The Derp Driver

Everyonce in a while I get a really dumb idea behold the derp drive:

Derp Drive 99% 'what if?', 1% something else

So one day I decided that I wanted to make a cutesy little bench top supply because bench top supplies are balls expensive. I'm going to talk about the rest of this venture at a later date. But today I will talk for too long about the gate drive and how it is for the most part an exercise in curiosity.

Imagine for a moment that you wanted to switch a supply at 500kHz on a full bridge in an odd synchronous fashion. Your are strapped by cash and need synchronous rectification on the output because you're going to be dealing with high current and low voltages. You also want the output floating because if the output floats that's handy it means you can have +V or -V or stacked or paralleled if the outputs can share current. 

But doing synchronous rectification requires gate drives, some weird boot strapping some form of isolation. If gate drives go in that means an auxiliary floating set of voltage rails for the drivers and some sort of sensing the voltage state which means chips and rails and money and stuff. 

It would be nice if all the switches were driven by floating things, you know kinda like a transformer.

But transformers cost money they are big and slow and inductive... but that is a fallacy. Lots of fast things use transformers for signals, pulse transformers are things, USB uses transformers, ETHERNET uses transformers. Ain't nobody calling my gigabits slow. There are some highly legitimate reasons why signals like usb and ethernet utilize magnetic coupling for signals:

  1. It can be isolated electrically. My laptop could sit at a completely different potential than your laptop, powered by batteries or off the line and they could still yarble at one another over an CAT5e cable thanks tot he awesomeness that is transformers.
  2. A local return path/ reference. Any signal sent out needs to be with reference to something and have a return path and to be a fast signal inductance needs to be minimized. This is why PCIe, usb, ethernet, SATA and ect. are all differential pairs for high speed signalling. Transformers by nature create a differential voltage source. As a statement of Gauss's law and conservation of charge they must create a + and relative - to maintain the E field flux but I'm going off on a limb. The important bit is that by creation of a differential signal a return path exists that does not necessarily need to be referenced to ground or anything in particular besides the other end of the wire in the transformer.
  3. You can have common modes impedance without differential mode impedance. if your output if flapping around a some voltage this can help keep noise from getting into the primary side of the circuit. because transformers aren't perfect and they have some amount of capacitance going from the secondary to primary side. 
Transformers are pretty nifty for a variety of reasons, but they have their limitations. That being said in the system of this gate drive I am worried about the volt-seconds that the cores will be able to withstand before saturating. 

Once the cores become saturated the primary becomes decoupled from the secondary in at least  one direction and the ability to keep the switches in a specific state compromised (to some extent). The ethernet transformers I have seen largely don't come with a primary volt-second spec. However the ethernet transformers I have seen seem to posses a magnetizing inductance of ~300uH which I thought was surprisingly high, and I think puts the idea of the gate driver on the edge of potentially acceptable/ practical.

As far as practical implementations go all of the ethernet transformers are center tapped on the primary and secondary side. By running a push-pull converter using the primary centertap the transformer is a 2:1 step up. This allows you to effectively drive a fet using logic level voltages, once you get above 10V on the gate of a FET the reduction in Rds on is generally marginal. I think the advantage this system would get from running at higher voltages is the faster turn on time of the switch but once the switches are on it won't matter much.

This brings into discussion a few things which I have marginally modeled in spice. What limits the turn on speed of the device? Like I got a voltage source with some impedance. The switches require some base line amount of voltage to turn on. The FET gates are a derpy capacitor, in order to turn on the switch requires some amount of charge slapped on the gate which roughly translates into a given amount of energy (roughly). The goal of a fast gate driver must be to deposit that energy onto the gate as a fast as possible. Any impedance between your roughly ideal voltage source of a decoupling cap which is hopefully sized significantly larger than your gate capacitance otherwise what the fuck are you doing, will restrict the flow of power. There were many many assumptions made in that last sentence. 

But what it boils down to is what is throttling the current? resistance or inductance? with a transformer inductance can be a severe issue at high frequency. Even if the fundamental frequency of the switching isn't that fast what is important its about the rise time of the gates voltage waveform. The long the turn on the longer the losses and that what I care about reducing.

But holy fuck the post is longer than I expected and I'm just glossing over things. may I'll post the spice sim next post.


On the Mother board and stepper carriers

The mother board I was referring to in my last post got made and seems roughly functional.
Despite me screwing up the stepper driver to some extent.

The mother board should be able to interface to six of the stepper carriers as soon as I produce a stepper carrier that isn't inherently screwed up ...besides the first round of stepper carriers which worked just fine ironically.Tango 1.1.3 has some dumb issues as i turns out not all 780x devices in the same packages have the same pin out and I was playing a bit fast and loose with the foot prints and mixed up the 5V with the ctl power in on the 5V regulator I was using. This may have resulted in burning out the gate driver chip immediately since I wasn't able to coax any PWMing out of the tango driver after fixing the foot print mess up. Le sigh.

Time to populate/kludge another carrier board. If this time round I get the board built up right and touble shoot it it will be pretty exciting to have a board carrier for so many boards. It will make a nice test platform.

The mother board is laid out in a fashion that allows for n arduino mini or some other equivalent micro to be mounted onto the board and have its outputs connect to all the varirious driver boards through the parallel busses laid out on the mother board. There are up to ~8 accessible signals on each bus/ for each driver board (the GND/SIG/GND/SIG/GND/SIG traces reffered to in the last post). This was intended to leave the future possibilities open. Currently the stepper driver would only use 2 signals/ driver leaving lots of possibilities.

Hopefully I'll get something done on the stepper fron again soon. I've been a bit distracted by other electronics and that whole having a job thing.


Motherboard/ Driver carrier board preview

Here is a preview/test route of the motherboard to hold the tango drivers:

The traces go kind of like this:
gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd, signal, gnd
I'll talk about why in my next post and some adjustments I made to the tango board before sending it to the fab, I need sleep.


Stepper Driver v1.1.3: tango: ready for manufacturing

So the next iteration of the stepper driver for the millathe has been laid out. It it fixes a few quirks on the waltz board such as some mussed up foot prints and lack of labelling on the dip switches. Also it takes up much less copper are and uses a different card edge connector, actually made to fit a 64 pin PCIe connector. Why? Because as it turns out PCIe connectors are
1. apparently way cheaper than other car edge connectors mostly likely because they're made in stupid quantities.
2. capable of carrying fairly high currents 1A+/pin*64 pins = more amps than a board this size should carry.
3. It's metric  and has a 1mm pin pitch, the 1mm pitch is nice and small shrinking the space the signals take up vs .1" pitch. Also I abhore standard units and .1" spacing is the bane of my existance.
4. A 2x32 pin connector is just about the right size for this board.

That being said lets take a look at this board, behold Tango:

Top + bottom

You might have noticed I did not skimp on the copper. all of the large current carrying traces (motor voltage, pwrgnd, all motor out/inputs) are straight up polygons. I figured there really was no harm in increasing the power sinking capabilities of the board. Actually there is some harm assembly will be slightly more painful, there are no thermals on this board, it is made to be soldered in an oven or with the aid of a hot air gun. The terminal connectors on the left hand side of the board just won't go on with out the soldering iron turned to the max other wise. However besides that one downside there isn't much of a case against using all of the power side copper that is is available in this scenario.  I would rather maximise dissipative capabilities and minimize dissipation.

As far as gate driving is concerned this circuit is still using the original A4989 output as the driver despite (in my opinion) it's relative low current output in the 10's to -barley 100's of mA range for maybe a 20Ohmish resistive values on the gate output. One this I might want to experiment with in this scenario is the ringing effects of an under damped gate vs. the losses from turning on slower due to the damped circuit. In order to minimize inductance ( and therefore overshoot/ringing) in the system each gate drive trace was routed in a pair with a trace going to the source of it's corresponding mosfet, similar to a parallel port matching each signal is matched with a return ground for lower inductance. With the smaller loop it will also lower noise from inductive coupling from the currents on the power side of the board.

In comparison with the last stepper board this one boasts a larger volume pair of decoupling caps for the motors, they could probably be smaller and they take up an ass load of room where assload is like 1cm^2 each... maybe not huge but for this board that is a big component. If you put a rectangle arround the board there is around 35cm^2 of area on this board making them take up around roughly 5.7% of all possible physical space.
But  on the plus side with the increased size of the capacitors allows me to use ones with higher voltage ratings/ capacitance ratings. That was a major limiting factor of the driver last time. Now the main voltage limitation is the power mostfets.

So about them
mosfets. this time around going with the DPAK FDD8778 again same fet as last time. One of the main reasons I'm going with it again is it's relatively low gate charge and reasonable on resistance (14mOhm@25C/10vgs). When operating at higher voltages my calculations say that according to the equation:

\\[ Psw = I_{ds} V_{ds} /2(Q/ I_{hl} +Q) \\
but apparent the latex add in I have isnt working... I'll edit this bit later.

Either way the important take away is this equation:

Psw = Ids*Vds/2*Qg*(1/ihl+1/ilh)

It represents the switching power losses occuring in the system.
As far as numbers go for me.

Ids = 10A
Qg(that I care about) ~ 9nC
ihl/ilh~100mA with a complete short on the driver (I'm going to look at how low a value of gate resistors this circuit can get away with)
f=some weird poop

Now the stepper frequency, it has a linear correlation with your switching losses and is therefore important highly dependent on your control method. In the tango driver, the A4989 controller provides a hysteretic-constant off time current control method. A hysteretic controllers are also called bang-bang controllers, they are not constant frequency and the dependent on the system load. Often thermostats contol temperature using a bang-bang controller. Generally with a hysteretic controller you have a high limit and a low limit, when the output sense is below the lower limit the controller turns on the power full blast, then once the out put sense reaches the higher limit it turns off the power.

Your thermostat is set to 70 with +/-5 degree limits on the temperature, the temperature is dropping in your house because it's winter and cold outside. The temperature in your house drops to 65. The thermostat senses this and cranks on the heater; depending how large your house is and how powerful the heater affects how quickly your house heats up. Regardless of how long it takes, your heater is going to try it's damnedest to heat up that house as fast as it possibly can. Once your house is 75 degree the thermostat shuts off the the heater and your house begins to cool again. This cycle then repeats.

In the control of the stepper motor the controller is similar, but rather than having a lower limit it just turns the controller off for a fixed time. With this controller, the controller clock is 4MHz (set through a resistor) and the fixed off time is 87 clock cycles. This sets an upper limit pwm frequency of ~46kHz, giving us a decay time of 21.75microseconds. The load dynamics decide how long the on-period will be. The load: a big ass stepper motor, motors are commonly modeled as a voltage source, inductor and resistor. What matters most in this case is this motor inductance.

For a decay time of 21.75us assuming we are dominated by the motor inductance... the time constant is 2.8mH/.73Ohm = millisecond range = way longer than we care about (go on wikipedia and read about RC and L/R time constants if you are curious). Assuming steady state operation with the stepper motor which is ironically is a pretty bad case for this controller in terms of power dissipation, but that is another interesting discussion that is highly related but I don't want to get into right now.

If the motor is drawing 5A RMS on a phase


You know what, maximum pwm frequency in the steady state is 35ish kHz and I'm tired of writing about this right now this post really went on a tangent. More about controls pwm frequency and losses later and how it relates to this controller.

 Either way the drivers should have pretty much the same amperage rating as the last ones at (+10A) but with more voltage up to 25V till the fets poop them selves giving it 200W/ driver minimum for a bit of margin on the driving voltage, I'll be on the look out to better suited fets to increase the power density of the system since that is a real limiting factor at the moment.

I'm gonna go order the boards now.


Waltz Stepper Drivers: Functional Yet Noisy

After assembling one of the Waltz boards and only screwing up the the tiny driver once the board works!
There were only a few quirks in getting the board to run. A grounding issue between my laptop and the Arduino and the scope which made things flip out whenever the scope connected. Which means something isn't as well grounded as it should be, since half the outlets where I live don't have a ground that might have been the issue... connecting all grounds explicitly in the circuit fixed the grounding issue. From that point out operating the motor was pretty smooth. The board was hooked up to an arduino with modified 'blink' code to step the motor continuously in a given direction. Here's a video of the waltz board driving a stepper:


It worked well up to several kHz of step frequency if the step frequency got too fast the motor would just sit there and make a very annoying whining sound. The controllers were tested up to around 10A with no heat sinking or additional cooling. Using the 'will this burn my finger?' test methodology the main power fets remained cool enough up to around 6A that you could continuously keep you finger on them. At 10A they had a temperature of  'owfuckshitthathurts' after around 5s of keeping your finger on a fet, but it wasn't that 'burn on contact' kind of hot. In conclusion I should get some thermocouples/ temp sensors also the boards seem quite capable of being pumped over 10A. Once they are upgraded to D2pak FETs rather than the jankily soldered on Dpak FETs this might improved heat sinking and current capacity.

On the next rev of the stepper driver the caps are going to be swapped out so the motors can run at higher voltage. Hopefully this will push the motor PWM frequency out of the audible range. The way the A4898 does current control is a fixed off time system, this makes the output PWM of the system variable with various operating conditions such as  the voltage supply inductance of the motor and the decay rate settings on the controller. The noise and vibration also seems to vary greatly with the step frequency of the motor and the microstep settings. Motor operation seems much smoother and less noisy at increased speeds, this might be partially due to rotor inertia/velocity matching up with the commutation of the motor. There will have to be some characterization to find optimal operating points.
The motors being used are 27.4kgf*cm (381oz*in), 3.5Arms/phase, NEMA 23 hybrid steppers: data sheet. When stepping the motors vibrate enough on any hard surface to be really obnoxious, that's why the motor is sitting on a cushion in the video. 

Next steps for the millathe project will include measuring up the millathe itself sizing it for a new set of ballscrews and mounting hardware. The next set of boards needs to be designed as well as the motherboard to hold all of the smaller stepper boards.


Stepper Boards have Arrived: Waltz v0.2

Woooooo the boards arrived like several weeks ago they arrived, but what ever they're getting posted about now so ...woooo stepper boards. They're all pretty n'purple n'stuff.
So far I've only assembled one and there seem to be no fatal errors on the board that will prevent it from functioning. Couple minor package errors and what not but whatever.

here's a pic of the board:
I went with purple because why the hell not?
I've been slightly distracted from the project by work. However there will be more posts soon since there needs to be board testing.

In the mean time pretty boards are pretty, but pretty useless boards are pretty useless until they are tested otherwise.


Millathe Stepper Driver v0.2: waltz

In order to power the millathe's axes I decided to create a stepper driver with a higher current capacity than most hobby stepper controllers out there. Driving a mill seems to be above the capabilities of standard hobby stepper motors. Below is a picture of the board. I named this version 'waltz'.

Waltz v0.2


The waltz boards are currently being manufactured overseas in China at Myro PCB. Unfortunately as it turns out the first week of October is 'National Day' in China and the the factory closed down for the week a few days after the order was put in. On the flipside this gave me time to finalize/order components before the boards arrived. 

waltz v0.2

The waltz board is designed to fit in a 20 position 0.1" spacing card edge connector, this makes the board 53x62.5mm. Each board has two full bridges made to power a single bipolar stepper motor. The control of the motor is handled by an Allegro A4989. The FETs to be used in the half bridge are FDD8778 in a TO-263 package, each has 14mOhm of on resistance. These were chosen because of their relatively low gate capacitance/charge to reduce switching losses, however the majority of losses in this system will be ohmic. The Allegro A4989 was chosen because it  seemed like a good all in one solution for various  features such as current control, fast current decay and, up to 16th microstepping all while still supporting external FETs. The only two control inputs to the chip required are step and direction.

This brings us to how this system is going to be controlled. The plan is that several waltz boards will be plugged into a single motherboard carrying an arduino nano. Card edge connectors seemed like a modular way to stack multiple stepper drivers on a single board while having large amounts of connector contact area for the power paths and conveniently allowing for data to come in the same connector. The Arduino nano will partly be the brains of the operation. A computer will stream the control information (step and direction) for all of the waltz boards to the Arduino which will act as a buffer/ demultiplexer and timer making sure all of the outputs are  switched synchonously and with proper timing. This type of setup will take heavy calculation off of the arduino however the tool paths/ step patterns will have to be preprocessed on a computer before being sent to the arduino.

Concerns /Issues/Thoughts/Modifications:
-The current ripple for this system could be quite large depending what motor is chosen. That being said each capacitor is rated to pretty high ripple >4A ripple. However I would like to raise the input voltage to the motors, which would make this more of a concern.
-I would  like to increase the voltage of the system allowing for faster stepping and more awesomeness, this requires new bus caps as they are the limiting factor for the motor voltage, currently they are only rated to 16V.
-Screw these capacitors.
-I Screwed up the zener gate clamp pinout on using a 3-pin package...derp. In the mean time the diodes will have be added in a weird orientation. can be fixed on next rev.

That is all on the waltz board for now more updates when they come in and the mother board is ready to go.