How to Design a Four-in-One Boost Inductor for a 250 kW PV MPPT

1. Freeze the operating envelope: 250 kW is not constant power over the full input range

The example uses a four-phase interleaved three-level boost, with a 350–1000 VDC input range, 1100 VDC maximum and a 1100 V calculation bus. Switching frequency is 32 kHz. Each channel is limited to 100 Arms and 62.5 kW: current-limited below 625 V and power-limited above. Four inductors share a potted 200 × 150 × 100 mm aluminum enclosure at 60 °C ambient, with a temperature-rise target below 30 K.

2. Four independent UU paths and inductance at 100 A

Each channel uses a 60µ FeSi powder UU core, with Ae = 1169 mm² and le approximately 194 mm. Two nine-turn coils are connected in series for 18 turns, using edgewound 2.0 × 10 mm flat copper. The four paths are independent, with a design coupling coefficient ≤0.05; combined-ripple cancellation relies on control phase shift.

Zero-bias model: L0 = N² · µ0 · µi · Ae / le ≈ 147.2 µH. At 100 A, H = N · I / le ≈ 9278 A/m. The document fits DC-bias roll-off to a material curve. The following values are calculated and exclude the temperature dependence of permeability.

The 147 µH zero-bias value cannot replace acceptance at 100 A. For incremental inductance that falls with current, review stored energy using W = ∫ i · L(i) di instead of only substituting the operating-point inductance into ½LI². Measured room-temperature and hot L-I curves take precedence.

3. Separate channel ripple, combined ripple and phase shift

Under the article's three-level modulation assumption, the inductor voltage switches in Vbus/2 steps and ripple frequency is 2fs = 64 kHz. Verify this against the actual PWM timing. The linear approximation is ΔIpp = (Vbus/2) · x · (1 − x) / (L · 2fs), where x = Vin/(Vbus/2), subtracting 1 when x exceeds 1. The document integrates nonlinear L(i) for its numerical results.

At 350 V and 100 A, lower-bound inductance gives approximately 20.5 A peak-to-peak channel ripple. At 825 V and full channel power, current is approximately 75.8 A and ripple is 19.3 A: 19.3% of rated 100 A, but 25.5% of actual current. A “channel ripple <20%” limit must specify its denominator.

With a Ts/4 shift between channels but a Ts/2 inductor-ripple period, four channels form only two ripple phases. Assuming equal currents and identical parts, the source calculation reaches approximately 6.4% combined ripple with Ts/4 and ≤1.5% with Ts/8. This is conditional on the modulation; inductance tolerance, sharing error and gate delay require measurement.

If each MPPT channel connects to a separate PV string, their input currents do not sum at one node. Define whether the requirement applies to individual input-capacitor current or bus-side current before using a combined-input-ripple limit.

4. Loss budget: include low-input-voltage operation

DC copper loss follows Pdc = I² · Rdc(T). The document uses 3.24 mΩ at 25 °C, approximately 4.18 mΩ at 100 °C, giving 41.8 W DC copper loss at 100 A. Core loss uses iGSE with typical material data, and AC copper loss uses a one-dimensional Dowell estimate. These are development-stage models.

Lower-bound inductance and 100 °C winding temperature. At normal current, 350 V / 100 A gives the worst total loss, approximately 257 W across four channels. Core loss is higher at 825 V, so that operating point also needs thermal testing.

5. Thermal paths and insulation at 60 °C ambient

A 257 W loss budget and 30 K rise require a total hot-spot-to-ambient resistance of approximately 0.12 K/W or less. The ideal surface area of a 200 × 150 × 100 mm enclosure is approximately 0.13 m². Removing all heat through that surface would require an equivalent heat-transfer coefficient of about 66 W/(m²·K), before internal winding-to-case gradients.

Airflow, effective surface area and the baseplate interface are not yet fixed, so “forced air” alone does not establish compliance. The document proposes a conductive base path to a heatsink or cold plate and control of winding–potting–case, core–base and base–heatsink thermal resistances. Instrument inner windings, cores, terminals and the base.

The development drawing lists winding-to-core tests of 3 kVAC / 60 s with leakage ≤5 mA, insulation resistance ≥100 MΩ at 500 VDC, and Class F or higher insulation. Applicable standards and editions, switching overshoot, overvoltage category, pollution degree, altitude and materials remain project inputs. These tests do not replace a complete insulation design review.

6. Sample validation and RFQ inputs

• Measure each channel's room-temperature and hot L-I curve at least at 0, 76, 100, 110 and 120 A; verify ≥80 µH at 100 A.
• Use four-wire DCR measurements corrected to 25 °C, recording terminals, joints and channel spread.
• Test 350 V, 625 V and 825 V with controlled airflow and base conditions to thermal steady state; verify overload for its specified duration.
• Measure individual and combined currents under actual PWM timing; confirm phase shift, denominator, measurement node and coupling.
• Complete dielectric, insulation-resistance, thermal-cycle, vibration, acoustic-noise and potting-quality checks.

For an RFQ, provide topology, channel count, shared or separate inputs, voltage range, channel current and overload, switching frequency and modulation, inductance at a stated current, coupling, ripple definitions, cooling and mounting conditions, insulation requirements and annual volume.

FAQ

Why review a 147 µH inductor against 80 µH at 100 A?

The 147 µH value is at zero bias. DC bias reduces permeability, so acceptance uses loaded inductance with temperature and material-batch boundaries.

Does four-phase interleaving guarantee combined ripple below 5%?

No. Confirm the ripple period, channel phase shift, input summation, part tolerances and actual measurement node.

Is 257 W a measured loss?

It is the document's four-channel estimate using lower-bound inductance, a 100 °C winding and other stated assumptions. Actual loss and temperature rise require sample tests.

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