The battery gets the headlines. The inverter decides how much of that battery reaches the road.

In a sub-£30,000 electric hatchback, every kilowatt-hour is budgeted. Our reference platform runs a 55 kWh "Sovereign" sodium-ion pack at 152 Wh/kg at pack level. That is a deliberate cost decision. It also means the powertrain cannot waste energy. This Built in Britain instalment covers the component that recovers that energy: an 800V Silicon Carbide traction inverter.

Powertrain specification at a glance

  1. Switching devices: 1200V-class SiC MOSFETs, replacing silicon IGBTs.
  2. Inverter loss reduction: up to 70%.
  3. Powertrain system efficiency gain: 4–6%.
  4. System voltage: 800V nominal.
  5. Battery: 55 kWh sodium-ion, 361.8 kg pack, 152 Wh/kg at pack level.
  6. Net effect: a lower system cost, despite a higher device cost.

The core efficiency swap: IGBT to SiC MOSFET

A traction inverter loses energy in two ways: conduction losses while a device is on, and switching losses each time it changes state. Silicon IGBTs are penalised on both counts.

Where silicon loses

  1. A fixed knee voltage. An IGBT has a collector-emitter saturation voltage of roughly 1.5–2 V, whatever the current. At light load, that fixed drop makes up a large share of total loss.
  2. Tail current. Minority carriers take time to clear at turn-off. That tail current turns directly into switching energy.
  3. Diode reverse recovery. The silicon freewheeling diode adds recovery charge and further loss at every commutation.

Where silicon carbide wins

  1. Wide bandgap. At about 3.3 eV against silicon's 1.1 eV, SiC withstands roughly ten times the critical electric field. Drift layers are thinner, and specific on-resistance is far lower at 1200V ratings.
  2. Resistive conduction. A MOSFET behaves like a resistor, with no knee voltage. At the part-load currents that dominate real driving, conduction loss falls sharply.
  3. Unipolar switching. There is no minority-carrier tail, so turn-off energy drops by a large factor.
  4. Synchronous rectification. The MOSFET channel conducts in reverse during freewheeling, which avoids most of the diode loss.
  5. Higher junction temperature. SiC tolerates junction temperatures of 175°C and above, which widens the thermal margin.

The result: inverter energy loss falls by up to 70%. A silicon inverter averaging around 97% efficiency over a drive cycle becomes a SiC inverter at around 99%. Two percentage points may sound small. It means about two-thirds less heat.

From inverter to system: the 4–6% figure

The benefit extends beyond the inverter.

  1. A cleaner current waveform. SiC switches faster at lower loss, so switching frequency can rise. Current ripple falls, and so do iron and copper losses in the motor.
  2. Part-load operation. Urban and motorway cruising sit at a small fraction of peak power, and that is where the IGBT's knee voltage costs most. SiC gains most in exactly the conditions that make up most driving.
  3. Lower auxiliary load. With less heat to reject, the pumps and fans run less.

Together, these add up to a 4–6% gain in overall powertrain system efficiency. For this platform, it is the most cost-effective efficiency lever available.

Squeezing the 55 kWh pack

As the battery packaging instalment established, the 55 kWh "Sovereign" sodium-ion pack was chosen for cost, sovereignty and cold-weather resilience. Its trade-off is energy density. At 152 Wh/kg at pack level, with lower volumetric density than nickel-based lithium-ion, the floorpan leaves no room for a bigger pack.

So every watt counts. The inverter is where the platform recovers range without adding cells.

The range arithmetic

Illustrative figures, assuming a real-world baseline of 4.0 miles/kWh:

  1. Silicon IGBT baseline: 55 kWh × 4.0 = 220 miles.
  2. SiC at +5% system efficiency: 55 kWh × 4.2 = 231 miles.
  3. Extra range: about 11 miles, from the same cells, the same mass and the same floor height.

The pack you do not have to build

Now reverse the calculation. To reach 231 miles with a silicon inverter, the pack must grow by about 5%, or 2.75 kWh.

  1. Extra cell mass: 2.75 kWh ÷ 200 Wh/kg = 13.75 kg of active cells.
  2. Extra pack mass: at 76% Cell-to-Pack efficiency, about 18 kg in total.
  3. Extra pack cost: 5% of the £2,258 factory-gate pack, or about £113.
  4. Extra height or footprint: in a 110–130 mm pack envelope that is already full, there is none to spare.

The SiC inverter replaces that battery growth. Efficiency does the work that extra cells would otherwise have to do, which is cheaper and lighter. It also avoids a compounding penalty: a heavier pack needs more energy to move, which calls for more battery again.

The 800V thermal and weight dividend

The architecture runs at 800V. The physics is straightforward: P = IV. For the same power, doubling the voltage halves the current.

Current, copper and heat

Take an illustrative 150 kW peak traction demand:

  1. At 400V: 150,000 ÷ 400 = 375 A.
  2. At 800V: 150,000 ÷ 800 = 188 A.

Conductor cross-section is sized by current, through thermal rise and voltage drop. Halving the current allows a drastic cut in the copper cross-sectional area:

  1. A 400V DC link might need cables of around 95 mm². At 800V, 35–50 mm² will do.
  2. Copper weighs about 8.96 g/cm³. A 95 mm² conductor carries about 0.85 kg of copper per metre, against about 0.45 kg/m at 50 mm².
  3. Across the battery-to-inverter link, the DC fast-charge path, the busbars and the high-voltage auxiliary runs, that removes several kilograms of high-voltage wiring from the chassis.
  4. Resistive loss scales with I²R. Halving the current in the same conductor cuts cable heating by 75%, which gives engineers the margin to downsize further.

Thinner cables also have tighter bend radii, so they are easier to route and quicker to assemble. That cuts line time as well as material.

The 800V charging bonus

  1. At the same current limit, 800V doubles the available DC charging power.
  2. Sodium-ion's good rate capability makes full use of it, so a 55 kWh pack can charge quickly at high-power public chargers.
  3. For compatibility with the many 400V chargers on the UK network, the platform reuses the traction inverter and motor windings as a boost converter. No separate DC-DC stage is needed.

Engineering trade-offs to manage

  1. Device rating. 800V needs 1200V-class SiC. Silicon IGBTs at this voltage would perform poorly, so SiC is effectively required at 800V.
  2. Series cell count. Sodium-ion cells have a nominal voltage of around 3.0–3.1 V, so reaching 800V takes roughly 260 cells in series. In a 55 kWh pack, that means smaller-capacity cells and more BMS monitoring channels.
  3. Fast edges. SiC switches with a high dV/dt. That demands careful EMC design, motor insulation rated for the voltage stress, and bearing-current mitigation.
  4. Insulation and clearances. High-voltage connectors and creepage distances must be specified for 800V operation.

Budgetary impact: paying more to spend less

A SiC power module costs more than an equivalent silicon IGBT module. Substrate cost and wafer yield still carry a premium, although it keeps narrowing as 200 mm SiC wafer production grows.

The right comparison is not device against device. It is system against system. Here is the ledger:

Costs added

  1. SiC MOSFET power module: higher unit price than a silicon IGBT module.
  2. Gate drivers: faster, better-isolated drivers to control SiC switching edges.

Costs removed

  1. Radiator: about two-thirds less inverter heat, plus coolant at higher temperatures, which SiC's junction limits allow, means a physically smaller front-end radiator. It also needs less cooling-air aperture, which helps drag.
  2. Pumps: lower heat flux means lower coolant flow, so smaller thermal water pumps draw less 12V power.
  3. Harness: half the current means lighter, cheaper high-voltage wiring with less copper, smaller connectors and faster routing.
  4. Battery: about 2.75 kWh of cells, 18 kg of pack mass and £113 of pack cost that no longer need to be built.
  5. Passives: higher switching frequency allows a smaller DC-link capacitor and lighter filter components.
  6. Mass cascade: less mass means lighter brakes, suspension and body structure, and a further efficiency gain.

The net result is a system cost reduction. The device premium is recovered through the thermal system, the harness and the battery. That keeps the powertrain comfortably within the vehicle's £25,000 net manufacturing budget, which already carries the pack at just 9.0%.

Manufacturing context: assembling power electronics in the UK

An inverter is high in value and small in size. That makes it an efficient way to add local content.

  1. Midlands integration. Power module packaging, inverter assembly, end-of-line testing and electric drive unit integration take place within the Midlands automotive supply network, close to the vehicle plant and the Coventry battery operation.
  2. Imported dies, local value. Bare SiC dies can be sourced globally. The value-adding steps are done in the UK: die attach, sintering, substrate bonding, gate-driver PCB assembly, housing, calibration and test.
  3. A domestic semiconductor option. The UK's compound semiconductor cluster in South Wales offers a path to more local sourcing of wide-bandgap devices over time.
  4. Rules of Origin. Every pound of inverter value added in the UK counts as originating content. Together with the UK-converted sodium-ion pack, it keeps the vehicle comfortably inside the post-2027 TCA 45% cap on non-originating materials for tariff-free export to the EU.

Engineering summary

  1. SiC MOSFETs replace silicon IGBTs: up to 70% less inverter loss.
  2. Powertrain system efficiency: +4–6%.
  3. 55 kWh sodium-ion pack: roughly +11 miles of real-world range, or about 2.75 kWh, 18 kg and £113 of battery avoided.
  4. 800V: half the current, a drastic cut in copper cross-section, several kilograms of HV wiring removed, and twice the DC charging power.
  5. Net system cost: lower, with smaller radiators, pumps, harnesses and passives, and fewer cells.
  6. Local assembly: in the Midlands, supporting post-2027 TCA compliance.

Efficiency is the cheapest source of energy in the car. At 800V, silicon carbide is what turns a lean 55 kWh sodium-ion pack into a competitive entry-level EV.

Next in Built in Britain: Read Part 3: The Sovereign Motor Topology and Rare-Earth Magnet Strategy.