A buck converter is a DC-DC power electronics device which reduces input voltage to a desired level. The most basic implementation consists of a transistor, a diode, an inductor and output capacitor. The efficiency of a such circuit can often exceed 90 percent.
Device prototype assembled on a universal PCB.
The goal of this project is to design and build a DC to DC buck converter. The system will perform cyclic, synchronous measurement of the output voltage and current (using a series shunt resistor) and adjust these parameters according to values set by the user. A maximum steady-state error should not exceed 1% of the control range (300 mV).
The device should be based on the STM32 Nucleo board, and should have the following set of features:
- input voltage: up to 30 V,
- variable output voltage: 0 up to Vcc (minus the losses across the diode, shunt resistor etc.),
- adjustable current limit (two modes of operation CC (Constant Current) and CV (Constant Voltage)),
- a rotary encoder for changing voltage and current limit values,
- preset buttons for switching between desired voltages,
- an LCD for showing voltage and current,
- a photoresistor for controlling the brightness of the LCD,
- an SD card for logging data over time,
- a serial port communication setting and reading the voltage and current limits,
- a dedicated C# desktop application as an additional graphical user interface.
This project implements a buck converter circuit controlled by an STM32 Nucleo board, which measures the output voltage and current and adjusts the MOSFET accordingly. Both measured quantities pass through a unity gain amplifier and are filtered using a second-order active filter based on the Sallen–Key topology. A 16x2 LCD display shows the output voltage, current, power, and energy delivered to the load, with the backlight controlled by a transistor, adjusting based on ambient brightness measured by a photoresistor. The desired voltage is set using a rotary encoder, and four preset buttons allow users to store and recall voltage settings.
For prototype purposes, the device was soldered on a universal PCB.
The regulation of the system is implemented using proportional and integral terms. The microcontroller compares the output current and voltage readings with the user-defined values, and based on this comparison, it adjusts the MOSFET duty cycle, either increasing or decreasing it to maintain the desired output.
An API specification for communication between dedicated C# application and the device via UART interface.
The following set of messages can be received from the device:
- Voltage - measured voltage in mV
[int]. - Current - measured current in mA
[int]. - TargetVoltage - reference voltage in mV
[int]. - CurrentLimit - current limit in mA
[int]. - OutputMode - working mode (either "cv" or "cc")
[string]. - Output - output status (either "on" or "off")
[string].
Example message:
{
"Voltage": 3310,
"Current": 26,
"TargetVoltage": 3300,
"CurrentLimit": 1000,
"OutputMode": "cv",
"Output": "on"
}
The following set of messages can be sent to the device:
- SetTargetVoltage - sets the reference voltage in mV
[int]. - SetCurrentLimit - sets the current limit in mA
[int]. - SetOutput - turns output on or off (either "on" or "off")
[string].
Example message:
{
"SetTargetVoltage": 3300,
"SetCurrentLimit": 500,
"SetOutput": "on"
}
This project is licensed under the MIT License. See the LICENSE file for details.