oscillations with C3M0075120J

PS_OT

We are using two of the C3M0075120J SiC MOSFETs in a two-switch forward converter rated for input voltages in the range of 100 - 300 Vdc and 125 W output. Each of the MOSFETs is driven by a UCC5350SBDR gate driver chip located within 1 inch of the MOSFET gates. The turn-on and turn-off gate resistors are all 10 Ohm.

 We are experiencing oscillations at around 11 MHz with an amplitude of 8.8 Vpkpk on the gate of the lower switch (yellow), the drain of the lower switch (blue), and the source of the upper switch (purple). The oscillations are all synchronized. We believe that the oscillations are being driven by the gate of the MOSFETs.

 We can observe the same oscillations on the output pin of the gate driver with an amplitude of 4.2 Vpkpk.

 We can also observe the same oscillations on the power supply pin of the gate driver with an amplitude of 0.6 Vpkpk. The gate driver chips have an internal on-resistance from the output to the power supply pin of around 12 Ohms.

 Previously, when the oscillations had a smaller amplitude and duration, we were able to reach 125 W of output power with the 300 Vdc input and the MOSFETs would stay cool. Now we can only reach about 40 W of output power with the 300 Vdc input and the MOSFETs get extremely hot.

 We are using a Tektronix 6100 x10 voltage probe to measure the gate voltage, gate driver chip output and power supply pin (yellow) with a very short ground from the coaxial ring at the probe tip. We are using two Micsig DP10013 differential probes to measure the drain of the lower switch (blue) and the source of the upper switch (purple). The oscilloscope is a Tektronix TDS2014B.

 Any suggestions or recommendations for reducing/eliminating the oscillations would be greatly appreciated.

 Thanks.

 Phil

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