Buck Converter PDB

Background:

UCR RoboSub is a University of California Riverside competitive autonomous underwater vehicle(AUV) project that competes internationally in the Robonation RoboSub Competition. The RoboSub competition is held every year for AUV’s to test their autonomy by completing underwater tasks. The RoboSub has to be able to perform different tasks for the competition such as drop weights and autonomous navigation. The RoboSub therefore needs a peripheral power source to power the hardware for operating the system.

Objective:

The buck converter is a peripheral power distribution board that receives power from one of the two main batteries. It has a 14.8V input and an output of 12V and 5V. The switches that we are using for this circuit are the LM257602T-5 for a fixed 5V output and LM2576s-ADJ P+ for adjustable output voltage.

• 1000μF output capacitor
• 100μF input capacitor
• 67μH inductor
• 8k and 1k resistors
• LM257602T-5 and LM2576s-ADJ P+ switch
• 1N5822 Flyback schottky diode - referenced as catch diode

Overview :

There are two capacitors: an input capacitor and an output capacitor. The input capacitor should have a low ESR(Equivalent Series Resistance) between the input pin and the ground pin to have a stable operation. It prevents large voltage transients from appearing at the input. The capacitor must be near the regulator and use short leads. The output capacitor has two main functions: it filters the output and provides regulator loop stability. The ESR of the output capacitor and the peak−to−peak value of the inductor ripple current are the main factors contributing to the output ripple voltage value. The catch diode provides a return path for the inductor current when the switch turns off. Must be located close to the LM2576 using short leads and short printed circuit traces to avoid EMI problems. The inductor will be the cornerstone of all switching power supply designs. Using a poorly designed inductor can cause high voltage spikes generated by the rate transitions in current within the switching power supply. Finally, we chose these resistor values because they give us the values that we wanted as seen in the equation in the schematic above. The resistors work for the adjustable switch.

We prototyped this board by first creating the schematic for the non adjustable chip on a perf board. We connected all the parts exactly as they are on our schematic and then using a digital multimeter(DMM) checked the output voltage to make sure we got 5V. Once we knew the chip was working we created the adjustable chip version by changing which chip was connected and added the resistors, then we used the DMM to make sure we got the 12V output we expected.

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