To find the input current for a 150W rad hard dc dc converter operating at 82% efficiency on a standard 28V aerospace bus, the converted answer is exactly 6.50 Amps. This calculation is the baseline for sizing upstream fuses, bus traces, and EMI filters in satellite power distribution units (PDUs).
The formula used is Iin = Pout / (Vin × η). Substituting our baseline values: Iin = 150W / (28V × 0.82) = 6.50A. The assumption that fixes this answer is the steady-state efficiency (η) at the specific bus voltage; rad-hard modules suffer efficiency drops at extreme cold (-55°C) or when operating far from their nominal input voltage.
The Core Conversion: Output Watts to Input Amps
When designing a spacecraft power bus, you rarely have the luxury of oversizing conductors. Every gram counts. Below is a quick-reference table showing how the input current shifts across a ±20% load range for our 150W baseline converter on a 28V bus, assuming a fixed 82% efficiency.
| Output Power (W) | Load Percentage | Calculated Input Current (A) | Minimum AWG (Chassis Wiring) |
|---|---|---|---|
| 120W | 80% | 5.20A | 20 AWG |
| 135W | 90% | 5.85A | 20 AWG |
| 150W | 100% (Nominal) | 6.50A | 18 AWG |
| 165W | 110% (Overload) | 7.15A | 18 AWG |
| 180W | 120% (Max Transient) | 7.80A | 16 AWG |
Real-World Rad-Hard Component Specs & Bus Variations
Single-voltage answers are never universal in aerospace. While 28V is the legacy standard for Low Earth Orbit (LEO) satellites, modern high-power payloads and More Electric Aircraft (MEA) rely on 50V, 100V, or 270V DC buses. Higher bus voltages drastically reduce input current, allowing for lighter harnesses. According to the NASA Electronic Parts and Packaging (NEPP) program guidelines, selecting the right converter requires matching the specific radiation environment (Total Ionizing Dose and Single Event Effects) to the electrical topology.
Here is a data-dense breakdown of real-world rad hard dc dc converter architectures, showing how input voltage and efficiency dictate the upstream current draw.
| Manufacturer / Series | Nominal Vin | Max Pout | Typical Efficiency (η) | Calculated Iin at Max Load |
|---|---|---|---|---|
| Infineon IRHMJ71134 | 28V DC | 100W | 80.0% | 4.46A |
| Vicor V28B12C250BL (Rad-Hard) | 28V DC | 250W | 85.5% | 10.40A |
| Crane Aerospace (Interpoint) SMFL | 28V DC | 75W | 78.0% | 3.44A |
| VPT (Bel Power) VPT28-150 | 28V DC | 150W | 83.0% | 6.42A |
| Infineon IRHNAV270 | 270V DC | 150W | 88.0% | 0.63A |
Notice the massive drop in input current when moving from a 28V bus to a 270V bus for the same 150W output. The 270V converter pulls less than 1 Amp, which is why high-voltage DC buses are dominating next-generation ESA and commercial space platform designs.
AC Test-Bench Feeds: When 120V, 230V, and 3-Phase Apply
A common point of confusion for junior engineers is trying to apply AC mains math to a DC-DC converter. A rad-hard DC-DC converter does not run on 120V AC or 3-phase power. However, when you are testing this $8,000 module on the bench, you must feed it using an AC-DC Ground Support Equipment (GSE) power supply. To size the AC mains breaker for your test bench, you must convert the DC input power back to AC line current.
Here is how the answer shifts when calculating the AC breaker size for a test bench feeding a 500W total DC load (assuming a 90% efficient AC-DC bench supply):
- 120V AC (Single-Phase): At a Power Factor (PF) of 0.95, the current is I = 500W / (120V × 0.90 × 0.95) = 4.87A. You would use a 10A breaker.
- 230V AC (Single-Phase EU): The current drops roughly in half to 2.54A, allowing for much thinner bench wiring.
- 208V / 480V (3-Phase): For massive thermal-vacuum chamber test racks pulling 10kW+ of DC power, 3-phase is used. The current is divided by √3 (1.732). A 10kW load on 480V 3-phase (at 0.95 PF) pulls only 12.6A per leg.
For the AC test-bench side, if the bench supply's Power Factor (PF) is unknown—common in older, non-PFC switching supplies—calculating the exact AC line current is impossible, rendering the 120V/230V conversion meaningless for precise breaker sizing.
For the DC-DC stage itself, steady-state efficiency calculations become meaningless during Single Event Latch-up (SEL) or heavy-ion radiation testing. Under SEL, the converter's internal CMOS may short-circuit, drawing dead-short current until the upstream Solid State Power Controller (SSPC) trips. Sizing wires based on nominal efficiency will result in melted harnesses during a latch-up event; you must size for the trip-threshold of the SSPC.
Frequently Asked Questions
Do I need to derate the input current for vacuum environments?
Yes. In a vacuum, there is no convective cooling. A rad hard dc dc converter relying on conduction cooling (via its baseplate to a cold plate) must be derated based on the thermal impedance of the mounting interface. If the baseplate exceeds 85°C, the manufacturer's datasheet will mandate a 20% to 40% reduction in maximum output power, which proportionally reduces your calculated input current.
How does Total Ionizing Dose (TID) affect efficiency?
As a converter accumulates TID (measured in krads or Sieverts), the threshold voltages of its internal MOSFETs shift, and optocouplers (if used in the feedback loop) degrade. This typically causes a 2% to 5% drop in efficiency over the mission life. Always add a 5% margin to your end-of-life (EOL) input current calculations to account for radiation-induced efficiency degradation.
Can I parallel rad-hard converters for higher current?
Only if the specific module supports active current sharing or droop-share (trimming the output voltage based on load). Parabling fixed-output rad-hard converters without current-sharing circuitry will cause one unit to hog the load, hit its over-current protection (OCP) limit, and shut down, cascading the failure to the second unit.
