IR2113 For VFD use, one driver keeps failing

Good day,

I am trying to use three IR2113 drivers for a VFD. I have followed the application guide as attached. I am using six STF13N95K3 n-channel MOSFETs whose datasheet is also attached.
Upon inspection of why the drive outputs an unbalanced voltage I found the output of the third driver was giving a constant high output for Ho and a constant low output for Lo.
The bootstrap capacitor furthermore also seemed to not fully charge and Vb-Vs was at 5.66 V whereas the others were at 15.2 V. I replaced that driver and both the two MOSFETs it was connected to and all seemed fine until it happened again.

My switching frequency is 25.6 kHz, using a sine-wave PWM scheme. The bootstrap capacitor calculation yielded 100 nF but it seems it might not be big enough as some of the pulses seems to switch off and then back on rapidly as though the capacitor was depleted.
What could I look at specifically to try and mitigate the problem as why one driver stops functioning and yet the others do not? I tried using a 450 nF bootstrap on all three drivers and it did not revive the one driver.

Kind regards
Ruan Kruger
Attachment 4397
Attachment 4398

?????

• float_drive.pdf‎ (191.2 KB)
• stp13n95k3.pdf‎ (856.4 KB)