An ATtiny13A and MP18021 Gate Driver Make for a Handy, High-Performance Supercapacitor Charger
Finished design offers 6A charging current, surprisingly low temperature operation, and a scope-probing test-bench add-on.
Pseudonymous creator "sciencedude1990" has published build logs for a project designed to aid in the use of supercapacitors: A charging circuit using an ATtiny13A microcontroller and an MP18021 half-bridge gate driver — along with a few lessons on how not to blow things up.
"I wanted to learn more about supercapacitor charging," sciencedude1990 explains. "I had made a few boards using purpose-built ICs, but wanted to make it up of simple blocks that I could tweak and test. Also, I blew up a few of the purpose-built ICs, and wanted to slow the whole thing down."
"First, I designed an ATtiny13A eval board with a 20 MHz external clock. Then, I designed a 'testbed' with the basic parts — input capacitor, half-bridge, inductor, and output current sensor."
In a series of detailed project logs, sciencedude1990 walks through what's needed in a supercapacitor charging circuit, how it behaves, the design of the controller board and a test board with probe points for measuring the charge cycle. "Total capacitance, 450 F, total charge time from 0.7V to 2.8V, 2 minutes, 53 seconds," he notes. "So with the untuned controller, the average current was around 5.5 Amps."
"Note in the charging phase there are three steps. First, have the high side transistor on (to increase the inductor current), second have the low side transistor on (at the start, you actually don't turn on the lower transistor), and lastly, dead time. The dead time is super important because at the start, you want to gradually increase the lower transistor on time so that you don't discharge the supercapacitor if it has an initial charge."
"Since the switching node of the controller is okay with going a bit negative," sciencedude1990 notes, "you use the body diode of the lower transistor and the dead time to decrease the inductor current until the loops converge. When the loop converges, you shed the dead time in favor of the lower transistor on time."
The full write-up, with design files and source code for the microcontroller, is available on sciencedude1990's Hackaday.io page.