|Soft Switching R9/21 was not sufficient for IRS2127 leakage|
Kind of a modified voltage doubler, it utilized the soft switches of R9 and R21 to handle the 'boost'. I had hoped that the modest current demands of driving FETs would allow this simple approach to work. But it did not, the internal Vb impedance of the IRS2127 was such that too much of the charge was bled off, rendering the soft-switching idea unworkable...
So, I modified it replacing the soft resister switches with a hard FET. I found an interesting approach of configuring a FET to self-switch, eliminating the need for an inverted PWM driver with all its associated issues of race conditions. (reference: "A Self-Boost Charge Pump Topology for a Gate Drive High-Side Power Supply" by Park and Jahns. IEEE March 2005). Building upon this idea I now have:
|Revised hard switching Charge Pump|
There is a bit of inefficiency in this design (ala R19), but as the frequency is very low (455hz) we really do not need to worry about things like switch loss, etc too much. I have mocked up this modified charge pump and it works great. Spice modeling shows this will also more then handle 2mA of current demands, more then sufficient for any leakage in U3.
One side effect of this was needed to create a 12v voltage source. I did this by raising the intermediary voltage in the power supply from 8V to 12v (simply changing the per-regulator Zener). A down side of this is the 3.3v regulator now needs to do more work and is starting to get a bit on the edge for heat dissipation. So one will need to add either a small separate heat sink to it, or drill and mount it to the main heat sink.
Has been fun to come up with this, but will tell you: Sure wish the LT1910f would had about 10v more headroom in it. . . .