279 lines
15 KiB
TeX
279 lines
15 KiB
TeX
%! TEX root = ../thesis.tex
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\chapter{Theory}
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This Chapter will be discussing the fundamental principles used in the experiments. These will contain simplified circuits and their respective Equations as well as component behavior as specified in their respective datasheets by their Manufacturer
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\section{Hardware Component Behavior}
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Before discussing the experimental results it needs to be clear what circuitry is used in these experiments and what behavior we expect.
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Keeping in mind, that these are theoretical values and will most likely not be exactly the same as those found in actual hardware, as all values given will always be within some error as defined by their manufacturer.
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Each of the three voltage regimes that will be observed on the PowerIt Board, 48V 9.6V and 1.8V, has a voltage- and in the cases of 48V and 1.8V also a current-measurement circuit. Additionally we have a temperature sensor built into the STM32 Chip.
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\subsection{48V Input Voltage}\label{sec:mon48v}
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\begin{figure}[H]
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\centering
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\includegraphics[width=.9\textwidth]{./tikz/mon48v.pdf}
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\caption{Circuit for measuring the 48V input Voltage, consisting of input potential (left), two resistors as voltage divider, one fully differential isolation amplifier (left), one operational Amplifier (right), output voltage as well as the connection to the STM32-Chips input pin (right)}%
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\label{mon48v}
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\end{figure}
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The circuits for measuring input voltage and current are the most complex, because for Voltage measurement the circuit needs to
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\begin{itemize}
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\item divide our input voltage into a usable potential range
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\item decouple the input (48V) from signal potential (3.3V)
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\item amplify the voltage, to be in the STM32-Chips Voltage range of 0--3.3V
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\end{itemize}
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The already implemented Cicuit can be seen in \autoref{mon48v}.
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It consists of a 1:240 Voltage Divider, a full differential isolation amplifier taking in the roughly 200mV (nominal voltage range), and amplifying it by a factor of 8 (\(r_\text{diffOpAmp}\)~\cite{diffopamp}).
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It is also decoupling the input and output voltages, so our 48V and 3.3V circuit parts are electricly insulated.
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The remaining operational amplifier provides difference to single ended Conversion with an amplification or 1.1 (\(r_\text{OpAmp}\))
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This circuit results in the following equation for calculating the input voltage from a pin voltage:
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\begin{align}
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V_\text{48V in}\cdot\frac{R_1}{R_1+R_2} \cdot r_\text{diffOpAmp} \cdot r_\text{OpAmp} =&~V_\text{MONITOR\_48V}\nonumber\\
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\Leftrightarrow \quad \frac{V_\text{MONITOR\_48V}}{r_\text{diffOpAmp}\cdot r_\text{OpAmp}}\cdot\frac{R_1+R_2}{R_1} =&~V_\text{48V in}
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\intertext{and the extremes, when assuming 48V\(\pm 10\%\) are}
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V_\text{MONITOR\_48V, min} = \SI{43.2}{\volt}\cdot\frac{1}{240+1}\cdot 8\cdot 1.1 =&~\SI{1.5774}{\volt}\\
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V_\text{MONITOR\_48V, max} = \SI{52.8}{\volt}\cdot\frac{1}{240+1}\cdot 8\cdot 1.1 =&~\SI{1.9280}{\volt}
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\end{align}
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\subsection{48V Input Current}
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\begin{figure}[H]
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\centering
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\includegraphics[width=.9\textwidth]{./tikz/mon48i.pdf}
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\caption{Circuit for measuring the 48V input Current, consisting of the powerit Input Circuit, one shunt-resistor, one full diff isolating Amplifier, one operational amplifier, output potential, as well as the connection to the STM32-Chip input pin}%
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\label{mon48i}
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\end{figure}
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%In case of the current measurement circuit we require the following:
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THe circuit has to satisfy the following constraints:
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\begin{enumerate}
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\item use a shunt resistor, with minimal heat dissipation
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\item still providing a good resolution also within the STM32-Chips Specifications
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\end{enumerate}~\\\\
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To accomplish that, the circuit is measuring the voltage over a small (\(\SI{500}{\micro\ohm}\)) shunt Resistor, while a current is flowing, resulting in a proportionality, by Ohms Law. The obtained Voltage is then decoupled and amplified by a factor of 8, as well as converted from a difference to single ended Voltage, with a amplification factor of 1.1.
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Here we use the same Amplifiers as in \autoref{sec:mon48v} and so we can use the following equation for our input current:
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\begin{align}
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I_\text{48V IN}\cdot R_\text{shunt} \cdot r_\text{diffOpAmp} \cdot r_\text{OpAmp} = V_\text{48I pin}\nonumber\\
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\Leftrightarrow\quad \frac{V_\text{48I pin}}{R_\text{shunt}} \cdot \frac{1}{r_\text{diffOpAmp}\cdot r_\text{OpAmp}} = I_\text{48V IN}
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\end{align}
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And applying the extrema of 0A and 41.7A, a voltage range from 0V to:
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\begin{align}
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\SI{41.7}{\ampere}\cdot \SI{500}{\micro\ohm} \cdot 8\cdot 1.1 =& \SI{0.1833}{\volt}
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\end{align}
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can be observed.
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\subsection{9.6V Output Voltage}
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This Circuit consists of a a simple 1:3 Voltage Divider.
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\begin{figure}[H]
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\centering
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\resizebox{.55\columnwidth}{.12\paperheight}{%
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\begin{circuitikz}[scale=2]
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\draw[color=black, thick]
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(0,0) node[left]{GND}
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to [short, o-] (1,0)
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to [R, l_={R1 1k}] (1,1)
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(0,2) node[left]{9.6V}
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to [short, o-] (1,2)
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to [R, l={R2 3k}] (1,1)
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to [short, *-] (3, 1) node[right, draw=black] {MONITOR\_10V};
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\end{circuitikz}
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}
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\end{figure}
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And the following equation can be used to calculate that voltage:
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\begin{align}
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V_\text{10V PIN} = \frac{V_\text{10V IN} \cdot R_1}{R_1 + R_2}
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\end{align}
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\subsection{1.8V Output Voltage}
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To measure this Voltage the output is directly connected to a pin on the STM32-Chip.
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Until now the voltages and current could only be measured, now the mechanism for setting a resulting Voltage at the 1.8V Terminals is known.
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\autoref{fig:gen18v} is the circuit for generating 1.8V.
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It consists of a power module and a resulting resistance between two pins, defined by R$_\text{series}$, R$_\text{parallel}$ and R$_{pot}$.
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The Resistances job is to set the output to a given voltage of around 1.8V, but we can vary R$_\text{pot}$, because this resistance is set vie a digital potentiometer.
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\begin{figure}[t]
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\centering
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\includegraphics[width=.55\textwidth]{./tikz/gen18v.pdf}
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\caption{1.8V supply circuit, featuring a DC-DC Converter, a resistor chain, supply voltage (left) and resulting voltage (right)}%
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\label{fig:gen18v}
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\end{figure}
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The in \autoref{fig:gen18v} used 1.8V Converters have a characteristic formula~\cite{pth08t}, and the in this circuit used Potentiometer is a linear 10k$\Omega$ Rheostat resulting in the following equations:
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\begin{align}
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R_{potentiometer} =& P_{val} \frac{10k\Omega}{256} \label{eq:rpot}\\
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R_{SET} =& 1 / \left(\frac{1}{R_{potentiometer}} + \frac{1}{R_{parallel}}\right) + R_{series}\nonumber\\
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=& \frac{R_\text{potentiometer}\cdot R_\text{parallel}}{R_\text{potentiometer} + R_\text{parallel}} + R_\text{series}\label{eq:rset}\\
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V_\text{MONITOR\_1V8} =& \frac{30.1 k\Omega}{R_{SET} + 6.49 k\Omega} \cdot 0.7V + 0.7V\label{eq:vout}
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\end{align}
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%This equation is in contrast to all previous behavior models not of a linear nature but proportional to $\frac1x$ as visualized in \autoref{fig:beh1v8}
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\autoref{eq:vout} is visualized in \autoref{fig:gen18v}.
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Note that this Equation is proportional to \(\frac 1x\)
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\begin{figure}[H]
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\centering
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\hspace*{-.165\textwidth}
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\includegraphics[width=1.3\textwidth]{./data/theory/v18.pdf}
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\caption{Expected behavior of our output voltage by setting the potentiometer}%
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\label{fig:beh1v8}
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\end{figure}
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\subsection{1.8V Output Current}
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The circuit for measuring the outgoing current over 1.8V, consists of a current sensing IC, which is Hall sensor based. Each connection (digital and analog) has this ic in series to its load.
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\begin{figure}[H]
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\centering
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\resizebox{.55\columnwidth}{.12\paperheight}{%
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\begin{circuitikz}[scale=2]
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\draw[color=black, thick]
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(0,0) node[left]{GND}
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to [short, o-] (1,0)
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to [] (1,.5)
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(0,2) node[left]{1.8V}
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to [short, o-] (1,2)
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to [] (1,1.5)
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(1.2,1) node[draw=black, regular polygon, regular polygon sides=4, minimum size=2.7cm]{acs758}
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(1.7,1)
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to [short, *-] (3, 1) node[right, draw=black] {VDD\_1V8\_*};
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\end{circuitikz}
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}
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\caption{1.8V current sensing circuit, featuring a acs758, hall sensor based current sensing IC, input voltage (left) and output voltage (right)}%
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\label{fig:mon18i}
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\end{figure}~\\
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The IC is rated for a maximum constant current draw of 100A, and has the following behavior:
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\begin{align}
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I_\text{1.8V, in} \cdot \SI{0.004}{\volt\per\ampere} + \SI{0.12}{\volt} =&~V_\text{MONITOR\_1I8\_*}
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\intertext{which gives for the extremes:}
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\SI 0\ampere \cdot\SI{0.004}{\volt\per\ampere} + \SI{0.12}{\volt} =&~\SI{0.12}{\volt}\\
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\SI{100}{\ampere} \cdot\SI{0.004}{\volt\per\ampere} + \SI{0.12}{\volt} =&~\SI{0.52}{\volt}
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\end{align}
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\section{ADC Calibration}
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As mentioned beforehand, the actual hardware will differ in behavior from its theoretical counterpart.
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Therefore it is possible to say that all signals with a significant difference of behavior ($\approx 5\%$) will need to be corrected.
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To calibrate these readouts we need to employ some simple actions.
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\subsection{serial ADC readout}\label{sec:adc}
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The measurements will be done by the STM32-Chip, which uses 12bit ADCs.
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A single ADC will be switching between all connected pins.
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This Behavior can be problematic in regards to measuring accurately.
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The timing used to measure a single pin can be programmatically set from 3 up to 480 clock ticks\footnote{this clock is the internal adc clock, with a frequency of \SI{159}{\hertz}}
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\section{1.8V Output Regulation}
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For regulating the Output the method used is a relatively simple one.
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The voltage, wanted at the output terminal, will be calculated and then a corresponding potentiometer setting can be extracted, which changes the voltage produced (see \autoref{fig:gen18v}).
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The second part is already done beforehand and then available as a lookup table\footnote{mV-Scale, from 1.549V to 2.022V} to the firmware.
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On the other hand, to calculate the voltage to output, it is necessary to classify the connections between the PowerIts output terminals and the reticle.
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\subsection{Power Wafer}
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To test the 1.8V Regulation the so called Power Wafer is going to be used, it can be controlled similar to a in BrainScales used, functional, Wafer module.
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But it is fundamentally different, as it cannot be used for computation, but only to test for voltages and currents.
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Its internals behave like ohmic resistors, which provides us with a maximum power draw per Reticle of what is possible inside a usable wafer module.
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\begin{figure}[H]
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\centering
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\includegraphics[width=.8\columnwidth]{./data/theory/wafer.pdf}
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\caption{reticle diagram of a wafer in BrainScaleS. All 48 Reticles are shown}%
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\label{fig:wafer}
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\end{figure}
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\begin{figure}[H]
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\centering
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\includegraphics[width=.7\columnwidth]{./pics/waferpcb.png}
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\caption{part of the mainpcb on which a wafer is placed, visible are the 48 Reticles and two terminals each for 1.8V Digital (blue) and Analog (red), which correspond to the output terminals on a PowerIt}%
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\label{fig:mainpcb}
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\end{figure}
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It has the same layout as its system counterparts and each of the 48 Reticles can be accessed, digitaly as well as electricaly.
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Each Reticle is connected to its corresponding CURE Board.
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Those can read voltages of each reticle, but not at the same position as an AnaB.
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They read right before \(R_1\) in \autoref{fig:retmodel}.
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Another specialization of the Power wafer is, that all reticles analog and digital 1.8V lines are connected directly to pins on the Analog Readout Boards~\cite{anabpower}. There it is possible to measure a voltage, which is the one after the load resistors in \autoref{fig:retmodel}
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\subsection{Simple Wafer Resistance Model (SWRM)}\label{sec:swrm}
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The circuit in \autoref{fig:retmodel} can be used, as a first step, to describe the connections powering Reticles inside a wafer.
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\begin{figure}[H]
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\centering
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\includegraphics[width=.4\columnwidth]{./tikz/reticlepower.pdf}
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\caption{model of the to measure resistances and their currents, $R_0$ describes the resistance of a connection between the PowerIt Output and up to the FET (depicted as switch), while $R_1$ is a Resistance between FET and Reticles. The measurement is done between Output Terminals on the PowerIt and pins on a Analog readout board}%
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\label{fig:retmodel}
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\end{figure}
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SWRM allowes for two fixed resistance values and their respective currents. The current flowing through $R_1$ will be either 0 or a constant current $I_{ret}$. The current through $R_0$ will change depending on the number of reticles that are powered $n_{ret}$
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\begin{align}
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I_{ges} = n_{ret} \cdot I_{ret}
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\end{align}
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Therefore the voltage dip \(V_\text{dip}\) as measured by a Voltmeter (see \autoref{fig:retmodel}) can be described with \autoref{eq:vdip}
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\begin{align} \label{eq:vdip}
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V_\text{dip} =&\ V_{R_1} + V_{R_0} \nonumber\\
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=&\ R_1 \cdot I_\text{ret} + R_0 \cdot I_\text{ges} \nonumber\\
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=&\ I_\text{ret} \cdot \left( R_1 + R_0 \cdot n_\text{ret} \right)
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\end{align}
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Combining Equations~\ref{eq:rpot},~\ref{eq:rset}, and~\ref{eq:vout}, we gather \autoref{eq:fullreg}. This equation is a reversed function of the one given by the 1.8V supplying IC (see \autoref{fig:gen18v})
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\begin{align} \label{eq:fullreg}
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P_\text{val} = \frac{%
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R_\text{par} \left[ \left( \frac{0.7V \cdot 30.1k\Omega}{V_{O}-0.7V} - 6.49k\Omega \right) - R_\text{ser}\right]
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}{%
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R_\text{par} + \left( \frac{0.7V \cdot 30.1k\Omega}{V_{O}-0.7V} - 6.49k\Omega \right) - R_\text{ser}
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}\cdot
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\frac{256}{10k\Omega}
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\end{align}
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inside the code used for Regulation, \autoref{eq:fullreg} will be used to create a lookup table, while \autoref{eq:vout2} will be used at runtime, for which \autoref{eq:vdip} and~\ref{eq:voff} are needed.
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\begin{align} \label{eq:voff}
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V_\text{dip} =& V_O - V_\text{off}\\
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\Rightarrow V_O =& I_\text{ret} \cdot \left( R_1 + R_0 \cdot n_\text{ret} \right) + V_\text{off}\label{eq:vout2}
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\end{align}
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\subsection{Distance Wafer Resistance Model}
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Although the through SWRM gained functions are useful for determinig a theoretical Regulation procedure, it is still not near the realworld scenario.
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In a wafer, the distance between reticles and voltage connector (see \autoref{fig:mainpcb}) are resulting in additional resistance, proportionally.
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Therefore we adapt the DWRM after Circuit~\ref{fig:retmodelshell} in which each different Distance requires additional Resistors.
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\begin{figure}[H]
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\centering
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\includegraphics[width=.5\columnwidth]{tikz/reticlepower_2}
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\caption{retpow2}%
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\label{fig:retmodelshell}
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\end{figure}
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With this model the voltage is now expected to change depending on the reticles distance instead of being the same. The distances inside a wafer are visualized in \autoref{fig:retmodelrdist}
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\begin{figure}[H]
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\centering
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\hspace*{-.14\columnwidth}
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\includegraphics[width=1.2\columnwidth]{../pitstop/20180821/reticel_rtheo.pdf}
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\caption{Distances of reticles to the nearest voltage supplying connection for DWRM, distance is in reticle-side length}%
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\label{fig:retmodelrdist}
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\end{figure}
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