Sunday, September 9, 2007

DCDC Transistor Hack

After talking with various people and realizing that the over-current circuitry was making the FET gate pulses way too large, I decided to hack the DC-DC converter circuit.

Here's the new circuit (click to enlarge). I swapped out the IRFP350 FETs with IRFP460 FETs which have a higher voltage and current rating. To compensate for the larger gate capacitance and speed up the rise/fall time of the FET gates, I lowered the gate resistance from 27 ohms to 15 ohms.

The biggest hack, though, is tying the nShutdown pin to ground (disabling it) and adding a 2N3904 NPN transistor to bleed voltage off the soft-start capacitor when current is sensed too high. This makes a crude overcurrent sensor that limits the FET gate pulse without all the problems with pulsing the nShutdown pin. I adjusted the "amp adjust" potentiometer and was able to get the FET gate pulse to shrink if too much current was pulled.

Here is the top of the board. I've tied the emitter and collector of the 2N3904 to the leads of the soft-start capacitor. The base of the transistor is stretched to the left and goes down through a hole I drilled in the printed-circuit board.

Here's the back side of the circuit-board. I kept the .01uF capacitor to filter out the pulses on the current sensor and tied the lead of the 2N3904 (coming out through the hole) to the tap on the potentiometer. Note that I cut the trace just below the hole which originally tied the over-current circuit to the nShutdown pin, which I grounded with a large solder blob just to the right of the hole.

For the final test, I installed the unit back in the 914 EV, turned on the ignition and turned on all the accessories. The DCDC seemed to handle the current reasonably well without heating up much. The real acid test will be to drive around at night with the headlights on and see if the DC-DC fails again. Things look promising so far. I'm a bit miffed that CCPower could have solved this issue themselves with a cheap (69cents from Radio Shack, cheaper in bulk) transistor. I'll be driving around after dark this evening and will keep y'all posted.


Ross Cunniff said...

Very cool! It would be useful if you could come up with a "cookbook" of how to mod these things - although I really hope CC power fixes them, I'm not holding my breath...

pjorg said...


It is great that you have fixed this as well as debugging other aspects of the conversion kit. Now that you have been driving it for a while do you have any idea what your watt hours per mile may be? Have you had time to keep track of your cell temperatures? I have concerns with how much heat is building up in the batteries. Iam also wondering what the vibration problem is doing and if you have attacked that again.

Any one seen the infomation on the Nilar battery?