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Full changelogEngine 1.18.3

System Power Budget

Power & Regulators

Total input power, conversion loss and battery runtime across every rail on the board.

Inputs

V
The rated current of the adapter, bus or upstream regulator at Vin
A
Headroom for tolerance, ageing and loads not listed
%
Usable capacity, after cut-off and temperature derating
mAh
V
Rail / loadFed from (V)Rail V (V)Load I (A)Duty (%)ConverterEff. (%)
120 mA
60 mA
30 mA
250 mA
800 mA

Results

1 to check
Drawn from input Pin

Not judged: no supply current entered. Enter it to check this budget against the supply.

with 20 % margin: 5.179 W
4.316W
System efficiency
97.1%
Input current (with margin) Iin

at 12 V; rate the fuse and connector above this
431.6mA
All results (2)
Delivered to loads Pload
4.193W
Total conversion loss

2.9 % of input
123.6mW
LDO efficiency is below 60 % on Sensor 1V8. An LDO's efficiency is Vout/Vin, so consider a switcher ahead of it and leave the LDO a small drop.

Each row's input power is added as if drawn straight from Vin, so a row fed from another rail misses that rail's conversion loss. For an exact total, add its input current to the parent rail's load instead of giving it a row.

Totals are duty-weighted averages. Size bulk capacitance, connector and fuse for the peak: 800 mA on Motor driver.

Per-rail breakdown
RailFromVIDutyTypeηP loadP inLoss
MCU 3V312 V3.3 V120 mA100 %Buck89.0 %396 mW445 mW48.9 mW
Analog 5V12 V5 V60 mA100 %Buck91.0 %300 mW330 mW29.7 mW
Sensor 1V83.3 V1.8 V30 mA100 %LDO54.5 %54 mW99 mW45 mW
RF TX burst3.3 V3.3 V250 mA10 %Direct100.0 %82.5 mW82.5 mW0 W
Motor driver12 V12 V800 mA35 %Direct100.0 %3.36 W3.36 W0 W
Total4.193 W4.316 W123.6 mW
Where the heat goes
MCU 3V3
48.9 mW
Sensor 1V8
45 mW
Analog 5V
29.7 mW
RF TX burst
0 W
Motor driver
0 W

Parameter sweep

Vary one input over a range and see the answer and verdict at each step, as a table and a curve.Pro

Worst-case corners

Put a tolerance on each input and get the worst-case band around the answer.Pro

The principle

A power budget answers four questions: how much current comes in, how much heat stays on the board, how long the battery lasts, and which part loses the most power.

Load power on a rail
Pload=Vrail⋅Iload⋅DP_{load} = V_{rail} \cdot I_{load} \cdot D
D is the load's duty. A radio on 10 % of the time adds a tenth of its power to the average, but its full peak to the decoupling design.
Switching converter
Pin=PloadηPloss=Pload(1η−1)P_{in} = \frac{P_{load}}{\eta} \qquad P_{loss} = P_{load}\left(\frac{1}{\eta}-1\right)
Linear regulator (LDO)
Pin=VinIloadPloss=(Vin−Vout) Iloadη=VoutVinP_{in} = V_{in} I_{load} \qquad P_{loss} = (V_{in}-V_{out})\,I_{load} \qquad \eta = \frac{V_{out}}{V_{in}}
The load current passes straight through, so efficiency is set by the voltage ratio alone.
System
ηsys=∑Pload∑PinIin=∑Pin⋅(1+m)Vin\eta_{sys} = \frac{\sum P_{load}}{\sum P_{in}} \qquad I_{in} = \frac{\sum P_{in}\cdot(1+m)}{V_{in}}
Battery runtime
t=CmAh⋅Vbat1000⋅Pinhourst = \frac{C_{mAh}\cdot V_{bat}}{1000 \cdot P_{in}} \quad \t{hours}
  • η\etaconverter efficiency at the actual load current
  • DDduty of the load, 0…1
  • mmdesign margin
  • CmAhC_{mAh}usable battery capacity, after cut-off and temperature derating
More detail

Getting the numbers right

  • Efficiency at your load current. A buck rated 95 % may be 70 % at 10 mA.
  • Quiescent currents. Regulators, supervisors, dividers and pull-ups draw all the time. In a sleeping battery product they often dominate.
  • Average and peak. Average sets battery life and heat. Peak sets bulk capacitance, connector, fuse and trace width.
  • Inrush. Charging the bulk capacitance at power-on can far exceed the steady peak. It sets soft-start and fuse I²t.
Margin. 20 % is normal for a mature design; use 30–50 % on a first prototype.

Total loss is the heat left inside the enclosure. Compare it with what the enclosure can shed.

Workflow: budget → LDO Dissipation & Limits / Buck Converter Design for each stage → Junction Temperature & Safe Limits for each hot part → Trace Width for each high-current path.

Engine version ⁨1.18.3⁩