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Team 1: Grant Trudel

Regulators

� Definition � Purpose � How they work �  Types � Questions

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Regulators - Definition � As its name implies, a regulator is meant

to regulate something: provide a constant flow

�  In our case, a voltage regulator strives to keep a constant level or flow of voltage

� Can be used to regulate one or more alternating current (AC) or direct current (DC) voltages

� We will look at direct current regulators

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Regulators - Purpose

�  This is an integrated circuit voltage regulator

� Used to stabilize the DC voltages used by the processor and other elements in the computer

�  In our case in class, it provides a stable voltage output to our breadboard components of either 3v or 5v (see https://www.sparkfun.com/tutorials/57)

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Regulators - Purpose �  Basic regulator

configuration �  9v in, 5v (or 3v) out �  Linear regulator

�  Built around a large transistor controlling output

�  Extra voltage converted into heat

�  Replaced by a switching power supply in phone chargers and computer power supplies

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Regulators – How They Work

1.  Bandgap reference circuit 2.  7805 (LM109-derived: a rare model) (1) 3.  7805 (a more common design) (2)

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Regulators - Bandgap

Voltage that goes

down

Voltage that goes

up

Voltage that is stable

Temperature Changes

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Base-Emitter Voltage vs Temperature

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1.25 V

0 V 0o K 300o K Room Temperature

Fix Ic

Adjust VBE

Put a constant current, Ic, through a transistor and look at the voltage between the base and emitter, VBE


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1.25 V


Transistor with high current

Transistor with low current

VBE

VBE

ΔVBE

Set up a second transistor with a lower current in which VBE will drop faster as temperature increases


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1.25 V


Transistor with high current

Transistor with low current

VBE

VBE

ΔVBE

Notice how the difference between the 2 voltages increase as the temperature increases



1.25 V


VBE

VBE ΔVBE

Constant Bandgap Voltage

nΔVBE

Combining the VBE with the appropriate number, n, of ΔVBEs, we get a constant result, the constant bandgap voltage, regardless of temperature

Regulators – How They Work (1)

�  Bandgap (yellow) keeps the voltage stable 2/5/15 Regulators 12

Q = transistor R = resistor


�  It takes the scaled output voltage as input (Q1 and Q6) and provides an error signal (to Q7) if voltage is too high or low




�  The error signal from the bandgap reference is amplified by the error amplifier (orange)




�  The amplified signal controls the output transistor through the large driver Q15.




�  The startup circuit (green) provides initial current to the bandgap circuit, so that it does not get stuck in an off state.




�  The circuit in purple provides protection against overheating (Q13), excessive input voltage (Q19), and excessive output (Q14).




�  If there is a problem, these circuits reduce the output current or shut down the regulator to protect it from damage. 2/5/15 Regulators 18


Regulators – How They Work (1) �  Large VBE Q3 �  Another large VBE across Q2 �  Drops of small VBE Q4 & Q5 �  2ΔVBE R6 �  Current through R7 = current

through R6 �  R7 = 16.5 x R6 so by Ohm’s law,

voltage across R7 = 16.5 x R6 = 33 ΔVBE

�  Voltage goes up by VBE across Q8 �  Another VBE across Q7 �  Then R7 voltage �  And another VBE across Q6 �  Final voltage will be 3 bandgap

voltages, or 3.75V



�  Large VBE (red arrow) Q1

�  Small VBE (green arrow) Q2

�  ΔVBE R3 �  Generates a current

R3, Q2, R2 �  R2 = 20 x resistance

R3 so 20 ΔVBE appears across R2


Regulators – How They Work (2) �  Follow blue arrows up

from the ground: Q3, Q4, R2, Q5, Q6

�  Σ voltages = 4VBE + 20ΔVBE.

�  There are 4 VBEs so circuit must be 4 x bandgap voltage (approx. 5v = stable point)

�  Amplifying transistors (Q4/Q3) will respond to any variation


Regulators – Types Linear Switching

Best when output noise is low Best when power efficiency is critical (e.g. in portable computers)

Best when fast response to input and output disturbances are required

Required when only power supply is DC voltage and a higher output voltage is needed

Cheaper More expensive (except at high levels of power: above a few watts)

Occupy less circuit board space Occupy more circuit board space


Regulators – Types (Combination or Hybrid)

Switching Regulator Linear Regulator Power Supply

Exact voltage with nearly all noise eliminated


Questions?


References

�  http://www.righto.com/2014/09/reverse-engineering-counterfeit-7805.html

�  http://en.wikipedia.org/wiki/78xx �  http://en.wikipedia.org/wiki/

Voltage_regulator �  http://www.mccsemi.com/up_pdf/

MC7805CT%28TO-220%29.pdf


team 1: grant trudel - clemson university · team 1: grant trudel . regulators ! definition !...

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