Direct Answer: For a 100W output radiation hardened DC-DC converter operating on a standard 28V DC spacecraft bus with an assumed 85% efficiency, the steady-state input current is 4.20 Amps.
The foundational formula used to derive this is $I_{in} = \frac{P_{out}}{V_{in} \times \eta}$. Substituting our benchmark values: $I_{in} = \frac{100W}{28V \times 0.85} = 4.20A$. This calculation assumes a steady-state DC input, a fixed nominal voltage of 28V, and an 85% power conversion efficiency. In aerospace power distribution, nailing this baseline number is critical for sizing upstream fuses, EMI filters, and harness wire gauges before the hardware ever sees a thermal vacuum chamber.
Core Conversion Formula and Neighboring Load Values
Spacecraft payloads rarely sit at a perfect 100% static load. Transceivers key up, reaction wheels spin, and telemetry bursts occur. To properly size your upstream protection, you need to see how the input current shifts across a ±20% power range around the 100W nominal target. The table below maps the input current draw assuming the same 28V bus and 85% efficiency baseline.
| Output Power ($P_{out}$) | Input Voltage ($V_{in}$) | Assumed Efficiency ($\eta$) | Calculated Input Current ($I_{in}$) |
|---|---|---|---|
| 80W (-20%) | 28V DC | 85% | 3.36 A |
| 90W (-10%) | 28V DC | 85% | 3.78 A |
| 100W (Nominal) | 28V DC | 85% | 4.20 A |
| 110W (+10%) | 28V DC | 85% | 4.62 A |
| 120W (+20%) | 28V DC | 85% | 5.04 A |
When selecting a fuse or solid-state power controller (SSPC) for this branch, you would typically apply a 125% derating factor to the maximum expected steady-state current (5.04A × 1.25 = 6.3A), pointing you toward an 8A or 10A rated protection device, depending on the specific inrush profile.
Real-World Spec Sheet Data for Space-Grade Converters
Theoretical efficiency is one thing; silicon performance in a high-radiation environment is another. When sourcing a radiation hardened DC-DC converter, you must balance power density against Total Ionizing Dose (TID) survivability and Single Event Effects (SEE) immunity. Below is a data-dense comparison of real-world architectures you will encounter when querying suppliers like Crane Aerospace (Interpoint), VPT, or Vicor for 100W-class flight hardware.
| Manufacturer / Series | Input Bus ($V_{in}$) | Output Config | Max Power | Peak Efficiency | TID Rating (Si) |
|---|---|---|---|---|---|
| Crane Interpoint MSAG | 28V DC | 3.3V / 5V / 12V | 120W | 88% | 100 krad |
| VPT VHF28 Series | 16-50V DC | Single / Dual | 100W | 85% | 100 krad |
| Vicor Radiation-Tolerant | 28V / 100V DC | 48V / 12V | 150W+ | 92% | 50 krad (Tolerant) |
| IR (Infineon) ESA DC-DC | 28V DC | 3.3V / 5V | 80W | 82% | 300 krad |
Notice the efficiency delta. If you select the Vicor module for a high-voltage bus application and achieve 92% efficiency, your 100W load at 28V only draws 3.87 Amps instead of 4.20 Amps. That 0.33A difference translates to significantly less heat dissipated inside a sealed satellite chassis, which is often the limiting factor in NASA electronic parts packaging thermal designs.
How Input Current Shifts Across Spacecraft and Terrestrial Buses
A common mistake in systems engineering is treating the 28V calculation as universal. The input current shifts drastically depending on the power bus architecture. Furthermore, during Ground Support Equipment (GSE) testing, you are often feeding the test rack from terrestrial AC mains, which requires an entirely different conversion approach.
- 28V DC Bus (Standard Mil-Aero): As calculated, 4.20A. This is the legacy standard (MIL-STD-704) for most LEO satellites and tactical aircraft.
- 100V DC Bus (High-Voltage Spacecraft): For larger GEO commsats, 100V buses reduce harness mass. At 100V and 85% efficiency, $I_{in} = \frac{100}{100 \times 0.85} = $ 1.17 Amps. Wire gauge can be drastically reduced.
- 270V DC Bus (More Electric Aircraft): Emerging in next-gen aerospace. $I_{in} = \frac{100}{270 \times 0.85} = $ 0.43 Amps.
What about 120V vs 230V vs 3-Phase AC?
Radiation hardened DC-DC converters are strictly DC-input devices. They will catastrophically fail if fed AC directly. However, if you are calculating the upstream AC mains draw for the Ground Support Equipment (GSE) power supply that ultimately feeds your 28V flight unit, you must account for Power Factor (PF) and phase geometry.
For a single-phase 120V AC GSE supply (assuming 90% AC-DC rectifier efficiency, 85% DC-DC efficiency, and 0.95 PF):
$I_{AC} = \frac{100W}{120V \times 0.90 \times 0.85 \times 0.95} = $ 1.15 Amps AC.
If your test facility uses a 230V AC 3-Phase supply to power a massive multi-unit burn-in rack, the formula shifts to include the $\sqrt{3}$ multiplier:
$I_{Line} = \frac{P_{Total}}{\sqrt{3} \times V_{Line-Line} \times \eta_{Total} \times PF}$.
Ignoring the $\sqrt{3}$ multiplier on a 3-phase GSE rack will cause you to undersize your facility breakers by nearly half.
When This Conversion Becomes Meaningless (Edge Cases)
The steady-state $I_{in} = \frac{P_{out}}{V_{in} \times \eta}$ formula is a reliable workhorse, but it becomes dangerously misleading under specific aerospace edge cases. If you size your wiring purely on the 4.20A figure without considering the following, your harness will fail qualification testing.
1. What happens during cold-crank or startup inrush?
Meaningless for inrush. When a radiation hardened DC-DC converter first energizes, it must charge massive internal input capacitance (often 1000µF to 3000µF to survive voltage ripple). Inrush currents can spike to 40A to 60A for several milliseconds. Your upstream fuse must be a slow-blow or time-delay type rated to survive this $I^2t$ let-through energy without nuisance tripping.
2. Does efficiency hold at 10% load?
No. The 85% assumption fixes the answer at nominal load, but converter efficiency typically drops off a cliff below 20% load due to fixed quiescent switching losses. If your 100W converter is idling at 5W in a safe-mode, efficiency might drop to 40%. The input current won't scale linearly with output power.
3. How does radiation degradation affect the math?
Over a 15-year GEO mission, Total Ionizing Dose (TID) exposure degrades the RDS(on) of the internal MOSFETs and increases the ESR of output capacitors. A converter that draws 4.20A at Beginning of Life (BOL) might draw 4.65A at End of Life (EOL) as efficiency degrades to 78%. Always run thermal and wire-sizing calculations using EOL efficiency figures provided in the manufacturer's radiation test reports.
4. What if the input bus experiences a dropout?
If the 28V bus sags to 22V during a solar array eclipse or thruster firing, the converter will pull more current to maintain the 100W output ($I = \frac{100}{22 \times 0.85} = 5.34A$). If your upstream wiring is sized exactly for 4.20A, this low-voltage/high-current state will cause excessive voltage drop and potential thermal runaway in the harness.
Ultimately, calculating input current for a radiation hardened DC-DC converter is just the starting line. The 4.20A figure gets you the right wire gauge for nominal operations, but surviving the vacuum of space requires designing for the inrush spikes, the end-of-life radiation degradation, and the low-line voltage sags that the basic formula conveniently ignores.
