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This project contains schematics and PCB source files of a simple solution to adjust remotely the positive voltage of
fixed isolated DC/DC gate driver supplies. Hence, the voltage can be tuned digitally, through a
UART bidirectionnal communication, isolated via a digital isolator. A linear regulator with fixed
value is used, connected to the DAC output of a microcontroller.
## Disclaimer
Warning: this board can be connected to deadly High Voltages (> 48V), and contains exposed pins.
Use at your own risks and use mandatory safety measures and protective equipments.
We do not provide any warranty of any kind at all. we shall assume no liability for any issue, damage or misuse.
Warning: MCLR pushbutton is located at floating voltage:
Do not manipulate during operation, where the isolated ground GND2 can be at hazardous voltages
(connected to the Kelvin Source potential).
Use MCLR pushbutton only when GND and GND2 are shorted, grounded and connected to earth protective connector.
The same method must be used to program the microcontroller.
Shared for educational purposes only.
## Description
*Why this project?*
Isolated DC/DC converters are typically used for gate driver power supplies, to drive power transistors
such as Si IGBTs and MOSFETs, SiC MOSFETs, GaN HEMTs and other FETs such as upcoming diamond transistors. The most commonly available isolated DC/DC converters are with fixed input and output voltages, typically with a fixed regulation at the outputs.
However, one would like to adjust online the gate voltages. This is particularly desired during
preliminary tests, such as Double Pulse Tests (DPT) or opposition method (full bridge). In these tests, the switching
losses are characterized, which are function of the gate currents.
One common method to investigate different switching speeds is to change the gate resistor(s), or use a linear regulator with a potentiometer in the feedback resistor divider.
This requires either soldering or manipulating by hand a pot, which can be referenced to high voltages.
The solution offers here online adjustement of the positive gate driver supply,
through bidirectionnal isolated series communication. The main benefits are:
- To use any available DC/DC isolated converter, whose output voltage is equal or higher than desired (e.g. +20 V, +15 V,...)
- To adjust online the isolated positive voltage, from the max output voltage of the DC/DC converter (e.g. +20 V, +15 V,...) down to as low as desired (e.g. +19 V, +15 V, +12 V, +8 V, ...)
- To measure both the fixed voltage from DC/DC output, and the output of the adjusted voltage
- To allow enable/disable of positive voltage
- To be integrated in any automated test circuit, thanks to the UART communication. Design of experiments are then eased
- To allow modulation of LED indicators
- To offer more functions thanks to the microcontroller referenced to the floating ground (typically connected to the Kelvin Source of the power FET)
- There is no need to redesign the isolated DC/DC converter, should the specifications changed
- Stock issues are less affecting this solution, since a broader range of DC/DC converters can be used, with various output voltages, for the same specifications
The proposed circuit is based on four main circuits (refer to schematics below):
- a fixed and regulated isolated DC/DC converter (U5)
- a bidirectional digital isolator for UART communucation (U2)
- a linear regulator with fixed reference voltage (U6)
- a microcontroller for DAC, ADC, UART and LED indicators (U3)
*Image 1: Schematics*
<img src="./Images/Schematics-v1.0.svg" width="600">
The PCB was fabricated and succesfully tested.
Two boards can be connected to drive the low side MOSFET and the high side MOSFET.
They can share the same TX UART host (e.g. a single [Adafruit MCP2221A Breakout](https://www.adafruit.com/product/4471) or any other USB <> UART converter).
See images 3 and 4.
*Image 3: Assembly of two boards, connected to an evaluation board with SiC MOSFETS in a half bridge configuration*
<img src="./Images/TwoBoardsWithSiC-HalfBridge.jpg" width="600">
*Image 4: Oscilloscope waveforms of adjusted isolated voltage (blue, Ch3), and PWM (green, Ch4), and DAC output (purple, Ch2).*
The principle of adjusting a fixed linear regulator with a DAC can be found in several application notes
such as [SBAA341A from TI, *Power-supply margining circuit for LDOs using a precision DAC*](https://www.ti.com/lit/an/sbaa341a/sbaa341a.pdf).
*Related publication*
The results and design approach will be presented at IEEE DMC 2024. Article will be available open access after the conference in [HAL open archive](https://cv.hal.science/nicolas-rouger).
## Installation & Fabrication
The PCB is a 4-layer type. Gerber files are provided.
The BOM is also provided.
The board requires two voltage sources, referenced to GND (typically grounded and connected to earth):
- a 3.3 V source for primary side of the digital isolator (*+3V3info*)
- a 5 V source for primary side of the isolated DC/DC converter (*+5V*).
This source can be of a large power, as a function of the DC/DC converter and the output circuit (e.g. > 2 W).
- Both 3.3 V and 5 V sources share the same ground (*GND*)
- This ground (*GND*) is isolated with the High Voltage secondary ground (*GND2*)
- The chosen references of isolated DC/DC converter and digital isolator offer an isolation of:
- 5.7 kV/3.0 kV rms, Common Mode Transient Immunity (CMTI) 180 V/ns (ADuM341E)
- 5.2 kV DC, CMTI 200 V/ns (MGJ2D052005SC)
## Startup and protections
The microcontroller is supplied by the secondary side of the isolated DC/DC converter, referenced to the floating ground (*GND2*). Since the positive output voltage at the secondary side (labeled *+20V*) can be relatively high (e.g. +20 V, +15 V) compared to the max supply voltage of the microcontroller (e.g. +3.3 V), the startup is of particular attention. This is also true for the startup of the linear regulator *U6*, whose adj. pin is controlled by the microcontroller, which could create some non intenational voltage at the output of the board.
Accordingly, the startup sequence at the secondary side is:
- The output voltage *+20V* at the secondary side of the DC/DC converter will increase towards its nominal value (+ 20 V)
- The fixed 3.3 V linear regulator *U4* will follow and maintain 3.3 V (labeled *3V3*) relatively to *GND2*
- The microcontroller will begin its startup sequence as soon as *3V3* voltage is higher than min supply voltage
- In the meantime, the linear regulator *U6* will follow *+20V* and its feedback loop associated with the output resistor bridge *R7,R10* might target a large value
- Once the microcontroller has initialized, the *RSTLDO* output can go high, to shutdown as fast as possible the linear regulator *U6* through its shutdown pin#3. The DAC output can also be initiated at a large value, corresponding to a lower output value of the linear regulator *U6* through *R9*
- ADC channels can be used to confirm the voltages and further reactivate the linear regulator *U6* by clearing *RSTLDO*
This startup sequence and the duration of each phase are function of several parameters such as capacitors, output load, ...
At the moment, there are no Zener neither protection diodes. They can be added in a future version to prevent overvoltages during transients or improve safety of the software.
Undervoltage and overvoltage protection can be implemented in the microcontroller code.
## Choice of Microcontroller
Feel free to use any microcontroller with:
- UART TX/RX
- Buffered DAC
- Multiplexed ADC or multi channel ADC
- 3.3 V (other voltages can be ok, as long as compatible with the digital isolator and linear regulator)
- PWM for modulation of LED indicators (e.g. blinking speed, intensity, ...)
The PIC16F1769 from Microchip was chosen for prototyping, mostly for the reasons:
- it has the required peripherals
- it was used previously on other projects
- it is compatible with both 5 V and 3.3 V supply voltage and I/O
- it has I/O slew rate control
- Simple 8 bit architecture and 32MHz internal clock
- available in both PDIP, QFN and SOIC, at a reasonable cost
Many other microcontrollers can be used in place of this reference.
PCB
- Kicad Source Files, which includes schematics and PCB layout
- Gerber files, for fabrication
- The footprint of the [Adafruit MCP2221A Breakout](https://www.adafruit.com/product/4471) can be downloaded from source files at [Adafruit Github] (https://github.com/adafruit/Adafruit-MCP2221-PCB) under a CC BY SA license.
***
Microcontroller
- The code, pinout for microcontroller can be found at [Gitlab rep. on the same group.](https://src.koda.cnrs.fr/laplace-groupe-cs/public_projects/isolated-power-supply-control/microcontroller-code)
- The microcontroller must be programmed, either before soldering or through In Circuit Serial Programming (Connector *J1*). The *GND2* must be grounded during programming - see disclaimer.
***
## UART COM for adjutable output voltage and reading values
For the code to control the microcontroller, refer to [Gitlab rep. on the same group.](https://src.koda.cnrs.fr/laplace-groupe-cs/public_projects/isolated-power-supply-control/microcontroller-code)
For the UART host, several options are offered:
- A USB <> UART converter can be used.
The footprint of Adafruit MCP2221A Breakout was integrated (*J2* and *J3*).
This circuit generates *3.3V3info* from its USB connector, which also provides bidirectional UART.
Only the following pins are used from the Adafruit breakout: GND, 3V, RX, TX.
- The UART TX and RX signals can be directly sent to the SMB connectors (*J7,J9* for TX, *J12,J10* for RX).
The socket for the Adafruit breakout can also be used to connect directly the UART signals with flywires.
Please note that the voltage levels for UART is compatible with 3.3 V and 5 V.
Without the Adafruit breakout, the *3.3V3info* must be supplied to the digital isolator, from *J14* or *J16*
- In case several boards are used, such as in half bridge, full bridge or three phase inverter, one UART TX can be used to drive
multiple channels, from any of the two previous options. Adressing can be done at code level, while each microcontroller being known by their unique address.
However, in the current state of the PCB, only one microcontroller TX cand be connected to one UART RX. This can be easily solved - see suggested improvements.
## Inputs / Outputs
- Voltage supply
- *+3V3info* referenced to *GND* for digital isolator primary side. Typically low power.
- *+5V* referenced to *GND* for primary side of isolated DC/DC converter. Can be of a few Watts.
- UART COM
- TX from UART host. A single TX can drive multiple boards. Typically with a max voltage of *+3V3info* referenced to *GND*. Compliant with 5V.
- RX at UART host. In the current version, only one RX host can be connected to one board. (see herinbelow for fix). *+3V3info* referenced to *GND*
- Isolated Power outputs
- First, select with a 2.54mm jumper the desired isolated output at *J13*. It will connect either the fixed output voltage of the DC/DC converter
(*+20V*) or the output of the adjusted linear regulator (*Vvar*)
- Then, select at *J11* the desired pinout:
- Either the top three pins (*-5V*,*GND2*,*Vvar* or *+20V* as a function of *J13*),
- Or bottom three pins (*Vvar* or *+20V* as a function of *J13*,*GND2*,*-5V*)
- Available extra pins
- For the microcontroller, *J15* can be used as inputs or outputs. This offer some flexibility.
- For the digital isolator, two outputs at *J5* can be used, as a function of inputs at *J6*. For exemple, the outputs can be connected to the inputs of the microcontroller available in *J15*.
- LED indicators are offered:
- *D1*, blue led, to confirm *+5V*, referenced to primary side *GND*
- *D2*, blue led, to confirm *+20V*, referenced to secondary side *GND2*
- *D3*, blue led, to confirm *+3V3*, referenced to secondary side *GND2*
- *D4,D5,D6*, controlled by the microcontroller, referenced to secondary side *GND2*. Refer to the microcontroller code for proper reading.
## Support
Feel free to contact us, we would do our best as a function of our availability.
## Authors and acknowledgment
Nicolas Rouger @nicolas.rouger.7
CNRS senior scientist at Laplace lab.
Contribution: Preliminary schematics and tests, supervision, microcontroller codes, project managing.
Fabien Devilliers
Intern at Laplace lab.
Contribution: Kicad schematics and PCB layout, design and choice of components, microcontroller codes, soldering and assembly, tests and measurements.
This project was developed during Fabien Devilliers' internship at Laplace / CNRS in Toulouse (Summer 2024, Univ. Toulouse III, UPS, Master).
We acknowledge the partial support of the [PEPR project #FRENCHDIAM](https://www.pepr-electronique.fr/frenchdiam/).
## License
All source files (schematics, gerber, kicad, pictures) are under Creative Commons Attribution - Share Alike 4.0 INTERNATIONAL.
CC BY-SA 4.0.
See license.md
## Version
August 2024 V1.0 initial release
- PCBs have been fabricated and validated experimentally
- Gitlab repository has been created and updated with Gerber files and Kicad files
- Initial release of Readme, License and pictures
## Roadmap & Possible improvements
Many improvements and further developments can be done:
### Schematics and layout
- [ ] The PCB has two connectors names J4 (USB and socket for adafruit).
The schematics has been updated J4 > J2 at socket.
The footprint of the USB is also changed in the schematics compared to the fabricated PCB.
- [ ] Reduce parasitics in the routing
- [ ] Add more space between the socket for Adafruit USB <> and the isolated DC/DC
- [ ] Use cheaper connectors and cables for linked RX and TX (rather than SMB)
- [ ] Reduce currents from LEDs with higher bias resistors
- [ ] Allow multiple channels from microcontrollers TX to a unique UART host RX.
This can be done by diodes and pull-up resistor on the same bus
- [ ] Add zener and protection diodes
### Functionnalities
- [ ] Adapt to adjust also the negative voltage of the isolated gate driver supply
- [ ] Having a microcontroller reference to the floating and isolated ground *GND2* offers many opportunities which can be explored (sensing gate voltage, temperature sensor, ...)
- [ ] See further improvements on the code subgroup related to this project