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Magnetics for charging, transport and solid state transformers

DC fast charge stacks, solid state transformer cells and fuel cell converters. Power density is the binding constraint here, which is why planar construction and litz windings earn their cost in this sector when they would not in another.

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Where the magnetics sit

The stages, and what goes in each one.

Isolated bidirectional stage

CLLC or dual active bridge, the workhorse of a charge stack and of an SST cell.

The magnetic: Medium frequency transformer where the leakage inductance is the power transfer element. It is specified, designed and measured, not discovered.

Resonant tank

Series and parallel elements set the gain curve and the switching behaviour.

The magnetic: Resonant inductor, either discrete or integrated into the transformer leakage, whichever gives the better loss and the better tolerance.

SST cell isolation

Cascaded cells, each isolated, stacked to reach medium voltage.

The magnetic: Cell transformer designed for the isolation between cells and for partial discharge behaviour, not just for hipot pass or fail.

Fuel cell converter

Low voltage, high current input boosted and isolated.

The magnetic: High current input inductor and an isolation transformer, both dominated by conduction loss and by the thermal path out.

What decides it

What actually drives the design here.

Power density

Every millimetre is contested in a charge stack. Planar construction, litz and careful thermal design are what buy the volume back.

SiC and fast dv/dt

Fast edges push common mode current through the interwinding capacitance. That capacitance becomes a design parameter, traded against leakage inductance.

Partial discharge

At medium voltage a part that passes hipot can still be discharging internally. The insulation system has to be designed so it does not.

Isolation and the tank at once

Leakage inductance is wanted for the resonant tank and unwanted for coupling. The winding arrangement has to serve both, and we show you where the compromise landed.

The shape of it

Standards, range and typical parts.

Isolation
IEC 61851 and the isolation class your architecture calls for
Frequency
Medium frequency stages typically 20 kHz to 500 kHz, and up to 2 MHz where the design wants it
Construction
Planar on your PCB, on a dedicated winding board, or in stamped copper; litz and triple insulated wire
Typical parts
CLLC and DAB transformers, resonant inductors, SST cell magnetics, high current input chokes

Questions

Charging, transport and SST, the usual questions.

Can the resonant inductor be integrated into the transformer?
Often yes, and it is usually the better answer for power density: you get the inductance from the leakage rather than from a second component. The cost is that the leakage now has to be controlled tightly, because it sets the gain curve. We tell you which route gives the better tolerance for your tank.
How do you control interwinding capacitance?
With the winding arrangement, the interleaving and the insulation thickness. Reducing leakage inductance by interleaving raises capacitance, so the two move against each other. With SiC edges the capacitance is often the one that hurts, and we design to that rather than to a default.
Do you build planar transformers on our PCB?
Yes, on your board, on a dedicated winding board we design, or in stamped copper. The advantage of planar here is that the leakage inductance is fixed by the artwork, so it repeats unit to unit instead of depending on how the wire was laid.
Can you test for partial discharge?
Yes, partial discharge is part of what we test and report, along with hipot and interwinding capacitance. For a medium voltage cell it is the test that tells you whether the insulation system is really right.
What is the lead time for a charge stack prototype?
We aim to send an engineering answer with a first design proposal inside 24 hours, then 4 to 8 weeks from an agreed design to a prototype, subject to raw material stock. No minimum order quantity.

Start here

Send us your charging or SST spec.

A schematic and a waveform are enough to start. You get a first design proposal with estimated losses, the inductances, the dimensions and the expected hotspot temperature, before you have spent anything.

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All sectors we supply