TIDUF28 November 2023
The efficiency testing was done at 27°C lab temperature with a Tektronix PA4000 power analyzer. The TIDA-010255 PCB without heat sink was placed horizontal on the workbench as shown in Figure 4-4, with natural convection only. The F28379D MCU software was configured to create a 3-phase AC voltage at 1-Hz frequency with configurable amplitude. The PWM carrier frequency was set to either 16 kHz or 8 kHz.
The following figures show the TIDA-010255 PCB power losses without heat sink versus the 3-phase motor load current in ARMS in the steady state, when the PCB and the GaN-FETs reach their steady state temperature, typically after around 5 minutes. The power losses are dominated by the switching and conduction power losses of GaN-FET, while the phase current shunt power losses are negligible.
The TIDA-010255 board power losses at an output current of 7.7 ARMS at 16-kHz PWM were 16.09 W, and 11.2 W at 8-kHz PWM.
The theoretical maximum peak efficiency at 320 VDC with a maximum phase-to-phase of voltage of 130 VRMS (Space Vector PWM with 3rd harmonics) and a power factor of 0.9 is 99.4% at 16-kHz PWM and 99.6% at 8-kHz PWM.
To see the effective parasitic capacitive losses, the TIDA-010255 PCB power losses were measured at zero load current with 50% PWM duty cycle and PWM switching frequencies from 8 kHz to 64 kHz, as shown in Figure 4-21. In the first test the inverter output was left unconnected. The losses at 64-kHz PWM were 21.7 W. The total losses are per Equation 5, where COSS_HB is the effective parasitic capacitance per half-bridge of around 1.1 nF, which gives around 550 pF per each of the six power switches including TIDA-010255 PCB parasitic capacitance. Assuming a 50-pF parasitic PCB capacitance, the estimated time related effective output capacitance CO(tr) of the LMG3422R030 from 0 V to 320 V is around 500 pF, which is around 15% higher than the 430-pF CO(tr) from 0 V to 400 V.
In the second test an AC induction motor with a 1-m cable was connected to explore the impact on the overall zero load current losses. Again, the PWM duty cycle was set to 50%, hence no motor current was driven. The losses at 64 kHz increased to 22.7 W. The additional parasitic load capacitance with the 1-m cable an AC induction motor was calculated around 50 pF per phase.