PyPSUcurvetrace

PyPSUcurvetrace is a software toolbox for I/V curve tracing of electronic parts using programmable power supplies. In short, PyPSUcurvetrace is a curve tracer.

For two-terminal devices under test (DUTs) like resistors or diodes you need only one power supply unit (PSU). For three-terminal DUTs like transistors you need two PSUs.

PyPSUcurvetrace is developed using Python 3 on Linux. Other software environments may (should) work, too, but have not been tested so far.

Currently supported power supply types:

• Voltcraft PPS
• Korad / RND (confirmed: models KA3005P and KWR103, other models not tested)
• BK Precision (confirmed: model BK9185B, other models not tested)

Power supply types on the radar for future support:

• Units with a SCPI interface

PyPSUcurvetrace is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. PyPSUcurvetrace is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with PyPSUcurvetrace. If not, see http://www.gnu.org/licenses/.

How does it work?

Overview

The following figure shows the basic test setup for a three-terminal DUT with two PSUs:

For two-terminal DUTs, only PSU1 is needed and PSU2 can be ignored. If negative voltages are required at the DUT terminals, the respective PSU terminals are connected with inverted polarity.

Tests are run using the curvetrace program. curvetrace tests the DUT by varying the voltages V1 and V2 at the PSU terminals, and by reading the corresponding currents I1 and I2. The results are shown on the screen and saved in an ASCII data file for further processing. curvetrace optionally inserts idle periods in between the individual readings, or a "pre-heat" period before starting the test, where the voltages (V1, V2) and currents (I1, I2) applied to the DUT are set to predefined ``idle'' values (this can be useful to control the temperature of the DUT during the test).

The procedure implemented in the curvetrace program is as follows:

1. Read a configuration file with the types and serial ports of the programmable PSUs, and then establish the serial connection to the PSUs.
2. Interactively ask the user for a name or label of the test data, and then open an ASCII data file with that name (an existing file with the same name gets overwritten!).
3. Determine the test conditions, either by interactively asking for user input, or by reading a configuration file with the test parameters:

• Start voltages, end voltages, and number of steps for each PSU
• Max. allowed current and power applied from each PSU to the DUT
• Polarity of how the PSU terminals are connected to the DUT terminals
• Optional: number of readings at each voltage step (results will be averaged)
• Optional: idle time and pre-heat time
• If idle or pre-heat times are not zero: idle voltage and current conditions for the PSUs

4. Check the test configuration versus the limits of the PSUs, and adjust the configuration where needed.
5. Show a summary of the test configuration and ask the user if it's okay to start the test.
6. Run the test. The voltages are stepped in two nested loops. Voltage V1 is varied in the inner loop, V2 is varied in the outer loop. The measured data are shown on the screen and saved to the data file.
7. Once the test is completed, turn off the PSUs.

Details

• For each step of the test procedure (measurement, idle, or pre-heat), curvetrace sets the voltages at the PSUs according to voltage values requested for this step. The current values of the PSUs are set to the minima of the current and power limits of the DUT and the PSUs at the given test voltages. If the currents established by the DUT are less than the current limits set at the PSUs, the PSUs will operate in "voltage limiting" mode, so that the voltage values required for this test step will be present at the DUT terminals. If the currents established by the DUT reach the current limits set at the PSUs, it is the task of the PSUs to avoid the currents from exceeding the limits by switching to "current limiting" mode, lowering the voltage values applied to the DUT. curvetrace reports the occurrence of "current limiting" mode in the data output using "Current Limit" flags.
• Once the inner loop (iteration of V1 steps) reaches a point where either I1 or I2 run into the current or power limits of the DUT or the PSUs, curvetrace stops the V1 iterations of the inner loop and returns to the next V2 iteration of the outer loop.
• Once a test voltage has been programmed at a PSU for a DUT measurement, curvetrace reads the voltage at the PSU terminals and waits for stabilisation of the read-back output voltage at the set point before continuing. This ensures that measurements are taken only after the voltages applied to the DUT have stabilised at the requested values.
• Some PSU types provide unreliable readings of the voltage or current values if the readings are taken too early after programming a new set point. To improve the reliability of the data obtains from such PSUs, curvetrace can be configured to take repeated readings with short idle periods in between. Readings are taken continuously until a specified number of consecutive readings are consistent within the readback resolution of the PSU, and the mean of those readings is returned as the measurement result. This method helps to achieve stable, low-noise readings. For configuration of this feature, see the "PSU configuration file" section below.

Software installation and configuration

• Download the code from the GitHub repository, either using GIT, SVN or as a ZIP archive. I like SVN (subversion):

svn co https://github.com/mbrennwa/PyPSUcurvetrace.git/trunk path/on/your/computer/to/PyPSUcurvetrace

• Install the Python-3 Serial package. On Debian, Ubuntu or similar APT-based Linux systems:

sudo apt install python3-serial

• Connect your PSUs to the USB or serial ports of your computer and determine their serial port interfaces on your system. On Linux:

ls /dev/serial/by-id

• Copy the config_PSU_TEMPLATE.txt file to config_PSU.txt. Modify the file to reflect the details of the USB/serial interfaces of your your PSUs. See "Notes" section for further information on the PSU configuration file.
• NOTE: By default, most Voltcraft units use the same serial device ID. Therefore, it is not possible to access more than one PSU unit via /dev/serial/by-id/. If you want to use more than one PSU unit, you will need to modify the device IDs of the serial interfaces so that they are different from each other. See "Notes" section below for details.
• Make sure your user account has permissions to access the serial ports of the PSU units. See "Notes" section below.

Usage

To run the software, execute the curvetrace program from a console terminal.

• The easiest method is to run the program without any arguments and just follow thew the on-screen instructions for fully interactive user input:

curvetrace

• You can also use provide a configuration file containing the test parameters (see example files provided). For example:

curvetrace -c examples/test_config/2SK216_config.txt

See "Notes" section for further information on test configuration files.

Examples

IRFP150 N-channel power mosfet

This example shows curve traces obtained from an IRFP150 N-channel power mosfet (drain current I_D vs. drain-source voltage V_DS, measured at different gate-source voltages V_GS). The IRFP150 pins and PSU outputs were connected according to above diagram.

• DUT source pin to the negative terminals of PSU1 and PSU2 (joined together)
• DUT drain pin to the positive terminal of PSU1
• DUT gate pin to the positive terminal of PSU2. A gate stopper resistor was used to avoid oscillation.

The power limit for the test was set to 100 W, because the DUT was mounted on a heatsink that did not provide enough cooling for more power. You can see how the IRFP150 tends to run away at higher power levels, which is due to the temperature increase. It would be better to mount such high-power DUTs on a large copper block or a water cooling system to achieve more stable temperature conditions during the test.

2SJ79 P-channel mosfet

This example shows curve traces obtained from an 2SJ79 P-channel mosfet (drain current I_D vs. drain-source voltage V_DS, measured at different gate-source voltages V_GS). The 2SJ79 was connected in the same way as the IRFP150 in the previous example, but with the polarity of the PSU terminals inverted to obtain negative test voltages:

• DUT source pin to the positive terminals of PSU1 and PSU2 (joined together)
• DUT drain pin to the negative terminal of PSU1
• DUT gate pin to the negative terminal of PSU2. A gate stopper resistor was used to avoid oscillation.

6C33C power triode

This example shows curve traces obtained from an 6C33C power triode (anode current vs. anode-cathode voltage, measured at different negative grid-cathode voltages V_GS). The 6C33C was connected as follows

• DUT anode pin to the positive terminals of PSU1
• DUT cathode pin to the negative terminal of PSU1 and positive terminal of PSU2 (joined together)
• DUT gate pin to the negative terminal of PSU2. For this test, only one high-voltage power supply (PSU1) was available. The second power supply (PSU2) was therefore configured as a 90 V PSU made up by a series connection of a 30 V PSU and a 60 V PSU (see below for configuration of such a series combination).

Notes

PSU configuration file

The configuration file config_PSU.txt contains the configuration details of your power supplies (PSUs). Take a look at the config_PSU.txt for details of the file format. There are separate sections for PSU1 and PSU2. Each section contains the following fields:

• COMPORT: virtual file corresponding to the serial port of the PSU
• COMMANDSET: the "language" for communication with the PSU. Currently supported COMMANDSETs are
  • VOLTCRAFT for Voltcraft, Manson, etc.
  • KORAD for Korad, RND, etc.
  • BK, BK9184B_LOW, BK9184B_HIGH, BK9185B_LOW, and BK9185B_HIGH for BK Precision.
• NUMSTABLEREAD (optional): number of consecutive readings that must agree to within the measurement resolution in order to achieve stable and low-noise readings

If only one PSU is used (PSU1), the PSU2 section can be deleted.

Note that it is possible to connect multiple PSU units in series to each other to accomplish a higher voltage range. Such a series combination of multiple PSU units can be configured as a single PSU object by specifying their COMPORT and COMMANDSET fields as follows:

• COMPORT = ( "/dev/serial/by-id/<xxx_psu1>" , "/dev/serial/by-id/<xxx_psu2>" )
• TYPE = ( "<commandset_psu1>" , "<commandset_psu2>" )

Test configuration file

...(under constrution -- take a look at the example files in the examples directory.)...

Set user permissions to access serial ports (Linux)

To allow accessing the serial ports, add your username to the dialout group. For example, if your user account is johndoe, execute the following command:

sudo adduser johndoe dialout

Then log out and log in again to the user account johndoe for this to take effect.

Modify serial device IDs of the Voltcraft PPS power supplies

In their stock condition, the Silabs CP2102 USB/serial interfaces of the Voltcraft PPF power supplies all use the same ID. The serial interfaces of multiple PPS units connected to the same computer therefore show up at the same file node under /dev/serial/by-id/ (usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0001-if00-port0 or similar). In order to simultaneously use more than one PPS unit, the serial interface IDs therefore need to be reconfigured to use unique IDs. This is achieved using the cp210x-cfg program:

• Download the cp210x-cfg code:

svn co https://github.com/DiUS/cp210x-cfg.git

• Install USB library stuff needed to compile the cp210x-cfg program:

sudo apt install libusb-1.0-0-dev 

• Compile the cp210x-cfg program:

cd path/to/cp210x-cfg/
make

• Display HELP information for cp210x-cfg, and make sure you understand how the program works:

./cp210x-cfg -h

• Make sure only one Silabs CP210x interface is connected (the PPS unit one you want to reconfigure), then show its information:

./cp210x-cfg

• Change the serial ID of the device (don't mess this up!):

./cp210x-cfg -S 0002

• Plug in the other PPS device and make sure that both serial interfaces now show up separately at /dev/serial/by_id:

ls /dev/serial/by-id/
usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0001-if00-port0
usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0002-if00-port0