I am using the ADuM3223 gate driver to drive the gates of an IGBT H-Bridge. The control circuit is similar to the one shown in Figure 1 of the application note "Inside iCoupler Technology: Driving an H Bridge with ADuM3220 Isolated Gate Drivers" where two control signals are used to fire the four gates.
I have an issue with the gate pulse width. The attached document "ADuM3223 Anomaly" illustrates what I am seeing. If the gate pulse width is less than 4us, the gate doesn't appear to properly turn on the IGBT as seen in the top scope waveform of the ADuM3223 Anomaly document. If I increase the gate pulse width to about 4.5us, the gate control works as expected as seen in the bottom scope waveform.
I am suspecting the bootstrap charging of the high side gate driver. At present, I have 2.2uF in parallel with 0.1uF ceramic capacitors to charge the high side gate drivers with an isolated supply of +12V.
Could the bootstrap circuit be a likely cause of this problem?
Thank you for the excellent scope shots. As you have already pointed out, the bootstrap configuration is a good starting point for us to look at. Is there any sizable resistance in the path from the 12V source to VDDB that could be causing a slow charging of the bootstrap capacitor? Which UVLO grade are you using?
Is it possible to get a scope capture that contains VDDA? This will either require a floating differential probe, two probes in a math subtract mode, or even just a probe on VDDA referenced to GNDB. This will allow us to see if the on-time of channel B is long enough to fully charge the bootstrap capacitor on the high side.
It is recommended to use at least a 10µF capacitor for decoupling.
It sounds like you have probably found the cause. The 11V UVLO is usually chosen to accommodate threshold voltages commonly seen in IGBTs. Generally, the secondary voltage UVLO is chosen to be slightly above the minimum acceptable voltage the designer wishes the gate of the switch to be driven at.
If you are using IGBTs, your idea of increasing the secondary isolated supply is a good choice. If you are using standard MOSFETs, using a lower UVLO grade ADUM3223A or B is probably the more robust option.
Don't forget the diode forward voltage drop when estimating the gate voltage that will be seen on the high-sides .