What worked out well with the initial build:
- Converter maintained voltage regulation well beyond the 30 mA output design limit
- Output ripple was well-controlled
- RCD snubber proved to be unnecessary to protect the MOSFET
- Load transient response demonstrated good loop stability
What was a disappointment:
- The no-break injection circuit didn't work at all and destroyed both the MOSFET and diode on one prototype
- Input voltage ripple shows large spikes
- Efficiency remains close to, but below, 80% under full load
- Thermal problems at high load and low input voltage
Since everyone is still going nowhere, I sent off a second board revision for manufacturing with a couple of changes:
- I removed the no-break injection circuit and replaced it with the more "traditional" way of doing voltage injection by breaking the loop across a small series resistance. This method is documented in TI's AN-1889 and requires a wideband injection transformer. These are a bit of a back-catalog item of test equipment, but it turns out they are eminently homebrew-able with excellent results.
- The ceramic input capacitors have been moved next to the transformer, with a solid ground plane underneath. Additionally, I added a couple of footprints for some 1 µF or smaller capacitors. Throwing additional capacitance in has diminishing returns, but it's easy to leave them unpopulated if they also prove to be of no help. This change also means I can trim 5.5 cm^2 of board space without increasing component density.
The efficiency of the converter could be improved by using a lower switching frequency to reduce switching losses. The choice of switching frequency was driven by the limit I chose on inductor current ripple. Lowering the switching frequency will increase the current ripple and increase the minimum load where the converter begins to operate in DCM. It will also precipitate re-evaluating most of the passive component values, so I'll leave this be for the moment.