AERO
ASTRO
16.682 - Prototyping Avionics
Spring 2006
LECTURE 5
February 22, 2006
DEPARTMENT OF AERONAUTICS AND ASTRONAUTICS
Alvar Saenz-Otero
Outline
•
Power Regulation
– Transformers
– Rectifiers
– Linear Regulators
– Switching Regulators
•
How much power?
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16.682 - Prototyping Avionics
Transformers
• Exchange voltage for current to reduce potential difference
(voltage) across two points
– The difference in number of turns between the two windings determines
the ratio of voltage input to voltage output
v1 N1
=
v2 N 2
i1
N
= − 2
i2
N1
v1
i
=− 2
v2
i1
i1
i2
T
+
+
v1
v2
N1
N2
– Note: no conductive element between v1 and v2!
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16.682 - Prototyping Avionics
Transformers: Multiple Types
•
+
Single-tap
– One voltage output
+
T
V+
v1
Vpos
v2
-
•
T
Multiple-taps/windings
V
N1
N2
Single Tap
– Provide multiple voltage outputs
Vneg
– Multiple taps
V+
• Must share common point
+
– Multiple windings
• Can use in series (like a tap) to get
multiple voltages or in parallel to
increase current output
T
+
v2
v1
vref
-
v3
-
T
Vpos
V
V0
Vneg
Center Tap
Two Windings
configured for +V & -V
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16.682 - Prototyping Avionics
Rectifiers
• DC vs AC
V
– DC = Direct Current: used in most electronics & batteries
1.5
1
+V
– AC = Alternating Current: used during common electricity 0ac
generation (power plants, motors with rotating magnets) -Vac
0.5
0
0
2
4
6
8
10
12
14
t
-0.5
-1
-1.5
• Preferred method to transmit over long distances
Note: Volts AC is
given in RMS value,
which means the
peak is at
1
V pp =
Vac
0.707
• For DC projects must convert AC to DC: use rectifiers
Vac+
T
D
Vac+
T
D
Vdc
Diode
Vd
AC Voltage Plot
Vac-
VacGND
Half Wave Rectifier
Full Wave Bridge
– Many types:
GND
V
+Vdc
• Most common: half wave and full wave
0
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t
16.682 - Prototyping Avionics
Full Wave Rectifier
• A full-wave bridge rectifier effectively provides abs(Vac) at the output
– A capacitor “flattens” the sinusoidal AC signal for a relatively flat DC output
– The output will always have some ripple, must use a regulator to fully flatten the
signal
D
Vdc
+
T
1.2
1.2
D
1
1
1
0.8
0.8
0.6
0.6
0.4
Vpos
0.2
C2
1.2
0.8
0.4
0.6
0.2
+
0.4
0
0
0
2
4
6
8
10
12
14
0
-0.2
2
4
6
8
10
12
14
0.2
-0.2
0
0
-
vac>0
2
4
6
8
10
12
GND
14
vdc>0
Capacitor
GND
T
D
1.2
0.2
0
0
2
4
6
8
10
12
Vpos
14
-0.2
1
0.8
-0.4
0.6
-0.6
0.4
-0.8
0.2
-1
1.2
0
+
-1.2
1
0
2
4
6
8
10
12
14
-0.2
0.8
0.6
0.4
vac<0
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GND
vdc>0
6
0.2
0
0
2
4
6
8
10
12
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16.682 - Prototyping Avionics
Linear Regulators
•
Regulate an input voltage to:
U
Vin
1
– Reduce voltage to exact needs
Vou
IN
C1
C2
2
– Provide constant output
OUT
GND
3
• No ripple
• Regardless of load (even changing load)
•
GND
Simplest design of all regulators
– Only needs capacitors as external components, no high-frequency elements
•
Input voltage (abs) MUST be higher than output voltage (abs)
– Linear regulators cannot increase or invert voltage
•
Power lost in regulator is linear WRT current pull and voltage drop (Vin-Vout)
– Useful for small currentand/or small voltage drop
•
Efficiency is linear WRT the voltage drop (V in -Vout)
– Most efficient with small voltage drop...
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16.682 - Prototyping Avionics
Linear Regulator Model
•
The simplest model of a linear regulator is a variable resistor:
iout =
Rreg
Vin
Vou
vout
Rload
iin = iout
Rload
Preg = iout (vin − vout )
eff =
GND
Pout
iout vout
v
=
= out
Preg + Pout iout (vin − vout ) + iout vout vin
– The effective resistance of the circuit, Rload, determines the necessary current
• The input current must be the same as the output current
•
Example:
– Vin=7V, Vout=5V, i=100mA
• Preg = 0.2W, Ptot = 0.7W,, eff =71% reasonable use, no heatsink
– Vin=7V, V out=5V,, i=1A
• Preg= 2W, P tot= 7W, eff=71% NEED heatsink to dissipate 2W!
– Vin=20V,Vout =5V, i=100mA
• Preg= 1.5W, P tot = 2W, eff=25% UNreasonable use, a lot of power wasted, low efficiency!
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16.682 - Prototyping Avionics
Switching Regulators
VCC(15V)
•
Regulate an input voltage to:
C17
4.7uF
C18
0.47uF
C19
4.7uF
– Decrease, increase, and/or Q1
Current
“switch”
NDS9407/SO
7
• Can only increase or
decrease voltage, not both
2
– Practically get rid of ripple
•
Complex design
3
8
10
VH
4
voltage differentials
IN
U6
Allow for large input/output
3/5
L1
EXT
9
D1
Inductor
EC2IQ506
22uH
/SHDN
Regulator CS
VL
Controller
OUT
REF
6
5
R8
40m
Feedback
Resistor
VCC(3.3V)
GND
•
Input Caps
MAX1744
C20
4.7uF
C21
0.1uF
C22
220uF
1
invert a voltage – Uses many external components
•
Output
Capacitor
Design uses an inductor to create the necessary voltage by driving current through it
at high frequencies (~100-500kHz)
– Utilizes feedback of both current (through sense resistor) and voltage (at output)
– Output voltage has small noise at operating frequency, usually insignificant
•
Usually 80-95% efficiency, dependent on current pull
pul
– Driving the high-frequency signal requires a minimum constant power input, even if the
output is disconnected (no current)
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16.682 - Prototyping Avionics
Switching Regulator Model
• No simple model
– It is a feedback loop where the sensor drives the “current switch” as necessary
• For design purposes can assume that power in equal power out, plus the
efficiency factor
– For design one must know the necessary current supply for Vin
viniin = Pin = Pout = vout iout
iin =
vout iout 1
⋅
vin eff
• Example: from datasheet, since the model of
a switching regulator is not always the same
– Note different curves for different input voltages
– Note very low efficiency for low output currents
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16.682 - Prototyping Avionics
How much power do you need?
•
Remember it is very simple:
– P = IV, always as far as we’re concerned
– Power can be added
• If you know the power of individual components, and are given the voltage &
current for others, add the power together!
– Account for regulator efficiency
• Select the best type of regulator for your needs, consider both design (linear
is simple) and efficiency/power dissipation (usually switching is better)
– Allow for at least 20% margin
• After considering regulator efficiency
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16.682 - Prototyping Avionics