Digital power without the need to learn programming

Author : Rolf Kowalsky, Arrow

30 January 2013

Caption: Demo screen before “Program Chip” button is clicked

For some years already digital power has been increasingly visible, whether in reports, product announcements or at electronics trade fairs. However, this technology, which replaces analogue control with a digital algorithm, has not yet gained wide acceptance. This is partly because those developers working in the area of power supplies are generally specialists in analogue technology, and seldom bring with them any programming knowledge.

It is also often the case that the benefits provided by a digitally regulated and controlled system are not visible at first sight. On the other hand, frustration is often experienced when people all too naively believe that a digitally controlled power supply can be made to work simply by an experienced programmer setting a few parameters. Precisely because the power module remains pure analogue technology, which demands great care and experience.

Two concepts
Two main concepts have emerged, each of which has its own raison d’être. The first involves the use of a DSP or microcontroller, which is freely programmable, and is supplemented by special peripherals, such as PWM units, and a software environment optimised for power electronic applications. This approach is suited, for example, to complex systems, which may include a PFC or bridge topology. In this case, both blocks can be regulated by means of a processor, while at the same time communication with a higher-level system can be implemented. However, this solution is very software-intensive and requires an in-depth knowledge of programming.


The other concept uses a component with a digital core, which is already programmed and “only” has to be adjusted to the requirements of the existing application by creating a set of parameters. To enable this to be done, software that is executable on a PC is also supplied, so all parameters can be managed and optimised for the application. This requires no programming know-how from the user. Engineers without any special knowledge of software can thus operate and optimise a digital system and easily find their way around, since many parameters are identical to those from analogue control technology. This concept will be presented using the example of a component manufactured by Exar.


Unpack, switch on… and it works

To save on costs and reduce effort to a minimum, since this is only meant as a first test, an Exar XRP7714 demo board was ordered free of charge via Arrow’s Testdrive board evaluation programme. All the other devices needed are normally present in any laboratory.
The Exar XRP7714EVB-DEMO-2P kit includes a quad-output power supply and a USB interface board. The software for loading new parameters is also supplied on a USB stick.

Software installation and first-time operation
The software supplied on a USB stick for board operation is very easy to install. Click the installation file PowerArchitekt_setup.exe and follow the instructions that appear on your PC monitor.

The hardware consists of the actual quad-output voltage controller board and an interface board, which is used to reset the PC's USB to the XRP7714’s 12C. All that remains is to connect the Vin socket of the power controller board with 12VDC.

Now the interface board is connected with the PC via the supplied USB cable and the previously installed Power Architect software is started. The XRP7714EVB can now be parameterised and controlled from the PC. The welcome screen displays the hardware configuration of the evaluation board and asks the user to select a suitable configuration file. The first time select the XRP7714EVB configuration. This already contains all the parameters needed to operate the board. The following output voltages are preset: 3.3V; 2.5V; 1.8V; 1.0V. After the initial configuration has loaded, the board can be controlled from the “demo” screen.

Now the first hurdle (and the only one identified by the author) immediately arises: For the board to function, it is first necessary to click the “Program Chip” button. Unfortunately, this is not mentioned in the documentation provided. The “Program Chip” button has to be clicked after every parameter change for the values to be transferred to the chip, even when the auto-update function is active.


Programming a typical multichannel POL supply

As a description of every single parameter and every possible function of the chip would go beyond the scope of this report, a number of typical and repeatedly used functions of a POL power supply, as found, for example, in FGPA or processor systems, will be shown.
The following functions are presented:

- Assigning functions to the freely programmable GPIO pins

- Setting the up ramp for an output voltage and Vin UVLO warning and stop threshold

Caption: Main screen with the defined GPIO configuration. GPIO4 and 5 are reserved for the I2C communication with the PC.

- Setting output voltage overcurrent warning and stop threshold

- Setting time-based sequencing with individual switch-on and switch-off ramp per channel

All settings can be performed in the "Digital Design" and "Power Design" screens using dropdown menus or radio buttons. First, the function of the four available GPIO pins is selected from a dropdown menu, and the “Active High” or “Active Low” logic is specified:

Then the thresholds for the input voltage monitoring and the time for the up ramp of the output voltage for CH1 are set on the "Power Design" screen:

The set values are now transferred to the chip by clicking the “Program Chip” button.

The configuration that has been defined can now be seen on the main screen:

The following images appear on the oscilloscope:

Now the functioning of the overcurrent detection will be checked. For this an overcurrent threshold of Iout = 2A is set for shutdown. The warning threshold is selected as 30% lower. The functions “Overcurrent Warning” (GPIO0), “Overcurrent Fault” (GPIO1) and “Power Good Flag” (GPIO2) are assigned to the GPIO pins. Image 9 shows how the chip responds to an increasing output current:
Now simple time-based sequencing of all four output voltages will be implemented. Each of the output voltages will be switched on with an individual delay and ramp. Here too the desired parameters are easily set in the "Power Design" screen:
Image 5 shows the result:

Summary
This report demonstrates how easy it is to work with available digital power components. Little work is involved in setting the parameters for POL power supplies and adjusting them to existing environments thanks to the supplied graphical user interface. In addition to the examples of basic functions shown here, it is also possible to adjust the PID regulator parameters to almost maximum power output any of your own. Exar provides good application notes for the design of the power module. All settings and parameters selected during optimisation and commissioning can be saved and documented in a parameter file. This file can then be used e.g. in production for end-of-line programming. Without programming skills, it is possible to design a digital power POL power supply.
Of course, it remains necessary to proceed with great care with the layout and the selection of components when designing the power module. However, the user gains more freedom than he might have imagined in the regulation and control module.

Image 1 – Caption: Demo screen before “Program Chip” button is clicked

Image 2 – Caption: Main screen with the defined GPIO configuration. GPIO4 and 5 are reserved for the I2C communication with the PC.

Image 3
- CH1 (yellow) Increase in output voltage with ramp time of 10ms
- CH2 (blue) Power Good flag (GPIO2)
- CH3 (pink) Softstart in Progress flag (GPIO3)

Image 4
- CH1 (yellow) Drop in input voltage Vin
- CH2 (blue) Undervoltage Lockout Warning flag (GPIO0)
- CH3 (pink) Undervoltage Lockout Fault flag (GPIO1)
- CH4 (green) Power Good flag (GPIO2)

Image 5 – Caption: On and off sequencing with individual ramps for all 4 output voltages


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