Driving a Capacitive Load with an H-bridge

Posted on Feb 6, 2014

Alright, so I had some post-publication weirdness following my last post, and it took me a few days to figure out exactly what was up, but I think I finally have all the details, and I learned a lot in the process. I noticed some weirdness towards the end of writing my 8, 000+ behemoth of a post last week. For those of you just checking in, I built

Driving a Capacitive Load with an H-bridge
Click here to download the full size of the above Circuit.

a simple Class D amplifier to provide an EL panel with a +/- 120V square wave. A Class D is basically an H-bridge attached to a DC/DC boost converter. Here`s the schematic of my H-bridge along with the output bypass capacitance of my DC/DC flyback (boost) converter on the left. The two tabs in the middle connect to my EL panel: I spent most of the post designing my boost converter based on my expected load. My load was an EL panel which I measured with an LCR meter and got the following: Based on some rudimentary research, I had assumed that an EL panel could be modeled as a resistor and capacitor in series. The resistance would drop as the frequency increased, but because I was planning on driving no faster than 1kHz, my load would primarily be a 56k as my desired load and did all my component sizing based on driving that steady load at 120VDC. Once I finally put everything together though, I noticed that my positive rail wasn`t doing a very good job maintaining that 120V: Now, I had some explanation for this. My flyback converter was controlled by a special chip that tries to maintain the output voltage by controlling the duty cycle going into the flyback. It`s possible that the flyback controller was simply not anticipating the sudden increase in load on the rail and therefore was unprepared when it hit. It was essentially going from an infinite resistance (fully charged capacitor) where it had probably dropped the duty...

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