEE Concordia. All Rights Reserved.

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 regulator
Computer power supply, laptop/phone charger:
magic switch-mode power supply
Ideal 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 project
Design & 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 regulators
Switched-mode converters: buck & boost
Safety & advanced topologies
Applications 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). [2] [1] [PD] JAK83 [2] [PD] Marc-Alexandre Chan. Original.
Two NEMA 5-15 sockets [1]

9. AC-DC converter. [PD] Marc-Alexandre Chan.
Solution: Convert AC to DC (rectification)
Parts: Transformer, rectifier, filter
AC-DC converter. [PD] Marc-Alexandre Chan.

10. 1 AC-DC: Transformer “Transform” AC voltage
Step-up → voltage ↑
Step-down → voltage ↓
Does not work with DC
Turns ratio 𝑁 𝑆 : 𝑁 𝑃 (of wires)
Ideal: 𝑉 𝑠 = 𝑁 𝑆 𝑁 𝑃 𝑉 𝑃
Real: 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) [1];
Basic transformer showing wire windings and electrical-to-magnetic energy conversion [2].
[1] [CC] ZngZng
[2] [CC] BillC

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 value
Example: 120VAC to e.g. 9VAC
Reminder: VAC is the RMS voltage = 𝑉 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 2
So 9VAC is ≈12.7V amplitude

12. 1 AC-DC Converter: Rectifier
Input is AC: sine wave goes positive & negative
To rectify = to make one-sided
Output 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 gate
Ideal: a one-way “wire”
Non-ideal:
Constant voltage drop VD when passing forward current
Reverse leakage: small reverse currents possible (μA)
Different diode types: silicon, germanium, Schottky…
Better non-ideal model: exponential
𝑉 𝐷 = 𝐼 𝑆 𝑒 𝑉 𝐷 𝑉 𝑇 −1
Voltage increases a bit when current increases
Diode schematic symbol. Current can flow A→K, but not K→A.
𝑉 𝑇 ≈26𝑚𝑉 at 300K
𝑉 𝑇 = 𝑘𝑇 𝑞 ,
k = Boltzmann const,
q = electron charge

14. 1 AC-DC Converter: Diodes
Anode
Cathode
Diodes, oriented for current flow left to right. A line (black or silver) marks the cathode. a) DO-35 glass package [1]. b) DO-41 package [2]. c) Schematic symbol.
[1] [CC] Vonvon. [2] [CC] Vonvon.

15. 1 AC-DC Converter: Rectifier
Half-wave rectifier: one diode
Passes current one way only = no more negative voltage
“Blocks” half the sine wave: less available power!
Demo
TODO: Add sine graph
Half-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 diodes
Diodes force current into the + node and out of the – node for the sine wave (positive and negative)
Absolute value function
Diode directions
Full-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 filter
Big capacitor to store charge
Helps 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 ripple
Depends on capacitor size + load current
Need impractically big cap for high current + low ripple
Design: Given 𝑉 𝑚𝑎𝑥 ,𝑓, choose 𝑉 𝑟 , 𝑅 𝐿(𝑚𝑖𝑛) , and find 𝐶 needed
Average (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)
Ripple
𝑉 𝑟 = 𝑉 𝑚𝑎𝑥 1− 𝑒 − 1 2𝑓 ∗ 1 𝑅 𝐿 𝐶 for R load
𝑉 𝑟 = 𝐼 2𝑓𝐶 for constant I load
𝑓= AC frequency (60Hz here)

19. DESIGN: Full-Bridge Rectifier

20. 1 DESIGN: Rectifier Design Procedure
Choose V 𝑖𝑛 (AC), 𝑉 𝑟𝑖𝑝𝑝𝑙𝑒,𝑚𝑎𝑥 , 𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥
Choose diode and output capacitor
Thermal analysis: avoid going poof!
Done!

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 rectification
Depends on output capacitor / amount of ripple
Peak of output voltage “bumps” is 12.7V, average is less
𝑉 𝑟𝑖𝑝𝑝𝑙𝑒,𝑚𝑎𝑥 =1.0𝑉
This is big, but we’re learning regulators next!
𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥 =100𝑚𝐴
We 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 capacitor
Ripple equation (const current): 𝑉 𝑟 = 𝐼 2𝑓𝐶
𝑉 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 1000μF
For 25mA, 𝐶=208𝜇𝐹 → go for 220μF
Load: 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 model
Voltage = temperature(oC)
Current = heat output (W)
Thermal capacitance & resistance
Worst case (long-term):
DC steady-state
So ignore capacitors
Get 𝜃 𝐽𝐴 and max temp. from datasheet
𝑇 𝑗 = 𝑃 𝑗 ∗ 𝜃 𝐽𝐴 + 𝑇 𝑎𝑚𝑏
= 𝑉 𝐷 𝐼 𝐷(𝑚𝑎𝑥) 𝜃 𝐽𝐴 + 𝑇 𝑎𝑚𝑏
=0.75𝑉∗0.1𝐴∗100 °𝐶 𝑊 +27°𝐶
𝑇 𝑗 =34.5°𝐶
Max 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 analysis
Worst case scenario: is it well below the max temp?
Do we need to add heatsinking or cooling?
Forced air (e.g. fan) = lower temperatures
Should be measured in lab if a concern
More info:

27. 1 DESIGN Rectifier: Done!
𝑉 𝑖𝑛 =9𝑉𝐴𝐶, 𝑉 𝑜𝑢𝑡 ≈11.4𝑉 D1-4 = 1N4001, C = 1000μF or 220μF 𝑅 𝐿 = 100Ω or 470Ω 𝑉 𝑟𝑖𝑝𝑝𝑙𝑒,𝑚𝑎𝑥 =1.0𝑉 @ 𝐼 𝑚𝑎𝑥 =100𝑚𝐴

28. Let’s get building!

29. But first… what’s this breadboard thing?
Breadboard: Make connections without a mess!
No solder, no PCB design, no chemicals
Good for quick low-frequency prototyping
Lots 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 chemicals
Good for quick low-frequency prototyping
Lots 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 direction
Black line on glass body
Silver line on black body
The 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. Elcaps.jpg

33. Barrel connector X
Connect 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 Converter
The resistor simulates the circuit you want to power
Try different 𝑅 𝐿 and 𝐶 (R↓ or I↑ or C↓ → ripple ↑)
C = 1000μF or 220μF 𝑅 𝐿 = 470Ω or 100Ω
See the Handout

36. Break time! Want a snack? Wash your hands.
Lead-based solder at the lab.

37. Linear DC-DC Regulator2
Linear DC-DC Regulator
Image: LM2940L-5.0 LDO die photograph. Source: [CC] ZeptoBars,

38. 2 Motivation Electronics want a DC voltage
AC-DC converter has ripple noise
Causes signal noise, errors in digital data, IC resetting
Small ripple → huge, expensive capacitor
Other sources of problems
AC voltage maybe not constant (noise, surge/sag, etc.)
An existing DC supply is at wrong voltage or too noisy
Can we solve this any other way?

39. 2 Regulators Voltage regulator: Keeps output voltage constant
Uses a control system to read & adjust output
Input-output assumptions don’t affect accuracy
ELEC372 + ELEC level controls courses
Reacts 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 Resistor
Adjust resistance to keep output voltage constant
Output voltage function of 𝑉 𝑖𝑛 and 𝐼 𝑜𝑢𝑡
Can only reduce voltage
A 7805 (top) and LM137 TO-220 package. This package is often used for three-pin voltage regulators and high-current transistors. [CC] John Dalton 220_Package_Four_Different_Projections.jpg

41. 2 Linear Voltage Regulator
Advantages
Cheap, simple
Very 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

42. DESIGN: Linear regulator

43. 2 DESIGN: Linear Regulator
Design Procedure
Choose V 𝑖𝑛,𝑚𝑖𝑛 , 𝑉 𝑜𝑢𝑡 , 𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥
Choose a suitable linear regulator chip
Choose capacitors
Thermal analysis: avoid going poof!
Done!

44. 2 DESIGN Linear: 2 Specifications
Assume we need to power something with 5V
𝑉 𝑜𝑢𝑡 =5𝑉
Assume we have a ≈10 to 13VDC supply available
𝑉 𝑖𝑛,𝑚𝑖𝑛 =10𝑉
Assume our circuits will not use more than 80mA
To be safe, let’s add a 25% overhead
𝐼 𝑜𝑢𝑡,𝑚𝑎𝑥 =100𝑚𝐴
From datasheet: max current 1A → 100mA is OK
But check temperature later too!

45. 2 DESIGN Linear: 2 Choose a Chip
Common/”classic” chips
78xx family (e.g = 5V)
LM317 (adjustable output voltage)
Let’s choose the 7805 because fixed 5V is OK
Minimum voltage drop (“dropout”)
𝑉 𝑖𝑛 − 𝑉 𝑜𝑢𝑡 has to be bigger than minimum dropout
e.g. 7805: 𝑉 𝑑𝑟𝑜𝑝,𝑚𝑖𝑛 = 2V, 𝑉 𝑜𝑢𝑡 = 5V, so 𝑉 𝑖𝑛,𝑚𝑖𝑛 = 7V
We 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 possible
Smaller 𝑉 𝑖𝑛 − 𝑉 𝑜𝑢𝑡 (actual one, not the minimum) = more efficient & less heat
Look 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 divider
C1 filters any input noise
C2 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 frequencies
Typical circuit applications for a) 78xx [1]; b) LM317 [2].
[1] [O] LM7805 Datasheet (Fairchild Semiconductor). Educational use.
[2] [O] LM317 Datasheet (Texas Instruments). Educational use.

48. 2 DESIGN Linear: 3 Capacitors
Input capacitor
With AC-DC converter, filter cap is OK
Except if AC-DC and linear regulator far away (a few cm)
Generally, with “good” DC supply, 0.1–10.0μF
filter noise due to wire impedance/RF noise
Start with a value, and increase if needed
Also good to use datasheet or application note example circuits (see last slide)

49. 2 DESIGN Linear: 3 Capacitors
Output capacitor: general rule μF
Chip’s datasheet is a good place to start: 0.1μF
Filters noise and current transients
If sudden Δ𝐼 → helps in time before chip can react
Big cap = slow but filters big Δ𝐼; small = fast, filt small Δ𝐼
Simulation DEMO later
Change 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 )

50. 2 DESIGN Linear: 3 Thermal
Voltage drop
𝑉 𝐼𝑁 =12𝑉 (max)
𝑉 𝑂𝑈𝑇 =5𝑉
𝑉 𝑑𝑟𝑜𝑝 =12−5=7𝑉
Current output
𝐼 𝑜𝑢𝑡 =100𝑚𝐴 (max)
Power
𝑃=𝑉𝐼=7∗0.100=0.7𝑊
𝑇 𝑗 = 𝑃 𝑗 ∗ 𝜃 𝐽𝐴 + 𝑇 𝑎𝑚𝑏
= 𝑉 𝑑𝑟𝑜𝑝 𝐼 (𝑚𝑎𝑥) 𝜃 𝐽𝐴 + 𝑇 𝑎𝑚𝑏
=7𝑉∗0.1𝐴∗65 °𝐶 𝑊 +27°𝐶
𝑇 𝑗 =72.5°𝐶
Max temp is 125°𝐶 → OK

51. 2 DESIGN Linear: 3 Thermal
If 𝑇 𝑗 too high?
Add heatsink?
Add fan?
Heatsink: similar model
𝜃 𝐽𝐶 : from chip datasheet
𝜃 𝐶𝐻 : from thermal paste datasheet (if used) or heatsink datasheet or best guess
𝜃 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)

52. MAKE: Linear regulator!

53. But first: 7805 Pinout
Pinout 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.

54. 2 MAKE: Linear regulator!
See your handout

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 regulation
Input voltage down to 7V → output still 5V.
Minimum dropout
Below 7V input (2V dropout), loss of good line regulation.
Load regulation
With 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,

60. 3 Switching regulators Linear voltage regulators are inefficient
Reduce voltage by dissipating extra power as heat
What if we switch power on & off then filter it?
Switches are theoretically 100% efficient
Reminder: 𝑃 𝑠𝑤 = 𝑉 𝑠𝑤 𝐼 𝑠𝑤
When 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 1000W
A similar linear supply is:
Very hot, very heavy
Huge heatsink, xformer, fans
And 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 lightweight
Linear would be bigger, heavier and need cooling!
Not old DC wall warts
Heavy, blocky thing
Transformer, rectifier, filter
Not a switching supply
Various 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!
Disadvantages
More complex to design
Requires a minimum output load
Switching noise on the output (bad for precision circuits!)
EMI emissions

64. 3 Switching: Buck converter
Simplest switching DC-DC converter
Decreases DC voltage
Principle: turn input voltage on and off quickly
Output voltage = take the average output over time
Use PWM or PFM to change the average output voltage
Use a control system to get a desired output voltage
Use filtering to take the average (LC filter)

65. 3 Switching: Buck converter
10% average
50% average
90% average
PWM 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
VPWM
VOUT
VIN
A 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]

67. 3 Switching: Buck converter
Minimum load current is required
Inductor 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 inductor
Less switch off time
But more switching losses → lower efficiency
Same frequency, bigger inductor
Inductor holds more energy to release during switch off time
But bigger, heavier & more expensive

68. 3 Switching: Buck converter
If less than minimum load current
Good controller: works but less effective/efficient
Bad/no controller: 𝑉 𝑜𝑢𝑡 can go up to 𝑉 𝑖𝑛 & cause damage!
Reason #104 not to buy cheap knockoff phone chargers
How to ensure voltage is regulated?
Put a permanent resistor: always minimum current
Or put a “smart” load that turns on only when needed
Wastes power! (→ Why to unplug your phone charger!)

69. 3 Switching: Transistors
But wait! How do we make an electronic switch?
Transistors
Many 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 workshop
See ELEC311 and ELEC433
Aside 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.

71. DESIGN: Buck converter

72. 3 DESIGN: Buck converter
Design procedure
Choose a chip
Follow the chip’s design guide (datasheet)
Done!

73. 3 DESIGN: Buck converter
MC34063A: Old but reliable
Buck, boost, inverting
Cheap, easy to use, comes in DIP package
Less efficient, low frequencies
TI’s “Simple Switcher” series:
Five-pin chip, more expensive, efficient enough
Need 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…
Name
Value
𝑉 𝐶𝐸(𝑠𝑎𝑡)
1.0 V
𝑉 𝐹
0.4
𝑉 𝑖𝑛(𝑚𝑖𝑛)
10.0
𝑉 𝑖𝑛 𝑚𝑎𝑥
13.0
𝑉 𝑜𝑢𝑡
5.0
Name
Value
𝐼 𝑜𝑢𝑡 𝑚𝑖𝑛
10.0 mA
𝐼 𝑜𝑢𝑡(𝑚𝑎𝑥)
100
𝑓 𝑚𝑖𝑛
kHz
𝑉 𝑟𝑖𝑝𝑝𝑙𝑒 10 mV
VCE(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

78. 3 DESIGN: Buck converter
Now let’s do calculations!
𝑡 𝑜𝑛 𝑡 𝑜𝑓𝑓 = 𝑉 𝑜𝑢𝑡 + 𝑉 𝐹 𝑉 𝑖𝑛 𝑚𝑖𝑛 − 𝑉 𝑠𝑎𝑡 − 𝑉 𝑜𝑢𝑡 = −1−5 =1.35
𝑡 𝑜𝑛 + 𝑡 𝑜𝑓𝑓 = 1 𝑓 = 1 100× =10𝜇𝑠
𝑡 𝑜𝑓𝑓 = 𝑡 𝑜𝑛 + 𝑡 𝑜𝑓𝑓 𝑡 𝑜𝑛 𝑡 𝑜𝑓𝑓 +1 = 10𝜇 =4.255𝜇𝑠
𝑡 𝑜𝑛 = 𝑡 𝑜𝑛 + 𝑡 𝑜𝑓𝑓 − 𝑡 𝑜𝑓𝑓 =10𝜇−4.255𝜇=5.745𝜇𝑠
𝐶 𝑇(𝑚𝑎𝑥) =4× 10 −5 𝑡 𝑜𝑛 =4× 10 −5 ×5.745𝜇𝑠=0.230𝜇𝐹

79. 3 DESIGN: Buck converter
𝐶 𝑇 is our first component value! Let’s use 150pF
𝐼 𝑝𝑘 𝑠𝑤 =2 𝐼 𝑜𝑢𝑡(𝑚𝑎𝑥) =2×100𝑚=200𝑚𝐴
𝑅 𝑆𝐶 = 0.3 𝐼 𝑝𝑘 𝑠𝑤 =1.5Ω (safety: not critical to demo)
𝐿 𝑚𝑖𝑛 = 𝑉 𝑖𝑛 𝑚𝑖𝑛 − 𝑉 𝑠𝑎𝑡 − 𝑉 𝑜𝑢𝑡 𝐼 𝑝𝑘 𝑠𝑤 𝑡 𝑜𝑛 𝑚𝑎𝑥 =114.89𝜇𝐻 (wait!)
𝐶 𝑂 = 𝐼 𝑝𝑘 𝑠𝑤 𝑡 𝑜𝑛 + 𝑡 𝑜𝑓𝑓 8 𝑉 𝑟𝑖𝑝𝑝𝑙𝑒 =25𝜇𝐹 → 47μF

80. 3 DESIGN: Buck converter
𝑉 𝑜𝑢𝑡 = 𝑅 2 𝑅 1
Therefore 𝑅 2 𝑅 1 =3
Choose kΩ range
Closest common values: 𝑅 1 =10𝑘Ω, 𝑅 2 =3.3𝑘Ω
Needed because the controller tries to keep the “comparator input” pin at 1.25V
Therefore use resistive voltage divider: feedback
Get 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 cap
Long dist. → bigger cap or different parallel ones

85. Aside: Capacitors Here’s what happens with a buck converter
Lab PSU, 12VDC
~40cm of 18AWG wire
An hour wasted!
Figure is of VDD at input of chip
Top: No input cap, 4.8V to 17.8V ringing
Bottom: with 1μF, almost flat
[PD] Marc-Alexandre Chan. Original.

86. MAKE: Buck converter!

87. But first: MC34063A pinout For all DIP chips:
Pins numbered counterclockwise
Dimple shows chip orientation
AND/OR
Dot shows pin 1
MC34063A 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 converter
Uses energy storage of inductor
Buck: only as a filter
Boost: uses it to increase voltage!
Not “free” power
𝑃= 𝑉 i𝑛 𝐼 𝑖𝑛 = 𝑉 𝑜𝑢𝑡 𝐼 𝑜𝑢𝑡 − 𝑃 𝑙𝑜𝑠𝑠
If 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 output
Example: Joule Thief
𝑉 𝑜𝑢𝑡
Fig. 33 a) Minty-Boost [1]; b) Joule Thief [2]
[1] [CC] Adafruit [2] [CC] Electron9 File:Joule_thief.png

92. 3 Switching: Boost converter
𝑉𝑃𝑊𝑀,𝑏𝑜𝑜𝑠𝑡𝑒𝑑
𝑉 𝑜𝑢𝑡
Theoretical boost converter circuit.
[PD]

93. 3 Switching: Boost converter
Current in inductor must be continuous
More important than buck converter!
Means it can never go down to 0A
Consequence again: Minimum output current
To select inductor, given a minimum operational load:
𝐿 𝑚𝑖𝑛 = ( 𝑉 𝑜𝑢𝑡 − 𝑉 𝑖𝑛 )(1−𝐷) 𝑓 𝑠𝑤 𝐼 𝐿,𝑚𝑖𝑛
To determine the theoretical output voltage:
𝑉 𝑜𝑢𝑡 𝑉 𝑖𝑛 = 1 1−𝐷
If not continuous: lower voltage than expected!

94. 3 Switching: Boost converter
If discontinuous (current = 0 at some point)
Inductor “used up” all its stored energy
So the “flyback” doesn’t last entire switch-off time
Voltage 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. MAX1595
Can be used to increase or invert voltage
Principle: capacitor stores charge
Charge cap, ↑voltage on “low” side, “high” side is ↑ too!
Advantages: Caps cheaper/smaller than inductors, simple circuit, efficient, low EMI
Disadvantage: Low currents only
More 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 changes
When turning on
When output current changes
When input voltage changes
Transients can go above desired output voltage
Depends 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 devices
Usage details outside of scope of workshop
Critical to good & safe designs!
Fuse: physically open-circuits if current too high
Circuit breaker: like fuse but resettable
PTC: automatic resetting “fuse”
Higher temperature = becomes very high resistance
MOV: metal oxide varistor
Short-circuits if voltage above a certain value

99. 5 More: Isolation and safety
Safety design features: choice of topology
Isolated switching power supply
Output “floating” relative to input voltage: no common ground
Less dangerous to interconnect different devices/equipment
Protect 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 circuits
Maintain isolation for an isolated switching supply’s feedback
Current limiting/short-circuit protection

100. 5 More: Switching regulators
Better/safer topologies of switching regulators
Isolated switched-mode power supplies
Flyback, RCC, Half-Forward, Forward, Resonant Forward, Push- Pull, Half-Bridge, Full-Bridge, Isolated Ćuk
Forward converter used in phone chargers
Uses transformers (high frequency = small, lightweight, cheap)
Also more non-isolated SMPS
Split-pi, Ćuk, SEPIC, Zeta
Wall-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 rectifier
Full-wave rectifier using thyristors (SCRs)
Controllable diodes (control turn-on)
AC-DC with output voltage control
Can be used instead of a transformer
Needs better filtering, more complicated (control circuit)
Animation of changing output voltage with thyristors:

102. 5 More: AC-AC Converters
AC-AC regulator using a TRIAC
TRIAC: Bi-directional thyristor
AC-AC voltage control (not frequency)
Can “cut off” part of AC wave (voltage ↓)
DC-AC inverter: transistors
High-frequency PWM sine wave + filter
Control AC voltage and frequency
“H-bridge” circuit can be used for this
See ELEC433
TRIAC regulator: synchronous or mains motor control; light dimmer
Inverter: induction motor control
TRIAC symbol. [PD] No author.

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 LEDs
Simple up to 1A: LM317-based circuit
[CC] SilverSrv. 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.

105. 6 Applications to Real-World PSUs
Combine these converters and regulators together
Often need several voltages: e.g. 3.3V, 5V, 12V
May require low noise output (SMPS  too noisy?)

106. 6 Applications to Real-World PSUs
Switch-mode power supply (no safety/isolation)
Rectifier +
Filter
12V Buck Regulator
5V Linear
Regulator
7V Buck Regulator
Inverting
–5V Linear
Regulator
3.3V Linear
Regulator
Conceptual 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.
[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 supply
Load and line reg., transient response, min/max load, cooling requirements, protection measures etc.
Understanding the closed-loop controller
Control 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!