Thursday, May 21, 2015

OpenEVSE II update

OpenEVSE II is for sale, and the design is more or less finalized at this point.

The most recent fiddling I've been doing has related to the pilot generator. While the old one was operating within the J1772 specification for the slew rate, it was barely doing so. One thing I learned the hard way was that resistors slow things down. But more so, in the original design, there was a tension between the primary transistor pair base bias resistors and the pull-up. In order to insure that both transistors are always in either one state or the other, the pull-up resistor has to overcome the bias path from +5 through the -12V primary transistor, to its base, through its base bias resistor, through the +12 primary transistor's base bias resistor, and then through the base-emitter junction to ground. In order to make that work, the pull-up had to be one tenth of what the base bias resistors were. The latest design was 10k base biasing and 1k pull-ups, but again, that was barely fast enough. The only improvement would have been to use 4.7k base biasing and 560 or 470 ohm pull-ups. But with that, you'd be sinking way more current through the microcontroller when it's pilot output was low. It just wasn't reasonable.

The solution was to change out at least the primary pair for MOSFETs. MOSFETs don't have a conduction path from their gate to drain, as there is in BJTs from their base to emitter. So the base bias can be 0 ohms and the pull-up can be 10k. The secondary pair can either remain a pre-biased BJT pair (47k emitter to base and 10k base to input), or can be also swapped out for a MOSFET pair, but with source-to-gate biasing of 2.4k and gate-to-input biasing of 2.4k (this is only necessary to reduce the gate voltage swings. Otherwise the -12 secondary gate will see a gate voltage of 17 volts, which is quite a bit). Either way, the slew rate is around 1 µs, which is half of the spec maximum of 2 µs.

If there is a next thing in the design to tackle, it would be to attempt to further increase the sensitivity of the GCM. Right now, it's on the order of around 50k or so, but the spec demands a trip point of 100 ohms per volt (so 12k for L1 and 24k for L2). Figuring out how to achieve that reliably is going to be a tough tuning task, but it still shouldn't result in any incompatible design changes at this point.

The next step will be to update the design of the Hydra to include all of the lessons learned. Stay tuned.

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