FBH’s GaN-based 650 V switching transistors feature a particularly low gate charge and a low output capacitance that may result in 400 V switching transients of down to 200 V/ns slew rate. Already small parasitic inductances and capacitances may already trigger oscillations during the switching events and increase switching losses.
We developed GaN half-bridge modules with very low parasitic inductances by using FBH front-end and back-end processing technologies. Besides the GaN chips the module supports the gate driver ICs and DC-link capacitors in a hybrid assembly approach. The submount made of aluminium nitride ceramic allows for optimum cooling. The combination of active and passive SMD devices and wire-bonded GaN dies with two high-current metal layers enables minimum parasitics and very small gate loops as well as small power loops. Using the monolithic GaN half-bridge chips on this AlN-ceramic based power core further reduces the power commutation loops.
The AlN-based power core performance was quantified by simulating the parasitic stray inductance of the commutation loop, an essential parameter which affects transient turn-off voltage overshoot, the switching losses as well as the stability of the switching transitions. In comparison with a conventional PCB GaN half-bridge setup, the power core feature a notable lower commutation loop inductance . The table summarizes the layout related parasitic circuit elements like stray inductance of the commutation loop (Lσ) and parasitic capacitance of the switching node (Cpar), parameters that impact module’s switching operation. Additionally, because of AlN’s high thermal conductivity (170 W/mK), the hybrid modules have significant enhanced cooling capabilities achieving a thermal resistance 10 times smaller compared to the reference PCB setup.