Saturday, July 24, 2010

July 24th: General Update

Last weekend I received the BTS550 and BTS555 High Side Switches from Infineon. As usual, my comprehension of the datasheet prior to ordering didn't really prepare me for what I ordered. I missed that these are NOT microcontroller friendly devices. When they are OFF, the voltage at the microcontroller input is the car  power voltage, so thats 12+ volts. Good thing I was using the Rugged Circuits Gator+ and not a standard Arduino, or I would have taken out some pins. The BTS could handle more current, but still not enough to drive the big fans with just one device and a smallish heatsink.

That evening of tinkering was a pretty big setback and I ended up pretty discouraged. If only they made the IPS6011 with a higher current capability. Sigh. I took the rest of the weekend off from electronics and continued the garage cleaning. It was VERY productive and I'm almost done.

During the week, I decided to let the project sit in the background, maybe my motivation would return or I'd have some epiphany.

A few things did come to mind:
1) I should be able to gang the IPS6011 for higher current capability. I emailed International Rectifier to see if they had any design considerations for ganging them.

UPDATE - response from IR (on SUNDAY MORNING!!)

Because IPS parts have HEXFET outputs, it may appear possible to parallel them., because as you know the Rds(on) of MOSFETS has a positive temperature coefficient which aides current sharing in parallel part situations. However, with IPS parts there is a practical difficulty as a result of the built-in over current and thermal overload protections. These protections are not built-in to discrete MOSFETS. There will be small differences in the actual Rds(on), the over-current and thermal trip points due to normal parametric spreads in these characteristics between a number of IPS parts. Which means that in an overload situation one part will be inclined to shutdown before the others. With IPS parts the over current protection is reset when the input signal is cycled from on to off to on and over-temperature is automatically reset when the junction temperature cools by around 7*C. With one of the parallel parts shutdown, the total load current will be shared with fewer devices which in turn will carry an increase in current and tend to shutdown in a chain reaction. As the input signal is cycled, every part will try and turn on again. If the input cycling is automatically repetitive with the current overload condition still present, the junction temperatures will tend to rise progressively until the first device shuts down due to over-temperature. Eventually all devices will shut down initially due to over current and ultimately due to over-temperature. The time for all parts to shutdown will be a function of the magnitude of the over-current, the individual device characteristics and the thermal conditions of the application with respect to heat sink ratings and device local ambient temperatures.

We do have many customers using it in parallel.In a parallel configuration of devcies, the major consideration is effective current sharing.Also, as these devices operate with current sense feedback, extensive circuit testing would 
be needed to ensure proper operation. We do not have reference designs / data for such an
application. However, a point to note is that during circuit turn-on, for a slowly ramping 
current (such as for an inductive load), the paralleling works better. For fast ramping loads
paralleling may be more difficult. You would also have to ensure proper feedback for each of 
the devices in parallel.

2) I've been testing with my WORST-CASE load - a GIANT cooling fan. I can always relay these big inductive loads and go on with life. I need to move forward with my plan to measure each load in the car to figure out if a single IPS6011 circuit for each will suffice. With a small heat sink, I proved them for an 18 amp inductive load. With a larger heat sink and a non-inductive load, I may be able to push higher.

3) Big mosfets aren't usually shoved in little cases - in fact, the cases themselves are typically the heatsink - car audio amplifiers and car ECM / ABS modules come to mind - the cases for most car modules are thick aluminum and many have fins molded onto the case. I may hit a "u-pull it" junkyard for some case options...

Last night I finished pulling  the factory harness. I found even more crimp connectors, taped connections and wires to nowhere. The seats are out, the carpet is almost out. I need to pull a few more things, then get started putting Humpty Dumpty back together again. Once again I'm having that "why did I go this far" feeling. I sure like doing this to myself... I need to either get this wiring harness project moving or just order one to move on - but that sure wouldn't be as much fun...or frustration :)

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