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Laboratory Power Supply with Switch Mode Pre-Regulator [130234-I]

Status: Finished
April 25, 2013
in Elektor magazine | September 2014 | Find it here

Commercially available laboratory power supplies often have disadvantages regarding the user interface, the performance or the price.

In this project a laboratory power supply module with the following commonly used output characteristics is developed:

  • Uout = 0 … 30 V
  • Iout = 0 … 2 A

The described power supply module is equipped with a switch mode pre-regulator. The pre-regulator ensures high efficiency and low heat generation in any load point. In order to achieve a good output voltage quality and control characteristics a linear post-regulation is also implemented.

The Module itself provides galvanically isolation in the switch mode pre-regulator. Consequently, several modules can be connected to the same power source in parallel. The input voltage for each module is 12 V, thus a simple commercial power supply, like a computer power supply, can be used for mains connection of several modules.

The following features are already implemented and tested:

  • Switch mode pre-regulator
  • Linear post regulator
  • Output relay for save disconnect
  • High accuracy of the voltage control loop
  • Adjustable voltage and current presets when the output is inactive (no short circuit required for current limit adjustment)
  • Minimized overshoot of the output voltage at recovery from short circuit

These features shall be implemented or will be tested in the future

  • Galvanically isolated RS232 connection via RS485 for remote control (not tested jet, but implemented)
  • Control panel with 7 segment LED display and rotary encoder
  • Evaluation of a PCB integrated transformer for simple buildup of the module

Overall, this project focuses on usability for all tasks in everyday laboratory life. Consequently, the behavior of the module is optimized and tested in order not to destroy the connected circuitry like it sometimes happens with cheap power supplies from dubious manufacturers.

Update 07.05.2013

New measurements of short circuit current limiting and voltage recovery were done.

Added file:

Test conditions:

Current limit 350 mA, Output voltage 5V, short circuit was done with IRFP3206 MOSFET, several component values differ from the values given in the schematics, due to optimizations.

Screenshots show:

Channel 1: Linear regulator output

Channel 2: Switch mode preregulator output

Channel 3: Output current

Test results:

Current overshoot at short circuit: 4.2 A, used output capacitor 470 nF (different from schematic)

Current overshoot duration < 10 µs

Voltage recovery overshoot < 10 mV

Voltage recovery time < 40 µs

Under the tested conditions the power supply behaves nearly perfect. Especially the voltage recovery with nearly no overshoot is as desired.

The switch mode preregulator control needs further improvements to reduce the occurring oscillations in the preregulated voltage.



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