Sizing DC-DC converters and power distribution

1. Engineering problem

Each subsystem needs its own voltage (24 V motors, 12 V computer, 5 V sensors). A converter that is undersized, a fuse that is too large and a thin cable each cause a different failure: brownout, fire risk or a voltage drop that resets a device.

2. When is this needed?

• After the power budget lists consumers and voltages.
• When adding a sensor with a different supply voltage.
• When a device resets at high motor load.

3. Selection and design decisions

Choose between buck (step-down), boost, buck-boost and isolated converters by the input and output voltage ranges. Use isolated converters when the noise or safety design needs galvanic isolation.

4. Advantages and disadvantages

• One converter for everything is simple and couples noise; separate converters isolate faults and noise.
• High efficiency usually costs more; the loss shows up as heat.
• Headroom: running a converter near its rating shortens life and increases heat.

5. Requirements and constraints

• Converter input range covers the pack from empty to fully charged.
• Rated output current ≥ peak load with margin.
• Fuse protects the wire and converter, not the load alone.
• Datasheet efficiency curve is read at the actual load, not at the best point.

6. Architecture

battery ─▶ main fuse ─▶ distribution ─┬▶ motor drivers (24 V)
                                         ├▶ DC-DC 12 V ─▶ computer
                                         └▶ DC-DC 5 V ─▶ sensors, MCU

7. Implementation

1. Sum the load power per rail at peak.
2. Divide by the datasheet efficiency at that load and by the lowest pack voltage to get the worst-case input current.
3. Choose the fuse and wire gauge for that current.
4. Compute the converter loss and check the thermal design.
5. Compute the cable drop for the longest run.

def dcdc(p_out, eff, v_in_min, v_in_max):
    p_in = p_out / eff
    return {"p_in W": round(p_in, 1), "I at vmin A": round(p_in / v_in_min, 2), "I at vmax A": round(p_in / v_in_max, 2), "loss W": round(p_in - p_out, 1)}
r = dcdc(60, 0.90, 20.0, 29.4)
print(r)
print("fuse (1.25 x worst-case current):", round(1.25 * r["I at vmin A"], 1), "A minimum rating")
drop = 8.0 * 0.020
print(f"cable drop: {drop:.2f} V at 8 A over 20 mOhm")

Printed (checked by the site’s tests):

{'p_in W': 66.7, 'I at vmin A': 3.33, 'I at vmax A': 2.27, 'loss W': 6.7}
fuse (1.25 x worst-case current): 4.2 A minimum rating
cable drop: 0.16 V at 8 A over 20 mOhm

Reading it: a 60 W load on a 90 % efficient converter draws 66.7 W; at the lowest pack voltage (20 V) that is 3.33 A, at the highest (29.4 V) 2.27 A. A 4.2 A fuse or larger covers the 3.33 A worst case with a 25 % margin, and 6.7 W has to leave as heat. A cable resistance of 20 mΩ carrying 8 A drops 0.16 V. Efficiency, voltages and resistance are illustrative.

8. Real examples

• AMR: 24 V pack to the motor drivers through one fuse, a 12 V converter for the computer and a separate 5 V converter for the sensors.

9. Failure scenarios

• The fuse was sized on the average current and blew on start-up.
• A converter was run outside its input range at full charge.
• A thin power cable to a sensor dropped enough voltage to reset it at motor start.

10. Troubleshooting

1. If a device resets, measure its supply with an oscilloscope during the event.
2. If a converter is too hot, compare the loss with the thermal design.
3. If a fuse blows, log the current and check for inrush.

11. Validation

• The example prints the values above (executed by the site tests).
• Measured voltage at each consumer stays inside its input range under the worst-case load step.
• Converter temperature stabilises with margin.

12. Engineering checklist

• Rail list with powers
• Input range covers the pack range
• Worst-case input current computed
• Fuse and wire sized
• Loss and thermal design checked
• Cable drop computed
• Noisy and sensitive loads separated

13. Further references

• Datasheets of the converters used (input range, efficiency curve, output current, thermal derating) and of the fuses and wire.

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