Key Design of Airborne Power Supply: Calculation Method for 4kW LLC Resonant Converter

2026-05-13 17:27 Levanepower

Key Design of Airborne Power Supply:

Calculation Method for 4kW LLC Resonant Converter

In avionics, UAV power supply, airborne radar, and communication systems, airborne power supplies face challenges of small size, light weight, high efficiency, and extreme reliability. For a 4kW airborne secondary power supply, the LLC resonant topology has become an ideal choice due to its soft‑switching characteristics, low EMI, and high power density.

This article provides detailed calculation steps for the LLC resonant tank based on the actual requirements of airborne power supplies, helping engineers quickly complete parameter design.

I. Special Requirements of Airborne Power Supplies for LLC

Requirement	Description
Wide input voltage	Airborne DC bus (e.g., 270V with fluctuation 250V~350V, or 28V after voltage doubling) requires sufficient gain range from the LLC
High reliability	Reduce component count, avoid electrolytic capacitors, choose film capacitors + SiC devices
Limited cooling	High efficiency (>95%) reduces temperature rise; LLC inherently features soft‑switching
Low EMI	Resonant operation reduces voltage/current spikes, making MIL‑STD‑461 compliance easier
Power density	High frequency (100kHz~300kHz) reduces magnetic component size

This article takes an airborne power supply with input 350V~410V (nominal 390V), output 48V/4000W as an example to present the LLC calculation method.

II. LLC Resonant Parameter Calculation Steps

2.1 Determine Transformer Turns Ratio $n$

Half‑bridge LLC topology (commonly used in medium‑voltage airborne applications):
Primary voltage swing Vin /2, secondary rectified output Vo(neglecting rectifier drop).

n= Np/Ns =Vin,nom/(2*Vo)

n = \frac{N_{p}}{N_{s}} = \frac{V_{i n, n o m} / 2}{V_{o}}

SubstituteVin,nom=390V,Vo=48V:

n= 195 / 48≈4.06 ⇒ n=4

2.2 Calculate Equivalent Load Resistance (Primary Side)

Secondary load resistance:

RL,sec= Vo *Vo / Po = 48 * 48 / 4000 =0.576Ω

R_{L, s e c} = \frac{V_{o}^{2}}{P_{o}} = \frac{48^{2}}{4000} = 0.576 Ω

Reflect to primary side (fundamental approximation):

Rac=8/（ π*π） ⋅ n ⋅ n ⋅ RL,sec

Rac =0.8106 ×16 × 0.576 ≈ 7.47Ω

2.3 Choose $k$ and $Q$ Values

$k$ =Lm/Lr : Airborne power supplies require a wide gain range; take k=4 or $5$ . Here k=5.
Quality factor $Q$ : To balance full‑load efficiency and dynamic response, take Q=0.5.

2.4 Calculate Resonant Inductor Lr and Resonant Capacitor Cr

Resonant angular frequency ωr =2πfr, choose fr =100kHz

ωr =2π×100×10×10×10 =628318.5rad/s

From Q=ωr * Lr / Rac:

Lr = Q⋅Rac/ωr = 0.5×7.47 / 628318.5 ≈5.94μH

Then from fr =1/( 2π √Lr √Cr):

C r =1/ (ωr*ωr*Lr) = 1/ ((628318.5) * 5.94*10*10*10*10*10*10*10) = 427nF

C_{r} = \frac{1}{ω_{r}^{2} L_{r}} = \frac{1}{(628318.5)^{2} \times 5.94 \times 10^{- 6}} \approx 0.427 μ F = 427 nF

Engineering nominal value: choose Cr =470nF(multiple film capacitors in parallel); then Lr ≈5.39μH.

2.5 Calculate Magnetizing Inductance Lm

Lm =k⋅Lr =5×5.39=26.95μH

2.6 Gain Verification and Frequency Range

Normalised gain formula:

M(fn ,k,Q) =

M (f_{n}, k, Q) = \frac{1}{\sqrt{{(1 + \frac{1}{k} - \frac{1}{k f_{n}^{2}})}^{2} + Q^{2} {(f_{n} - \frac{1}{f_{n}})}^{2}}}

where fn =f/fr.

Calculate the required gain based on input voltage fluctuation (350V~410V):

Minimum input 350V: $M$ $max$ $= nV$ $o$ $/(V$ $in,min$ $/2) =192/175 \approx 1.097$
Maximum input 410V: Mmin = nVo/(Vin,min /2) = 192/205 ≈0.9366

Substituting k=5, Q=0.5:

At fn ≈0.85, M≈1.12 (satisfies Mmax) → fmin=85kHz
At fn ≈1.15, M≈0.94 (satisfies Mmin) → fmax=115kHz

For airborne power supplies, the frequency can be increased to 200kHz under light load to reduce switching losses.

III. Additional Design Points for Airborne Power Supplies

Component selection:

Resonant capacitor must be AEC‑Q200 grade film capacitor, voltage rating >600V, capable of high frequency and high dV/dt.
Secondary rectification: recommend SiC Schottky diodes or synchronous rectification to improve efficiency and reduce thermal stress.

Magnetic components:

Transformer core: choose wide‑temperature, low‑loss material (e.g., ferrite PC95), operating flux density <0.25T.
Separate resonant inductor requires an air gap to avoid saturation.

Thermal design:

Airborne environment often uses forced air or conduction cooling; overall efficiency target >96%, thermal resistance must be carefully calculated.

Protection features:

Overcurrent, over‑temperature, and undervoltage lockout (UVLO) must have redundant design, complying with DO‑160 standards.

IV. Conclusion

The calculation method above provides a complete parameter design path for a 4kW airborne power supply LLC resonant converter. By properly choosing $k$ , $Q$ , and the resonant frequency, high efficiency, low EMI, and high power density can be achieved over a wide input voltage range, fully meeting the stringent requirements of airborne power supplies.

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