# Gas station without pumps

## 2018 October 28

### Redrawing figure in SVG

Filed under: Circuits course — gasstationwithoutpumps @ 20:18
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There is a figure in my book of the cross-section of a power nFET.  Originally, I used a figure from wikimedia: https://en.wikipedia.org/wiki/Power_MOSFET#/media/File:Vdmos_cross_section_en.svg, but I wanted a color version, so I colorized it myself in Inkscape.  Unfortunately, the original SVG was poorly done—it looked like an inkscape conversion of a raster image to paths, which did not result in paths enclosing fillable areas with clean strokes around them, but separate paths filled with black for each stroke.  This made colorizing the image quite difficult.  I did (eventually) manage to make a colorized version, but I’ve never been happy with it.  The file is huge for an SVG file (over 81kB) and difficult to edit.  I’ve been wanting to do it right for some time, and I finally got around to it today.

What I did was to print out the version I’ve been using as a full-page image, then used a ruler to figure out how big each part was.  I then entered SVG code by hand to remake the image. I included comments to describe what each part did and used styles for the different materials, so editing is now easy.  I also made sure that the image is now symmetric and that all the rounded corners have smooth joins to the straight lines (the “q” command in the path “d” attribute makes that fairly easy. The new svg file is only 3425 bytes, even with the comments, and the pdf created from it is only 8kB, instead of 28kB.  Those size changes are not very important (the PDF for the book, after all, is now 25MB, up from 23.7MB last spring), but the image looks better now also.

WordPress.com does not let me upload svg files, but you can see the PDF produced from it by inkscape at https://gasstationwithoutpumps.files.wordpress.com/2018/10/nfet-cross-section.pdf

WordPress.com only lets me upload raster images for display, so I used inkscape to convert the hand-written SVG file to PNG just for this blog. The black line on the right edge seems to have been chopped off in the conversion, though the PDF conversion gets it right.

Here is the PNG generated by inkscape from the SVG file.

I tried uploading the SVG file to Wikimedia Commons, so it could be used on the Wikipedia Power MOSFET page, in place of the black-and-white image, but the uploaded file got rendered as a badly wrong black-and-white PNG file (with all colors converted to black), which is totally useless. I don’t have time to figure out how to tell it to do the conversion correctly, so I just asked them to delete the image again.

Correction 2018 Nov 21: I realized that the drawing from Wikipedia is missing something—the body should be lightly-doped P, with a P+ contact to the metal. The doping profile for the P layer also does not make physical sense here—how does it get shallower under the N+?  I have redrawn the figure for the book, but not corrected it here. I’ve removed the svg source from this post, because WordPress mangled it completely, despite the use of the “source” square-bracket tag.

## 2015 April 27

### Ideas for improving hysteresis lab

Filed under: Circuits course — gasstationwithoutpumps @ 08:38
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The hysteresis lab was the least successful one so far this year.  Students got the designs done and built, but they learned less than they could have from the lab (too much copying of “answers” without understanding) and the lab took too long for the time allotted.

I’m considering going back to a 2-day hysteresis lab, but expanding the lab slightly to include using an nFET to control a loudspeaker (probably with a series resistor to limit current to 500mA). Adding the nFET as a low-side switch would be a useful thing for them to know about (it is a standard arrangement for controlling heating elements, solenoids, and other devices in lab equipment), and would prepare them better for the class-D power-amp lab.

The problem here is that a good oscillation frequency for a touch sensor uses a small capacitor (hence a high frequency), while a good oscillation frequency for the loudspeaker is much lower (200Hz–4kHz).  I can have them add an external capacitor (resulting in little or no touch sensitivity) when driving the loudspeaker, or I can have them build two oscillators: a high-frequency one and a low-frequency one.

I’d want to redesign the board they solder the hysteresis oscillator on to make room for the nFET and loudspeaker connections,  plus giving them more holes for connecting up unused pins on the 74HC14N chip.  This will probably make the board bigger than 2.5cm×5cm, raising the price by 50¢.  They’d need an extra nFET in their kits (another 40¢) and perhaps another screw terminal (80¢).  An extra buck or two for the parts is no big deal, as the lab fee is already larger than what the students are getting, though the extra cost for having to use UC-approved vendors may be eating up most of the “excess” fees.

The lab could be split between the breadboard and the soldering phases, with students who are behind still completing the breadboarding in the second lab.

One question I’ve not resolved is how to reschedule the labs so that there is room for a 2-day hysteresis lab.  Is there any lab that currently takes 2 days that can be squished to 1 day (with less damage than squishing the hysteresis lab)?  How do I keep the reports on schedule so that I can grade over the weekend?

## 2014 May 24

### Class-D amplifier lecture 2

Filed under: Circuits course — gasstationwithoutpumps @ 17:41
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Yesterday, in the last lecture before the long weekend (hence before they start wiring their Class-D power amplifiers), I covered three topics:

• open-collector outputs.  I had hoped to avoid that this year by switching to the TLC3702 comparators, but I couldn’t get the TLC3702 comparators to work with the FETs, so I went back to using the LM2903 comparators that the students got in their parts kits.  Because I had already cut the open-collector information from the lab handout, I had to cover it in lecture, and I’ll have to write an addendum to the handout today.
• LC filters for the loudspeaker.  This was a pretty rushed job.  I didn’t even have time to get them to derive the LC filter behavior with no load (the standard voltage divider formula $\frac{Z_{down}}{Z_{up}+Z_{down}}$ becomes $\frac{1/(j \omega C)}{j\omega L + 1/(j \omega C)}=\frac{1}{1-\omega^2 LC}$, which goes to infinity at $\omega=1/\sqrt{LC}$ and 0 at DC and infinite frequency.  Instead I pointed them to a gnuplot script that they needtomodify to see how much power the LC filter delivers to the loudspeaker at different frequencies, with different choices of capacitor (no choices for them on the inductor—it is a 220µH 0.252Ω AIUR-06-221 inductor).

LC filter and loudspeaker model used for sizing the capacitor in the gnuplot script. The students need to replace the loudspeaker model with the one that they created in an earlier lab. I think that next year I may use a much simpler loudspeaker model in the script, so that I can tell whether the students have replaced the model or not.

The output of the gnuplot script looks something like this:

Output of the gnuplot script. Note that too small a capacitor results in a spike near 61kHz and nowhere near enough suppression of the 50kHz–100kHz PWM frequency. Too large a capacitor results in a big boost in gain around 3.8kHz, which would a different sort of problem. Click

• Block diagram for the class-D amplifier.  I had originally planned to spend almost the entire lecture having the students develop the block diagram, but the addition of almost 45 minutes on open-collector outputs and review of the FET driver stage left me with little time for the block diagram. I did get some participation from the class in developing the diagram, but almost entirely from one student.  I felt bad about presenting, rather than getting them to participate in creating the block diagram, but they had to have a block diagram to do their more detailed design over the weekend.One important point I that I emphasized, based on common problems on the second quiz, is that a block diagram is not a simple pipeline, but can have merging and splitting.  The class-D amplifier has merging of voltage generators for DC bias with the signal path and merging of the triangle wave generator with the signal path.
One good thing came out of the block diagram discussion: in putting together the design they had the output of the preamp centered at 3v but the input of the comparators centered a 0v.  I could point out that putting range information on the signal lines allowed them to catch this error early (before even doing the schematic).  Most of the class was able to come up with the standard solution for changing the C bias: adding a high-pass RC filter.  I don’t know whether they can choose a corner frequency appropriately, but we’ll see that on Tuesday.

I didn’t get a chance to teach them about real power, either, which the LC script computes.  I’ll have to go over that next Wednesday, between the two halves of the lab.  There is a writeup of the concept in the lab handout, but my experience has been that students in this class don’t learn from written material.

I’ve also told the students that they need to get all their required “REDO” assignments turned in by Wednesday.  It seems that this year’s class does not have the time-management skills to handle open deadlines—they keep putting off redoing the assignments.  Given that they sometimes don’t fix the assignments sufficiently when they turn them in again, leaving the assignments to the last week is really dangerous.  Next year I’ll have to make one-week deadlines for redoing the assignments, though I can be generous about extending deadlines on request.

Overall, the lecture was way too rushed, because of the extra coverage needed for open collectors. Next year I’ll have to make sure to allow 3 lectures for the class-D power amp, which means not having it on the week with Memorial Day.  I’ll probably want to move Quiz 2 a little earlier also, so that it isn’t the week before the class-D power amp.

## 2014 May 22

### Class-D lab revision didn’t work

In Long weekend, I discussed what I was planning to do about anticipated problems with the class-D amplifier lab, specifically

• Replace the AOI518 nFET with one that has a lower input capacitance, such as the PSMN022-30PL,127.  The gate charge at VGS of 4.5v is 4.4nC, about half that of the AOI518.
• Replace the open-collector comparator with one that has push-pull output, like the TLC3072, which can provide ±20mA current (more than the LM2903, even before we allow for the current through the pullup resistor).

I did a neat version of the schematics last night using the TLC3072 comparators and the AOI518 nFET. This year I remembered to include an adjustable gain stage in the preamp, so that I could more easily control the volume. Today in the lab, while the students were soldering up their instrumentation amps for the pressure sensor, I wired up the class-D amplifier, one stage at a time, confirming that each stage worked using the oscilloscope before moving on to the next. The build took me longer than I had expected—almost 2 hours.

Everything worked fine until I connected the drains of the two FETs together.  Initially it worked ok, but after about 20 seconds the shoot-through current increased, causing the current limitation of the bench power supply to kick in.  Then the voltage on the lower power rail moved up  close to ground, and the input voltage on the comparator was swinging below the negative rail.  I think that this damaged a couple of my TLC3072 chips—I’ve marked them, and I’ll have to test them before using them.

Replacing the AOI518 transistor with the smaller  PSMN022-30PL,127 did not help.

I finally borrowed an LM2093P chip from one of the students (I’d left mine at home, by mistake) and tried replacing the TLC3072 chip with the LM2093P. They have the same pinout, but the LM2093P is an open-collector output, so I had to add pullup resistors.  I guessed a couple of values, based on vague recollections of last year’s design, and the amplifier worked.

Initially I could only run the amplifier up to ±7v on the power supply, without the FETs getting too warm—there was still too much shoot-through current during transitions.  I switched to a lower resistance for the pullup on the pFET gate, to make the voltage swing less and the turn-off faster.  At that point the amplifier worked quite happily with a ±8v swing without the transistors getting warm.The circuit worked with either of the nFET transistors, so I’ll just have the students stick with the AOI518 in their parts kit.

I couldn’t crank up the volume on the speaker, though, because I got feedback squeal whenever the gain got too high.  Perhaps I should make a long speaker extension cable for students to do testing next week.  I seem to be out of speaker wire, though.

The class-D amplifier design will be a tough one for the students, and I’ll need to do a supplemental handout on open-collector outputs (I’d cut that material from the handout when I thought that we would be using the TLC3072 comparators).

Last week I thought that the students could start on the class-D amplifier in lab today, having finished the soldering for the pressure-sensor amp on Tuesday, but it took almost the whole lab time today for students to finish the soldering, even though everyone had working breadboards on Tuesday.  The lab ran over by almost 2 hours for one group, taking a total of 8 hours instead of 6 (last year the same lab took only 4 hours for the slowest group, probably because last year’s class came to lab more prepared).

The layout took longer than students expected, as did the soldering.  Everyone did (eventually) get working soldered instrumentation amps, though for a couple of groups I had to point out that their wiring did not match their schematic (they had called me over to help debugging).

In one case they had connected a resistor to the wrong point in their circuit.  I found the bug by tracing where their virtual ground was connected, and asking them to identify each component. Even after I showed them both resistors connected to their virtual ground, where only one was supposed to be, it took them a long time to realize what the discrepancy was. They had wired exactly what they had laid out, but the bug was in their layout, and they had not done a thorough enough job of checking against their schematic.

Another group had a working circuit but with too little gain. After checking a few of the DC voltage levels with them, I compared each of their resistors to the schematic.  In one place they had wired in a 1kΩ resistor where the schematic called for a 10kΩ resistor.  They unsoldered the incorrect resistor and soldered in the resistor from the design, which salvaged the circuit.

I also returned Wednesday’s quiz in lab today—pretty much like last year, the scores were much better on the second quiz than the first one, though still only half what I think the students should be able to do at this point of the quarter.  I’m once again assigning the students to redo the quiz as homework.  I need to decide soon whether to give them another quiz during the final exam time.

## 2014 May 19

### Long weekend

In Problems rewriting the Class-D amplifier lab I discussed the problems I was having with rewriting the class-D amplifier lab, and came up with the following ideas:

• Try the same basic design as last year, and see if I can get a power supply voltage and pullup resistor settings that work.  I’ll have to do this in the lab on campus, because I don’t have a power supply at home that provides the same voltage ranges.
• Replace the AOI518 nFET with one that has a lower input capacitance, such as the PSMN022-30PL,127.  The gate charge at VGS of 4.5v is 4.4nC, about half that of the AOI518.
• Replace the open-collector comparator with one that has push-pull output, like the TLC3072, which can provide ±20mA current (more than the LM2903, even before we allow for the current through the pullup resistor).
• Add a driver chip that is intended for driving a pair of FET transistors, like the FAN7382, which can provide very fast rise and fall times even into 1000pF loads. But the driver chips are intended for two nFET transistors, and the charge pump for the high-side driver would be difficult to explain in this course and requires external components (diodes, resistors, and capacitors) that the students would have no idea how to choose.
• Use an H-bridge or half-H-bridge chip instead of having students design the power stage with FETs.  Most are designed for controlling motors, and don’t give timing specs that would tell me whether they could handle a high enough PWM frequency for audio output.

Yesterday I ordered some TLC3072 comparator chips and some PSMN022-30PL,127 nFETs, also some MCP6004 op-amp chips, since I seem to have run out, and they are quite cheap.  (I probably need more instrumentation amp chips also, but they are more expensive so I’ll probably wait until I really need them). I hope that I can get the TLC3072 comparator chips before lab on Thursday, so that I can test out the circuit before the students need to build it.  Otherwise I’ll have to cycle into work over the weekend to test the lab. Digi-key shipped it this evening by first-class mail, which probably means I won’t get it until Friday—when they’ve shipped early Monday morning, I ‘ve gotten things by Thursday.

This weekend had me spending a lot of time on the circuits course—I estimate that I spent about 35 hours on the course from Friday night until Monday morning. In addition to figuring out how to rescue and simplify the class-D amplifier lab, I also had to rewrite all the tutorial material for the lab handout, create a quiz for Wednesday’s class, and grade their optical pulse monitor lab reports.

The grading is still a bit painful, as some of the students write nearly incomprehensible English and some are still making trivial mistakes on the schematics (like leaving out component values and shorting out components).  One error on this round was particularly worth commenting on to the class: consistently using mΩ when MΩ was meant.  What is a factor of a billion between friends?  About half the class still can’t explain sensitivity and gain computations, even when they get the right answer.  In some cases, they probably had not come up with the answer themselves, as they made mistakes that magically got cancelled and disappeared when they reported their final results. They keep such awful lab notebooks though, that it is quite possible that they had done the correct computation at one point, but could not reconstruct their work when they went to write it up a few hours later, so I won’t accuse anyone of cheating.

Today I collected prelab homework at the end of class.  The assignment of prelab homework to be turned in did get students to read the assignment and make a stab at doing the design work, and (for the first time) I got questions about the design on Monday rather than halfway through lab on Tuesday.  Most of the designs were incomplete and had serious errors, but at least an attempt had been made to understand the problem, and I was able to clear up several misunderstandings in class:

• Several students tried to put the op amps before the instrumentation amp.  I obviously had not made it clear what an instrumentation amp was for.
• Many of the students had not gotten the idea that an instrumentation amp is a single symbol in a schematic not a conglomeration of op amps and resistors.  This was probably my fault for trying to explain how an instrumentation amp works, rather than how to use one.  It doesn’t help that the INA126P data sheet shows only the innards, and not the external view of the amplifier. I think that next year I may skip explaining how an instrumentation amp works internally, though I think it is kind of cool.

Instrumentation amplifier symbol. I prefer putting Vref on the bottom, rather than the top of the amplifier, but either way works.

• I spent a fair amount of time trying to help the students understand the notion of output voltages being referenced to a specific other voltage. I think that next year I will make a point of always writing the gain equations as $V_{out} - V_{ref} = g(V_{p} - V_{m})$, so that the output is always a difference between two voltages, rather than as $V_{out} = V_{ref} + g (V_{p} - V_{m})$, like I’ve been doing this year.  Although the two statements are algebraically the same, I think that they’ll be better able to do multistage gain computations and less likely to forget to connect up Vref if they have the output expressed as a difference.
• I cleared up some small misunderstandings of how to do layout on the protoboard PDF drawings.

At the end of class I collected all the homeworks, and I “graded” them tonight.  I’m providing feedback and a done/not-done check, not a grade (which is lucky for them, since most were such sloppily done notes that I doubt that most of the students could reconstruct their thoughts from the notes).

I did get a chance to introduce the students to pulse-width modulation and FETs today, which are essential topics for the class-D power amp next week, but I’m not happy with the presentation I gave—I did a better job in the handout, which I suspect most of the students will never read.  The problem with working so much this weekend was that I was very tired all afternoon today, and was not able to put together as coherent a presentation as I wanted to.

I still have to do a design for the class-D power amp with the new comparators, but I’m too tired to do it tonight.  I’ll probably sketch it on paper tomorrow and do a neat schematic in SchemeIt tomorrow night. I’ll wire up all I can at home either tomorrow or Wednesday night, while waiting for the comparator chips and MCP6004 op amp chips to arrive.  (Actually, I’ll probably just use an MCP6002 chip, since I don’t need 4 op amps, and I still have a couple of MCP6002 chips left.)

Tomorrow I need to remember to bring in the cheap aquarium air pump, some tubing, and a clamp so that students can observe back pressure and the fluctuations in pressure from the pump (assuming that they get their amplifiers working).

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