DC Fast Charging Module Magnetics at 800 V: Isolation, Loss and Liquid Cooling

What Changed When Chargers Went to 800 V

A 350 kW DC fast charger module is a different magnetics problem from an on-board charger. The DC bus moved to 800 V, the switching frequency rose to shrink the passives, the module is liquid-cooled at 60–70 °C coolant temperature, and the isolation barrier now carries a working voltage that makes partial discharge a first-order requirement rather than a formality. Designs carried over from 400 V platforms fail these modules for predictable reasons.

The Magnetics Inside a Charging Module

Typical magnetics and their dominant constraint
Position	Function	Dominant constraint
PFC inductor	Boost or totem-pole PFC at 800 V DC output	Core loss at high ripple with large flux swing; powdered or gapped ferrite
DC-DC transformer	Isolated LLC or CLLC stage, 800 V to 200–500 V	Isolation and creepage at 800 V working voltage; leakage inductance set for resonance
Resonant inductor	Sets the resonant tank with the transformer	Gap stability and loss at high frequency, plus fringing loss near the gap
Output inductor	Filtering at the charge port	High DC bias with saturation margin, and thermal path to the cold plate
Gate drive transformers / current sense	Isolation for SiC gate drivers	Common-mode transient immunity, creepage at high dv/dt

Isolation at 800 V Is Not a Hipot Number

At 800 V working voltage the design question is not whether the part passes a routine hipot test, it is whether the insulation has voids that will discharge in service. Practically that means: a specified insulation system qualified for the class and the construction, a winding arrangement that keeps the highest field out of air pockets, vacuum impregnation or potting to eliminate voids, and partial discharge testing at a stated inception and extinction limit. A part that passes 3 kV hipot and shows PD at 900 V will fail in the field on a humid day.

Thermal Design Follows the Coolant, Not the Air

Conduction, not convection. In a liquid-cooled module the part is potted to a cold plate, so thermal resistance is dominated by the potting layer and the interface, typically 2–5 K/W rather than 10–15 K/W in free air.
Coolant at 65 °C leaves little margin. With a 65 °C coolant and a Class H system at 180 °C, the permitted rise is around 110 K — generous on paper, but the hotspot inside a potted winding is often 20–30 K above the measured surface, so the effective budget is smaller than it looks.
Potting compound choice matters. Thermal conductivity, modulus and CTE all affect both the hotspot and the mechanical stress on the winding during thermal cycling.

Design Points Buyers Should Specify

Working voltage across the barrier, and the standard the module must meet.
Partial discharge limit and the test method.
Coolant temperature, flow, and the mounting interface (cold plate flatness and thermal interface material).
Switching frequency range, because core loss and AC copper loss both scale with it and the supplier needs the worst-case point.
Leakage inductance target with tolerance, not just a maximum.
Duty profile: continuous at rated power, or a charging profile with a long plateau.