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Embedded Systems Design ELEN90066
Lecture 5 Power Supply
David Jahshan
Note from last lecture • Designators
– Are unique identifiers that identify each component.
– Assist in conveying information about the location of the components on the PCB.
• Today we will look at the first component and the underlying theory of its operation.
Project Specification
• 1. Power
• 1.1 The device will be powered from one 1.5v
AA battery.
• 1.1.1 The battery holder will be mounted on the PCB.
• 1.2 The device will boost the battery voltage to 3.3v using a boost converter.
How do we convert 1.5v to 3.3v?
• With a boost converter, but first we need a bit of background.
– Voltage divider
– Voltage reference
– Linear voltage regulators – Buck converter
– Boost converter
Voltage divider
• Two resistors back to back • Vout = Vin(R2/(R1 + R2))
• Voltage varies on Vout Load
Image: Wikipedia Creative Commons
Zener diode as a voltage reference
• Zener diodes permit current when a breakdown voltage occurs in the reverse direction.
• Current through load is limited by current through zener.
• Idiode = (Vin-Vout)/R Image: Wikipedia Creative Commons
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Linear Regulator
• Uses a transistor in emitter follower configuration.
• Vout = Vin – diode drop
• Voltage drop is across the transistor.
Image: Wikipedia Creative Commons
NPN
4 Basic Rules of NPN BJTs
1) Collector must be more
positive than emitter.
2) Base to emitter and base to collector circuits behave like
diodes.
3) You should not exceed the
transistors IC, IB, VCE, and
other maximums (ie temp)
4) When rules 1 to 3 true
transistor is in active region and IC is approximately hFEIB where hFE is the current gain (AKA beta ß)
Linear Regulator
• Linear regulation is not very efficient. • VinIin = VoutIout + Pheat
• What is the solution
– When a transistor (be it BJT or MOSFET) is off, no
power dissipation.
– When a transistor is on, little power dissipation.
• What if we can regulate power with on/off pulses?
Pulse Width Modulation
• The area under the PWM signal is the power out.
• A filter will smooth out the signal so that only the average voltage is at the output.
Image: Wikipedia Creative Commons
Buck Converter
• Uses the PWM principle to regulate output. • Pout = Pin x Efficiency
• Efficiency typically greater than 80%
Boost Converter
• Current through an inductor can not change instantly.
• What happens if you open a switch on an ideal inductor?
• Infinite voltage spike
• If you filter these spikes you can make voltage increase.
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Boost Converter • We will be using the SC120.
• Lets have a look at the datasheet.
Next Lecture
• We will look at the other components.
• Lab for this week is to start your design for the assignment.
– Read the data sheets of the components, see if you can work out how to use the devices.
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