Contents
Contents ................................................................................................................................................................... 2
Nomenclature ........................................................................................................................................................... 3
Overview................................................................................................................................................................... 3
Design ...................................................................................................................................................................... 3
Testing and Results ................................................................................................................................................... 6
Future Work and Improvements .............................................................................................................................. 8
References ................................................................................................................................................................ 8
Torque Coils’ Power
Supply Regulator
Spring 2008
This document explains the goals and functionality of the torque
coil power supply regulator. Design constraints, system design,
and future work are also discussed.
Wahhaj Irfan
05/08/2008
CUBESAT PROJECT
TORQUE COIL POWER REGULATOR, SPRING 2008
Nomenclature
EAGLE Easily Applicable Graphical Layout Editor: CAD
program used for creating electrical schematics and
printed circuit board layouts.
Li-Poly Lithium-polymer
MPPT Maximum Power Point Tracking (for solar cells)
PCB Printed Circuit Board
RMS Root Mean Square
Overview
The scope of this task is to supply variable current to the torque coils in the satellite. This is
accomplished by utilizing the provided input from the battery input voltage (6.0 V to 8.4 V) and
routing it to the torque coils with circuitry that steps it down if need be and converts it to a variable
current output theoretically from 0-200mA (but practically this current range will be 20-200mA).
Torque coils are a length of copper wire wound around like coils. The copper thickness, width,
spacing and the number of coils are parameters which are optimized and chosen based on the design.
There are three coils located on the three different axes of the satellite. The goal is to supply each coil
with a known current and consequently a known moment. So at a given instance, the three coils will
have different currents going to them and producing separate magnetic moments which add up to
give an overall moment vector which stabilizes the satellite.
Figure 1: Red arrows will be the effects of the magnetic moments [2]
Design
Linear’s LTC1779 step down converter is the main part of the design. The LTC1779 is a
constant frequency step-down DC/DC converter. The part’s operating voltage range is 2.5-9.8V which
is within the battery voltage range of 6.0-8.4V. The part is available in 1mm 6-lead ThinSOT package,
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so it will not take too much real estate on the final board. A typical application for this part is show
below:
Figure 2: Typical application of an LTC1779 chip [1]
In the figure above, the typical application of the LTC1779, the output is a constant voltage
and current of 2.5V and 100mA respectively. One of the most important pins in a regulator is the
feedback pin, in this case, it is pin 3. It receives the feedback voltage from an external resistive divider
across the output. The resistors are the 169k and 78.7k in the figure above and R2 and R1 in the
equation below. Typically, the LTC1779 functions as a variable voltage mode regulator meaning that it
supplies regulated output voltage determined by:
R2
Vout0.81 (1)
R1
For most applications an 80k resistor is used for R1 and R2 is chosen based on output. To
convert this typical application into a variable voltage mode regulator, a digital resistor is added in
parallel to the divider or in place of one of the divider resistors. The AD5246 was used as a digital
resistor. The AD5246 is a 128 step 50k-ohm digital resistor that can be controlled via I2C.
Figure 3: Digital resistor across R2
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Figure 4: Schematic for the power supply design
The right side of the above schematic shows the H-bridge. An H-Bridge controls the direction of the
current. The magnetic moments generated can be both negative and positive.
Figure 5: PCB layout
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Part Number in Eagle Schematic Part Selected Package Selected Part Value
U1 LTC1779 SOT23 --
U3 AD5246 SC-70 --
L1 Inductor DR73 22 µH
D1 Schottky Diode SMB --
C1 Low ESR Tantalum Capacitor C0603 100 pF
C2 Low ESR Tantalum Capacitor C0603 0.1 µF
C3 Low ESR Tantalum Capacitor C0603 10 µF
C4 Ceramic Capacitor C1206 47 µF
C5 Low ESR Tantalum Capacitor C0603 0.1 µF
R1 Surface Mount Resistor 1206 20 kΩ
R2 Surface Mount Resistor 1206 10 Ω
R3 Surface Mount Resistor 1206 412 kΩ
R4 Surface Mount Resistor 1206 82 kΩ
R5 Surface Mount Resistor (sense) 2512 0Ω
R6, R9 Surface Mount Resistor 1206 20 kΩ
R7, R8 Surface Mount Resistor 1206 100 kΩ
Q1, Q2, Q4, Q5 N-Channel MOSFET SOT23 --
Q6, Q7 P-Channel MOSFET SOT23 --
JP1 Jumper (Pin Header) 1X03 --
X1, X3 DF-11 Connector DF11-4DP-2DSA --
X2 DF-11 Connector DF11-8DP-2DSA --
Table 1: Parts List
The above table lists the parts used in building the board.
Testing and Results
Harmonics testing results:
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TORQUE COIL POWER REGULATOR, SPRING 2008
Efficiency plot
Efficiency Graph
100.0
90.0
80.0
70.0
60.0
Efficiency
50.0
40.0
30.0
20.0
10.0
0.0
165.2 164.9 164.7 164.6 164.5 164.2 164.1
Load Current (mA)
Future Work and Improvements
As far as the H-bridge is concerned, it was successfully implemented this semester. The demo showed that if
flipped the direction of current upon the switching of the fets.
The present plan did not meet the design goals fully as it was only able to implement a regulator that would
supply 0-200 mA to a 22-30 ohm fixed load (in our case the torque coils). If we need an output voltage of up to
5 volts we are fine but if we need any larger value then the digital resistor will not be able to handle the voltage
across it. This is proving a bottle neck for this design. A design needs to be implemented that takes into account
all these factors and is able to overlook the fact that the digital resisitor can only have 5 volts across it. Another
alternative would have been to look for digital resistors that can have more than 5 volts across its terminals but
all i2C digital resistors in the market seem to cap at 5.5 volts. So this is not a viable option.
So the best future concept will be one which takes these factors into consideration and be able to supply 0-
200mA to the fixed load (torque coils) using an output of upto the battery voltage (8.4V).
References
1 Linear's LTC 1779 datasheet.
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1032,C1064,P1985
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2 ION1 Torque Coil page
http://cubesat.ece.uiuc.edu/Attitude_Control.html
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