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X14 Conversion
| PCB Summary | |
|---|---|
| Vehicle | X14, ROV Sub-Optimal |
| Contributors | Minh Nguyen |
| Predecessors | X13 Conversion |
| Success? | {} |
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Convert 12V (~12-13V) from X14 Power Distribution Board to 3.3V at 1A and 5.2V at 3A using buck converter and linear regulator.
X14 Power Conversion Board is connected to X14 Power Distribution Board (placing on top) through 2 main connectors.
What priorities did you have in your design? What design considerations did you have? What methodologies did you follow? (routing a differential pair, keeping something separate for isolation, etc)
- Priorities:
- Stably convert 12V to 5.2V (not 5V) using an adjustable output buck converter while following the correct application schematic suggested by the manufacturer datasheet (especially using the correct inductor, input capacitor, and output capacitor) to reduce spikes
- Design considerations:
- Choosing between a fixed 5V buck converter (LM2678s-5.0/NOPB) vs adjustable version buck converter (LM2678S-ADJ/NOPB)
- Minimize the size of the board in length by removing unnecessary components (screw terminals, etc.) while leaving space for other components of other boards such as the distribution connectors.
- For an easier soldering process and to minimize the space of the board, the 7 pins TO263-7 package of the LM2678S-ADJ/NOPB buck converter is chosen.
- Methodologies:
- Placing all components related to the Switch (SW) trace of the buck converter as close together as possible while using polygons to create a better wire route.
- Using big polygons on the board for large power inputs and outputs such as 12V, 5.2V, and GND.
- Placing most of the components (buck converter and linear regulator) on one side of the board
- Avoiding 45 degrees or less angle for traces and polygons on board
- Create multiple test points on board
- Last year’s Power Conversion board is made to input from 12V - 48V and output 12V and 5V using a buck controller and a MOSFET; however, this year’s Power Conversion board only uses one buck converter which is quite similar to X12’s buck converter. With simpler functions, X14 Power Conversion is smaller in size than last year’s.
- X14 Power Conversion used the same linear regulator but it’s a more updated version of that linear regulator.
- Connectors to the distribution board and the dimension/shape of the distribution board
- The connectors on the distribution board which we have to shorten the length of the conversion board
The board mainly based on X12 and X13 Power Conversion from previous years: X12 Power Conversion Repository X13 Power Conversion Repository The LM2678S-ADJ/NOPB datasheet and simulator is important as well: Datasheet WEBENCH Simulator
- Receive 12V (~12 to 13V) from X14 Power Distribution Board but can receive a max of 15V due to the input voltage limit from the linear regulator.
- 3.3V at 1A output, converted by the voltage regulator, always DC output with little/rare ripples
- 5.2V at 3A output, converted by the buck converter, is expected to be a flat DC signal but it has some ripple/spikes with a peak to peak voltage of around 0.04V or 40mV.
- There’s no data signal, just the power sources getting converted into different voltages.
Why did you pick certain components for your board? (If you don't know the answer/were told, now is a great time to ask)
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I picked the LM2678S-ADJ/NOPB, especially the TO263-7 package is because:
- Easy to solder (only 7 pins)
- Complement parts (output capacitors, inductors, resistors, etc.) are much lesser than any other buck converters that I have researched
- The datasheet is straight forward and there’s an interactive simulator
- Manufactured from Texas Instrument
- Cheap and in stock at the time on Digikey
- Used and recommended by electrical members and leader
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I picked the Linear Regulator AZ1117CR-3.3TRG1 because:
- Highly recommended by an experienced electrical member (Bryce)
- Previous personal experience (used on Arduino Nano)
- Worked well with previous version Power Conversion boards
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I picked the 22uH instead of the 15uH because:
- More stabilized outputs which spikes are reduced and output stays right at 5.2V when applied on v2 board
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I picked three 150uF capacitors for output capacitors because:
- More stabilized outputs which spikes are greatly reduced comparing to two 100uF capacitors and one 150uF capacitor on v1 board
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Include links to components for future reference Buck converter LM2678S-ADJ/NOPB [Updated (Instock)] LM2679S-ADJ/NOPB Linear Regulator AZ1117CR-3.3TRG1
- In design:
- The Switch (SW) trace is very difficult to condense onto one spot
- The labels on the board are too small to read
- In assembly:
- The test points are too small to test the board in v1 board
- The board dimension needs to be minimized in v1 board
- The screw points should be removed in v1 board
- Not enough pinouts for 12V, 5.2V and GND in v1 board
- The ordered diode has the wrong package compared to the diode in design. It’s too small for the footprint on the board.
- Missing indicator LED for easier visualization of the board when testing
- Small components such as capacitors and resistors near the buck converter are too close together, making soldering harder
- The v1 Power Conversion board blocks the connectors on the X14 Distribution board as we have to cut the end of the board and eliminate the test terminals.
- The ordered diode has the wrong package compared to the diode in design as it’s too small to fit the footprint on the board. To fix this, I but more soldered on the pinouts of the small diode to reach the designed pads.
There’s a new version of the buck converter LM2678S-ADJ/NOPB which is in stock and in manufactured by Texas Instrument: [Updated (Instock)] LM2679S-ADJ/NOPB
- Design:
- Small components for the buck converters are placed very close together
- Some of the labels for the components are small and incorrect
- Test points are designed too small compared to other conversion boards
- Missing indicating LED for better visualization when testing
- The size of the board should have been minimized more in v1 board
- Ordering:
- Diodes was ordered with the incorrect package
- One capacitor (10nF for CB trace) is not ordered
- Buck converter LM2678S-ADJ/NOPB should be ordered more
- Assembly:
- The diode is soldered with incorrect polarity (fixed)
- Larger components were soldered first, leaving smaller components hard to solder
- Make the conversion board smaller - this board has a lot of spare spaces and can be minimized in the future
- Small components should not be positioned too close to each other
- Board labels should be bigger and clearer
- Assembly for each components should be double checked before ordering
- LEDs should be included into the board for better visualization
To calculate for Feedback resistors of the buck converter: (this is in the LM2678S-ADJ/NOPB datasheet) Vout =VFB*(1+RFB2/RFB1 ) → RFB2=RFB1*(Vout/VFB -1) With Vout =5.2V, VFB =1.21, and RFB1=1kΩ → RFB2=3.32kΩ Choosing the inductor for the buck converter: (this is in the LM2678S-ADJ/NOPB datasheet)
Any addition links if relevant
- Any fun side details
Search keywords.
The X14 Power Conversion board is positioned on top of the X14 Power Distribution board where it receives a nominal 12 V, stably converts to two signals, and sends back to the Distribution board. The Power Conversion first nominally outputs 5.2V at 3A using a buck converter, then outputs exactly 3.3V at 1A using a linear regulator.
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