| Table of Contents | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
| Info |
|---|
Warning: this document is purely my notes from my own research. Don’t take everything in it for granted especially since I’m a beginner to this sort of stuff, and double check everything, especially if using it for design purposes. I am not purporting to be good at this or to get everything right or to know everything, since this is my first board for Solar. |
Research/Design
Objective:
...
USBPD devices can either act as the source (charger) or sink (device) or DRP (both). For our purposes we probably want a sink-only chip since we'll be plugging a charger into this board and the output will be a 12V/GND line. We don't need this to act as a source from a wall outlet.
Compatibility with USBPD 2.0 and USBPD 3.0 would be nice to allow us to use a wider array of laptop chargers.
USBPD 2.0 allows for fixed voltage outputs of 5, 9, 12, 15, and 20V
USBPD 3.0 allows for incremental voltage outputs anywhere from 3.3V to 21V in increments of 20mV. This is called PPS (programmable power supply). We don't really need this since we only need to provide fixed 12V and 5V.
Perhaps considerations should be made for USBPD 3.1 as well, as that would allow for a higher power delivery of 240W and possibly support driving contactors.
3.1 is backwards compatible so designing a 3.1 sink should allow for interfacing with 3.1/3.0/2.0 source
Since we want to max out current I think 3.1 makes the most sense as USBPD 3.1 EPR (Extended Power Rating) cables have the capability of providing up to 240W @ 48V
This may have to wait as there are not many 3.1 sink chips available
According to this reddit post, USBPD3.1 @ 240W probably isn't possible since the sink functionality of it is not well supported. I took a look at a few chips such as ST-ONEHP and TPS25730USB.
USBPD Controller Chip Evaluations:
USB Type-C and USB power delivery power path design considerations
Designing a USB PD (Power Delivery) Trigger Board For My Devices
Infineon CYPD3177-24LQXQT
Used in this Phil's Lab video
In stock on Mouser as of 5/22/24
Price: $2.20
Operates on USB-PD 3.0 Standard
Capable of 100W (20V @ 5A)
Pretty simple to set VBUS Min/Max and Current Min/Max; just use voltage dividers on four config pins as shown above
Offers a fault indicator
Falls back to 5V in case negotiation fails
QFN package (more difficult to solder, but still manageable at Pickle esp. with someone with experience)
INJONIC IP2721
Used in this isaac879 video
In-stock at JLC and Alibaba as of 5/8/24
Price: $0.69
Capable of 20V
Connecting high to SEL pin will make it output 20V, which makes configuration easy
The downside of this (as according to this Hackaday post) is that even though USB-PD supports current negotiation (sink asking source what current it supports), the IP2721 doesn't. So if a charger is plugged in that doesn't support 5A, which we are ultimately going to request, then we still may not get the current we want and perhaps trigger some overcurrent condition on the source if we go over that amount.
TSSOP package, so easier to solder
Texas Instruments TPS25730
Price: $2.69
On backorder in Mouser as of 5/8/24
In TI's product selector for USBPD chips, this seems like the newest sink-only chip
It supports PD 3.1 in that you can do PPS (which we don't actually need anyways)
Capable of 100W (20V @ 5A)
It looks like an external microcontroller can be attached to I2C for configuration purposes, but doesn't have to be
Min/max voltage and operating current/max current configured through ADC1 through ADC4, can just hook up voltage divider
TPS25730D has an internal HV path, whereas S has a gate driver for an external HV path
QFN package; harder to solder as mentioned before
STMicroelectronics STUSB4500
In-stock as of 5/8/24
Price: $2.71
Capable of 100W (20V @ 5A)
Configurable through programming non-volatile memory (NVM) through I2C
Would make initial configuration of anything above 5V a bit more difficult, but would only need to be done once at the beginning with a programmer of some sort (Nucleo?)
QFN package
Buck Converter Research:
I suppose we'll need a high-efficiency buck converter to step down from max 20V@5A to 12V@8.333A after the whole USB PD negotiation is complete.
Another design consideration was made in this area about the buck converter pipeline. The two possible options were 20V → 12V → 5V (e.g. having chained buck converters) or 20V → 12V AND 20V → 5V (independent buck converters)
Chaining buck converters means that we can use the same 12V → 5V buck converter we typically use, muRata MYLSM00502ERPL. However, this means that if 12V or above is not in a source’s capabilities, then 5V will also fail. Basically 5V is entirely dependent on the 12V working, which limits the set of USB Type-C chargers we can use with this board.
Independent buck converters means that 5V can function without 12V, so if a PD charger WITHOUT 12-20V support is used, we can still test a low voltage device. This seems better but also makes our job as circuit designers more difficult in terms of validation, cost, or efficiency.
Matthew Yu’s notes on LDO vs Buck Converters:
Another one: Buck Converter 2: Electric Boogaloo
Other typical forms of voltage step-down (Linear Voltage Regulator (LDO), Voltage Divider) dissipate power as heat to step down voltage. We lose power in some capacity as heat.
Buck converters will decrease voltage AND increase current to maintain power (typically higher efficiency but less stability when compared to an LDO)
When the switch is closed, the inductor charges up
When the switch is opened, the inductor releases charge through the load resistor RL. The current flows through the diode and back into the inductor
We switch the switch (MOSFET) at a certain frequency and duty cycle in order to keep the average current relatively stable over one period
The capacitor is assumed to have a steady voltage over one period, so it just keeps the voltage drop across RL the same
Watch the Buck Converter 2 video for more explanation and the derivation for the following equation:
Vout = Duty Cycle x Vin (typically)
Datasheet will have more specific calculations for things such as voltage dividers which will ultimately set the duty cycle
A synchronous buck converter replaces the diode with another MOSFET so that there is less power loss since the MOSFET will have lower power loss while current is flowing thru it
Luckily for us, these buck converters come in convenient little ICs that will do all this switching for us
We need two of these, one for 12V and one for 5V
Integrated FET packages (the mosfet comes inside the IC)
Nothing on here that can do more than 6V output
TI TPS56A37
$2.87 on Mouser
4.5V to 28V input
0.6 to 13V output
Supports 10A continuous output current
Seems most appropriate for our 20V->12V use-case especially since other ICs can't supply as much current
QFN package
Analog Devices ADP2303ARDZ-5.0-R7
$4.08 on Mouser (we can do better on price)
3 to 20V input
5V output
2/3A
$0.99 on Mouser
3.8 - 30V input
5V output
3A
12 uA quiescent current
Champers Fu approved!
This one is really good, perhaps we should use it for more designs
External FET Buck Controller packages also exist and were looked into briefly (mosfet is outside the IC, might be more cost-effective and able to provide enough current)
TBH I kind of prefer an integrated package rather than this so might not really look at these any further
Is isolation required between the USB-PD provided power and the device power?
If so, an external FET buck controller package with a transformer may be required
https://www.eevblog.com/forum/projects/usb-type-c-pd-lab-power-supply/
Decided it's probably not worth it. From what I can tell from online + talking to Jacob Pustilnik and Matthew Yu, isolation is mainly used to protect the user from voltage shock and to protect the device from taking a large amount of voltage if the buck converter fails. Additionally, the cost difference after looking at some Murata buck converters is large enough to warrant no isolation ($83 is a LOT). We'll just A) be safe when using the device, perhaps put it in an enclosure of some sort, and B) make sure that there is voltage protection on new devices we produce so that a 5V device like the Peripheral SOM won't break if a spike of 20V is applied to it. We'll also include some overvoltage protection on the output to make sure this doesn't output more than 12/5 V.
...
The general principle is that measuring the voltage drop across a known value resistor (shunt resistor) will give you current.
Since I = V/R, measuring the voltage drop and dividing it by the resistance value will give you the current.
This voltage drop across the resistor can also be fed into a differential amplifier, magnifying the voltage drop to a reasonable level (since a shunt resistor tends to be a lower value, so less voltage drop).
The plan is to use a shunt resistor and a current sense amplifier and feed the output to a couple of pins, test points, or a connector (haven’t decided yet). This could then be measured by an oscilloscope and logged so that current monitoring is possible.
After some deliberation, I decided that it would be cool to add a seven segment display to the board in order to display the amount of current on the 12V bus or 5V bus.
There are two strategies to implement this:
Use a specialized IC to convert the analog voltage produced by the shunt resistor +amplifier into an output that can be read by a seven segment display.
Program a microcontroller with an integrated ADC to convert the analog voltage into an output.
Much of the time, the second solution can be more cost effective and configurable than the first.
PIC microcontrollers are relatively low-cost and do not need much external circuitry, especially if you use an 8-bit MCU. They often have integrated oscillators as well. We just need to find one that fits the bare minimum requirements of this product.
- If we are measuring current on the 12V and 5V bus, we will need two integrated ADCs. These will
Microcontroller & LCD Selection
If we are measuring current on the 12V and 5V bus, we will need two integrated ADCs. These will come in anywhere from 8-24 bit resolution.
We can scale the current sense voltage into a range of 0-5V and read it in provided the MCU’s operating voltage is 5V (from the output of our buck converter).
Just for example:
At the minimum (an 8-bit resolution) our ADC will be able to read 0-5V in increments of 5V/(2^8) = 19.53125 mV
At the maximum (a 24-bit resolution) our ADC will be able to read 0-5V in increments of 5V/(2^24) = 0.000298023224 mV
Since the current output of the 5V bus is a maximum of 3A, we can scale this 0-3A range to fit the 0-5V input of the ADC. 1V on the ADC is equivalent to 0.6A on the line.
This means that our current measurement resolution at an 8-bit ADC resolution is 19.53125mV x 3A/5V = 0.01171875 A = 11.71875 mA
Our current measurement resolution at a 24-bit ADC resolution is 0.000298023224mV x 3A/5V = 0.000178813934 mA
Let’s think about the display, since that may be the constraining factor on resolution displayed anyways. I kind of want to use this 4-digit seven segment display I found on Mouser. It’s about $1.67.
It has LEDs for decimal points as well!
For the 5V bus, since current goes from 0 to 3A, it would be nice to display current as X.XXX A. This implies a resolution of maximum 0.001A, or 1mA.
The 12V bus will be lower resolution since we have to display a range from 0 to 10A. Thus, we will display current as XX.XX A. This implies a resolution of 10mA.
The 12-bit ADC can do 1.22070312 mV with a 5V input.
For a 0-3A range, this means we can do 1.22070312 mV x 3A/5V or 0.732421872 milliamperes
For a 0-10A range, this means we can do 1.22070312 mV x 10A/5V or 2.44140624 milliamperes
This is probably good enough
Next, the number of pins. Our MCU will need Let’s think about the display, since that may be the constraining factor on resolution displayed anyways. I kind of want to use this 4-digit seven segment display I found on Mouser. It’s about $2.55.
Be careful. Almost selected a 5V A.C. seven segment display rather than a D.C. one.
It has LEDs for decimal points as well!
For the 5V bus, since current goes from 0 to 3A, it would be nice to display current as X.XXX A. This implies a resolution of maximum 0.001A, or 1mA.
The 12V bus will be lower resolution since we have to display a range from 0 to 10A. Thus, we will display current as XX.XX A. This implies a resolution of 10mA.
Next, the number of pins. Our MCU will need 1 pin for an ADC and some pins to interface with the selected seven segment display.
Note that on the datasheet we are presented with a schematic:
This brings total I/O to 17 pins. Also note that forward voltage is 2.6V at max, which means we’ll probably need to add some resistor in series to ensure that the output of our MCU is not too much for it to handle. Similar thing with current.
One more thing to note is that we can power this off the 5V bus to reduce voltage conversion bullshit, which means we’re looking for something that can handle a supply voltage of 5V.
Also, no QFN packages please 🙏🏽 (we already have enough on this board).
This brings us to our product of choice: the ATTINY826-XU
18 I/O pins
12-bit ADC at 375 kilosamples per second, or 17-bit ADC with oversampling
USART supporting one-wire mode (which is both something we’re constrained by as we only have one extra pin, but also we don’t really need RX capability only TX to output data to UART)
Let’s do the math to see if the resolution is fine for our four digit display
Shunt and Amplifier Selection
First, let’s figure out where to put the shunt resistor: https://www.allaboutcircuits.com/technical-articles/resistive-current-sensing-low-side-versus-high-side-sensing/
To do this, let’s talk about CMRR (Common Mode Rejection Ratio). Here’s Google’s Gemini AI engine to help (prompt: Explain CMRR and what a common vs differential mode signal is of an op-amp to me with an analogy like I am 15 years old):
Imagine you're at a concert with your friend. The band is the desired signal you want to hear clearly.
Common-mode noise is like a bad sound system rumble that affects both of you equally. It might be caused by a loose wire or some electrical gremlins.
Differential-mode signal is the actual music from the band. This is what you want to amplify without the rumble.
An op-amp is like a fancy headphone amplifier that boosts the music (differential signal) for you and your friend. Ideally, it should completely ignore the rumble (common-mode noise).
CMRR (Common Mode Rejection Ratio) is how good the op-amp is at ignoring the rumble. A high CMRR is like having high-quality headphones that block out most of the rumble, leaving just the clear music.
In reality, no headphones are perfect, and some rumble might still leak in. Similarly, even a good op-amp will have some CMRR, meaning a small amount of rumble might still be amplified with the music.
Ok, but what’s even the big difference between putting a shunt resistor on the low side vs the high side?
The common mode voltage (VCM) is literally the average of the two voltage inputs into the amplifier. VCM = (V1 + V2) / 2
Thus, when Rshunt is placed on the low side (and since the voltage drop across the shunt is negligible) VCM is far closer to GND
Conversely, if Rshunt is placed on the high side, VCM is far closer to Vsupply.
This means that using a high side shunt resistor will require an amplifier with a higher CMMR (common mode rejection ratio).
Ok, but then why not just put it on the low-side and go on with our lives?
Let’s say some system fault happens and the low side of Rshunt becomes grounded. Then, our system load never actually gets power! All of the current flows directly to ground.
With a low-side shunt resistor, this would never be detected, as we would just read in a Vshunt of zero or perhaps some floating voltage.
With a high-side shunt resistor, however, we read in a voltage of Vsupply, as the entire voltage drop happens across Rshunt.
This isn’t really a concern with our application as we don’t really care. If the user of this board happens to detect a fault condition, they will notice it immediately as their board will not get power.
Another issue is ground loops. The gist is that if some components on the system use one ground and some use another, there can be some path for current flow. Even if the grounds aren’t connected, there are many ways things can go wrong (for example, even things like stray electromagnetic fields can induce small currents).
In this case, the system load (the device being powered) would have a slightly higher ground reference than our buck converters or USB-PD chip, which means that we would need separate ground planes or paths.
On the other hand, if a high-side resistor is used, the grounds of everything is all the same, so no possibility of weird ground shit happening.
High-side shunt resistor is our answer then.
To select a shunt resistor, we also have to keep in mind our amplifier circuitry due to something called
Development
GITHUB LINK
Components and ICs:
...