Friday, March 7, 2014

Revised Charge Pump

Almost all boot-strap FET drivers rely on a less then 100% duty cycle in order to drive the charge-pump; actually have found only ONE  exception - the LT1910f.  A great part, worked clean and well - but would not support voltages sufficient to deploy in a 48v environment (Specifically to support Equalize mode on a High Drive configured alternator).   So I looked at ways to add an external charge pump to more traditional FET drivers by using an unused pin from the Amtel  CPU as the PWM driver and a voltage doubler.  My original idea was this:

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
In operation, when Q5 is enabled C12 will charge to around 11v via D9, R19, and D11.  When Q5 turns off, R9 then starts to bleed some of that 11v back to the net named 'Floor'.  This starts Q6 to turn on as we now have the 11v in C12 applied across Q6's Gate-Source (with D11 now blocking the bead-back)..  As Q6 turns on, it pulls the Floor up to the level of the main FETs Source (Vs on U3), pushing C12 up with it via R19.  Resulting in the 'Pump' net to ultimately be sitting 11v above the floor - transferring this charge pump voltage into C11 via D8.  R19 is in place to limit peak surge current, protecting both the FETs and the Diodes.

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. . . .

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