SLUAAB9A March 2021 – December 2021 UCC25800-Q1
With the development of wide-bandgap power-semiconductor technology, inverters start to use SiC or GaN MOSFETs as the switching devices. These wide bandgap devices have faster switching speed and create higher dv/dt at the switch node, sometimes well above 50V/ns. This high dv/dt can couple through the transformer parasitic capacitance, as shown in Table 5-1. Common-mode transient immunity (CMTI) test becomes a standard requirement for this type of application. A UCC25800-Q1 based open-loop LLC converter uses a transformer with low parasitic capacitance. This makes the solution demonstrate excellent CMTI performance. The CMTI test setup is shown in Figure 9-1.
A CMTI generator was used to create CMTI strikes. It operates with 5-kHz switching frequency and 50% duty cycle. Its rise and fall rate are 165V/ns and 155V/ns respectively. With such a high voltage slew rate, the UCC25800-Q1 still operates normally. Its SW pin voltage remains a fixed switching frequency square wave.
(a) CMTI strikes don't interfere with converter switching (CH1: CMTI strike; CH2: UCC25800-Q1 SW-pin voltage; CH3: Not used) | |
(b) CMTI rise rate (CH1: CMTI strike; CH2: UCC25800-Q1 SW-pin voltage; CH3: Not used) | (c) CMTI fall rate (CH1: CMTI strike; CH2: UCC25800-Q1 SW-pin voltage; CH3: Not used) |
(d) CMTI postie strike | (e) CMTI negative strike |
The open-loop LLC converter uses a transformer with such a low parasitic capacitance. The current injection from the high dv/dt switch node becomes greatly reduced when compared to legacy solutions. Therefore, this solution can easily pass the CMTI test.