Initial CCPower Schematic

In my efforts to debug the failures of the DC-DC converter, I tried to capture a schematic of the board. I didn't have time to create a component for the SG3525A controller, but all the pin numbers are listed. I replaced the IRFP350 FETs with IRFP460 FETs which have a higher isolation voltage and larger drive current; however, they also have about twice the gate capacitance, which I didn't think of.

Any suggestions for what could blow the FETs up would be helpful (please leave in comments section).


You can get a larger image by clicking on the thumbnail above.
Links to the various parts are:

IRFP350
IRFP460
SG3525A

The failing conditions are when the 12V accessories are drawing a lot of current (25 amps) and the output voltage has sagged down to 12V (DC-DC tries to put out 13.9V).

I'm guessing that as the voltage sags, the output (pin 6) of the 741 op-amp goes low and turns off the LED in the opto-coupler. With the opto-coupler off, the voltage goes low on pin1 of the SG3525A (inverting input to error amp) and increases the pulse widths going to the drive FETs. If the pulse widths are too long, they cause the FETs to stay on too long and saturate the inductors, which causes a high-current spike of +144V that blows the FETs.

Supposedly, the current sense loop voltage that is adjusted by the R8 potentiometer should send a positive pulse into pin10 of the controller to shut down the FETs, but this may not be happening quickly enough.

Initially, I cringed because there were no kick-back diodes in the circuit, but I think that C7 and R16 (2 watt resistor) send any kick-back currents into the opposite inductor when the FETs let go.

Any ideas? Thanks for your input.

Cheers,
Tim

Taking a Break

These last few days have packed with non-EV stuff, but I did take the DC-DC controller into the lab to see what was going on. First of all, there are no kick-back diodes on the inductors inside the converter. This kinda scares me since pulling on an inductor with a FET and then letting it go tends to create really high voltages (> 600V etc...).

I hacked together a 160V DC power source by simply tapping off the bulk capacitor inside one of our power supplies. It fired up the DC-DC converter, but the whole thing was moving up and down at 60 Hz, it was darn near impossible to look at things with an oscilloscope.

Next week I plan on taking five 32-volt isolated supplies (2.5 amp) and hooking them up in series to form a 2.5 amp 160V supply with a non oscillating ground so that I can probe all the internal nodes better. Fortunately, we use these supplies all the time for our inkjet printers, so there's piles of them around.

In the meantime, I'm e-mailing with Mike at Electro Auto to try and figure this out before he sends me another DC-DC converter to blow. I'm going to try and get by with charging up the 12V battery at the same time as the 144V pack and just running them both down at the same time. During the daytime, this isn't bad since the headlights consume most of the power. I hooked up a .01 ohm resistor to measure current draw from the battery and found that I can pull 30 amps with everything on, but this is rather atypical. Supposedly, the battery can supply 30 amps for an hour, so I should be okay with a 30 minute commute. This is still frustrating.

Remember, it's just a process...

PakTrakr is Online! DC-DC blown again...

I spent most of yesterday and a few hours today wiring up the PakTrakr system. It's a bit of a pain to disconnect the most negative lead before attaching the display, but that's minor nuisance. It seems to measure pack voltage well; however, the voltage measurement on each battery seems to be slightly inaccurate. The Zivan charger is balancing the batteries well and each of them is within an 8.48V-8.50V range (20mv difference is pretty good!).


I chose to mount the display on the central console as shown above. It doesn't have a backlight but is easy to read during the daytime and the placement makes the buttons easy to access.

I had to greatly extend one of the PakTrakr remote module cables so that the rear battery pack could talk to the remote modules in the front of the car. That involved cutting one of the 3-wire cables on one remote module and splicing in an 8-foot extension of additional 3-wire cable. I added heat-shrink tubing to make the splices clean so they could be pulled through the right-hand heater duct under the passenger-side door.

The system seems to work fine. Having a backlight would be nice, but not necessary. My biggest gripe is that double sided tape seems to have a big problem sticking to the back of the PakTrakr modules, even though they tell you to use it to mount the modules.


On a downer note, the CCPower DC-DC converter blew again (fuse was vaporized and FETs shorted out). I actually caught the system in a working condition just before driving home and found out that it had blown when I got home. The only difference was that the DC-DC converter was under a heavy load on the drive home because the headlights were on.

Based on the circuit inside the DC-DC converter, I suspect that there is a significant amount of current (required from the headlights) going through 144V side. When the FET opens, the current rushing through the inductor causes a high voltage spike (300-400V is not unheard of) that gets added to the +144V side and just wipes out the 500V FET. I'll have to double check to see if the inductive kick-back diode is properly functioning. The CCPower website only has the input voltage of their DC-DC converters listed as 80 volts so, although these are designed for EVs, the underlying design may not be able to handle the higher voltage, especially with spikes from regenerative braking.

In the meantime, I'm using a 12V charger to charge the accessory battery as well. Maybe I can integrate that into the overall charging circuitry when I plug in the car.

Talking with Mike at ElectroAuto

I just got off the phone with Mike at Electro Automotive. We chatted about the blown CCPower DC-DC converter and the engine vibrations. Since I fixed the DC-DC converter, we agreed that I would try to find out which conditions would cause it to blow again, if it does. It seems that Mike is more of a mechanical person than an electronics person. He graciously faxed me a circuit to generate a tach pulse from an ADC8" or 9" motor, but he didn't really understand the circuit. Unfortunately, the circuit relies on the fan inside the motor interrupting an optical emitter-receiver pair which doesn't exist on the AC24 motor in my vehicle.

He also agreed to provide a new motor to flywheel coupler if I still had engine vibrations. Mike commented that it was probably not the transmission, but probably had to do with the flywheel/clutch assembly. I'll have to do some more experiments to figure that out when I get the time and energy to take the motor/tranny back out.

I also said that I would look into the peak amps that the system draws if I could find a suitable ammeter. I still don't have the shunt or the console gauges since ElectroAuto is still waiting to see what peak amperage it has to deal with. When I told Mike that I easily pulled > 150 amps during acceleration, he seemed a bit surprised.

Apparently I'm the only person with an operational AC kit currently, although several other people are hot on the trail to finishing theirs. Mike doesn't have an AC kit 914 back at the ranch.

Regarding the 8V golf-cart batteries, Mike also mentioned that they tended to either last 3-4 years or die right away (infant mortality). I hope I don't have a batch that demonstrates the "infant mortality" behavior...

Okay, I'm going on a bike trip next weekend and need to get biking gear, so the 914 will have to wait for a bit.

Fixing the DC-DC Converter

I cringe at the thought of having to send anything back to Electro Automotive for replacement or repair, so I took a chance and dissected the DC-DC converter to see if I could fix it. The company that makes the DC-DC converter is CC-Power in the UK, so I don't think sending it back to them would be timely either.


Here's the DC-DC converter taken out of its black aluminum case. The clips that held the FETs and diodes in place were a bit annoying to take off, but I finally got them with some needle-nose pliers. The circuit is basically a heavy-duty buck step-down converter with a bunch of inductors. The output stage is a transformer in push-pull configuration with two diodes to handle both sides of the coil. An opto-isolator takes the output and transfers a reference current back to the primary stage to regulate the voltage.

To the best of my knowledge, the inductor sitting on the table is directly in line with the +144V and is used to limit inrush current. The transformer coil on the left does the same to the +13.6V side, basically just a series inductor. The transformer coil on the right is the isolation transformer with +144V and +13.6V windings to transfer the current. The two TO-220 case parts on the top are the diodes to rectify the current coming out of the output side of the isolation transformer. The two filter capacitors on the left smooth out the +144V and +13.6V. The two FETs on the bottom edge of the board pulse current into the input stage of the isolation transformer. The rest of the circuit is just integrated circuits to drive the FETs and monitor the output voltage.


Here is one of the two IRFP350 FETs that clearly blew up. If you look closely at the burn marks, there is even a hole blown in the part package. Both FETs had blown and were shorted out. Since the FETs are rated at 16 amps and the fast-acting input fuse was rated at 5 amps, I suspect the FETs sustained damage from a high-voltage spike that went past their 400 volt rating.


The explosion from the FET shown above was so powerful, it even blew a hole in the steel clip that held it against the heat sink. (I'm glad I wasn't around when it blew!).


I went on a quest this evening back to the labstock at my work and found some IRFP460 FETs in the same package. They are rated for 500V and 18 amps instead of 16, so I thought they would be a good replacement. Heck, I might as well at least try since I probably voided the warranty when I opened up the case.


Here is the DC-DC circuit board with the new FET leads sticking out the bottom edge for resoldering. The middle lead is tied to the back of the FET, making it a good heatsink and difficult to solder without creating a cold solder joint.

Okay, I did a quick assembly without heatsink clips and attached the 144 volt pack to the input with a new 5-amp fast-acting fuse, and voila, the DC-DC started working again. So, I went back and did a full re-assembly with putting the heatsink clips back on and attaching the 12V accessory battery. The DC-DC seems to be working fine again. It creates a small whine unlike the blown unit before and it puts out 13.9 volts to charge up the accessory battery.

I have no idea why or how the DC-DC converter blew, but I suspect it will probably happen again given that I've only really driven the car for four days. As long as I check the fuse I'll know if it went bad or not. The red LED on top of the DC-DC converter lights up with a 12V accessory battery attached, even though the unit is blown, so I can't rely on that. Based on an idea from Ross Cunniff, I could rig up an op-amp comparator circuit that lights up the the generator warning light whenever the 12V accessory battery is below a certain threshold. That way I know the DC-DC converter isn't working and I can check it out. The other idea is just to add a green LED to the 144V inside the converter just past the fuse so that I can see at a glance if the fuse has blown. There are many other projects that I'd like to do first though, so for the meantime, I'll just periodically check the DC-DC fuse.

Good Night.

Getting the EV Back to the Shop and Fixing Electrical Stuff

Just before noon, I drove with the ICE vehicle over to the 914, charged the accessory battery with jumper cables for about 20 minutes, then quickly drove the 914 back home to the garage for repairs. Since the stranded EV was only 2 1/2 miles away, I walked back to the ICE car and drove it home too. Since the DC-DC converter was blown and I didn't want to deal with it, I focussed on some other electrical problems I'd been having.

The first issue is that I get major voltage drops when I turn on 12V accessories like the headlights and the turn-signals. Because the accessory battery is now in the front of the car and tied directly into the fuse block, the actual current path to the ignition switch (and the lights) goes from the fuse block all the way back to the original 12V battery position (see step VI.F.7 in the kit instructions), through a bolt connecting the large red wires, and then back to the ignition switch. The accessories were causing the voltage to drop at the main accessory battery, but I noticed an additional 1.5 volt drop from the wire resistance going to the back of the car and front again.

To fix this, I found the two thickest red wires under the fuse block going to the back of the car. I cut them both near the firewall and used an ohm-meter to figure out which one went to the ignition switch. I then added a yellow 1/4" quick-connect connector on the end of the ignition switch input routed it along the wiring loom and plugged it directly into the fuseblock where the other thick red wire came from. This eliminated the 1.5 volt drop from the long wiring and the 12V system is much happier.

The second issue I wanted to address was the lack of a heater/cooler fan blowing out the ducts. To get at the contacts which turn on the blower motor and fan light, you simply remove the ashtray in the center of the dash all the way (press down on the center clip).


Here's a look into the ashtray hole down onto the electrical contacts (just behind the black/blue and brown wires near the center of the picture). These didn't seem to work at all, or only intermittently. I globbed a bunch of dielectric grease onto the contacts and used a long flathead screwdriver to scrape away all the corrosion and carbon buildup. There was a tremendous amount of dust and other dirt in there that I cleaned out with an old worn-out toothbrush.

After all that scraping (the dielectric grease acted somewhat as a solvent too), the fan operated at all three speeds and the fan light lights when the fan goes on. The next step for the climate control system will be to add hair dryers to heat the cabin. Unless I come up with a better idea, I'll probably tie the hair-dryers directly to the 144 volt system through relays controlled by the rear-defroster pull-knob.

DC/DC converter blown

Well, I ran out to Radio Shack this morning and got some more 5 amp fuses for the DC/DC converter. After plugging a fast-blow fuse in, powering up the car blows the fuse. I tried a slow-blow fuse and that went too. I then disconnected the 12V battery to see if that was causing a problem and it still blew a fuse.

What's sort of interesting is that these glass fuses didn't just melt and break, they really vaporized and completely coated the inside of the fuse glass with metal bits. It looks like there's a pretty serious short in the DC/DC converter.

The car is still stranded at Krista's place. I brought over jumper cables and tried to charge the battery with my ICE vehicle. The voltage drop across the 300amp jumper cables was 0.8 volts so I'm guessing the 12V battery was mostly dead. Since this is my day off, I might take a risk, charge it up some more and try to crawl home for repairs. The distance is only 2.2 miles and the main 144V pack is happy up around 151V. Grrr...