Introduction

As solar, wind and storage converters push the DC bus higher and the efficiency targets tighter, the three-level converter has become a mainstream choice. A three-level NPC or ANPC bridge reduces the voltage step across each device from the full bus to half the bus, which lowers the switching loss and improves the quality of the output waveform, allowing a smaller output filter. The price is more devices and a more complex gate drive. This application note explains the topology, the loss estimate and the layout for Semikron IGBT modules in three-level converters.

The Three-Level Idea

In a two-level bridge each device blocks the full DC bus, and each commutation steps the output by the full bus voltage. In a three-level bridge, the output can take three levels, and the voltage step across each device is half the bus, so the switching loss per commutation is lower and the output has less harmonic content. The NPC and ANPC circuits achieve this with clamping diodes or active switches, and the devices are arranged so that no single device sees more than half the bus in normal operation. The reduced step is what makes the topology attractive at a high bus voltage.

Loss Trade-Off

The three-level topology shares the switching loss across more devices, which lowers the junction temperature for a given output power, and the lower voltage step reduces the switching energy per event. The conduction loss depends on how the output current is distributed among the devices, which differs between NPC and ANPC. The net result is usually a higher efficiency and a smaller filter, at the cost of more devices and a more complex gate drive. The BeiLuo FAE team can run the loss estimate for the specific topology and operating point.

Module Selection

SEMITRANS 3+ is optimised for modern three-level topologies as well as the classic half-bridge, offering the IGBT 7 (Gen 7) chip with CAL7 diodes and a 175 degree junction in the proven 62 mm footprint. The lower stray inductance of the redesigned package reduces the turn-off overshoot, which matters when several devices switch in the same commutation loop, and the multiple chip versions help supply stability. The voltage class is chosen for the full bus with margin, because the module must survive the peak voltage of the topology, not just the nominal level.

Gate Drive and Layout

A three-level converter has more switches, each with its own isolated supply and gate drive, so the driver design is more involved than a two-level bridge. Using matched Semikron drivers keeps the isolation, the protection and the timing consistent across the stage, and the soft shutdown and active clamping protect the devices during a fault. In the layout, keep each commutation loop as small as possible and place the clamping components close to the modules, because the loop inductance sets the overshoot in the same way as in a two-level bridge. The BeiLuo FAE team helps with the module and driver selection and the layout review.

Validation

Validate the design on the bench at the worst-case operating point: measure the switching energy, the overshoot at the module terminals and the junction temperature, and confirm the output waveform meets the grid code. Because the topology shares the stress across several devices, a fault can propagate, so the protection sequence deserves the same attention as the power stage. With the right modules and drivers, a three-level converter delivers high efficiency and a clean output for solar, wind and storage.