Using Bosch BIP373 Ignition Modules with MegaSquirt

Updated 4/24/2008

The Bosch BIP373 transistor has served in many different Bosch ECUs and ignition modules. They've been available to ECU and ignition module manufacturers for some time in wholesale lots, but now we are making these transistors available in retail quantities for the hobbyist market.  We've done extensive testing on them and found these make a great drop-in replacement for the now-discontinued VB921 IGBT ignition drivers commonly used in MegaSquirt ECUs. Not only do they drop right in, we've found they are much harder to destroy if the output or dwell settings are incorrectly configured.  In fact we've been trying really hard and we haven't been able to burn one up yet!

Like the VB921, the BIP373 is a transistor in a TO-220 package designed to drive ignition coils.  The pinout is exactly the same as a VB921. There is one key difference in building a Megasquirt using BIP373s: The BIP373 is not in an insulated package, so you must use a mica insulator when mounting it on a heat sink.  Other than that, it works in any circuit that you can use a VB921 in.  When driving a single BIP373 from a processor output, we recommend using a 330 ohm resistor in the wire connected to the base for both MS1/Extra and MS2/Extra applications.  When driving two BIP373s from one output for firing coil-on-plug applications in a wasted spark mode, use 660 to 680 ohm resistors, one resistor per BIP373.

Bosch BIP373s have both output current limiting and thermal shutdown built in. The data sheet puts the typical current limiting at 11.5 amps with a maximum of 16 amps.  The thermal shutdown range is between 185 and 210 degrees C with a typical shutdown at 195C.   Basically that means the BIP373 will provide a hotter spark that a VB921, and if you configure it wrong and it gets hot, it shuts itself down instead of turning on the pyrotechnics. 

According to the datasheet and confirmed by our tests, the BIP373s can charge up more current than a VB921s could and can do so fairly safely due to the very functional current limiting and thermal shutdown.  There is however still a need to responsibly set your dwell and configure your spark output setting properly.  If you let it get out of control the BIPs will not be the weak point now, they'll get hot and thermally shut themselves down and stop sparking until they cool down.  You'll know something is wrong because your spark will go away and your ECU and particularly the BIPs will be HOT... but the spark will come back after they cool down.  Correct the problematic setting (spark output reversed or too much dwell) and try again.  Note that with the BIP373 IGBT component no longer being the weak link, the next link in the chain could be the coil itself if you overdwell it or invert the spark output.  Coils can melt too.  We tried really hard on our test rig and weren't able to melt one, but there are a ton of coils out there, your mileage may vary.

The proper way to set dwell is with a scope, using the trick we're about to share as a backup.  As a 'quick and dirty' method of setting dwell without a scope we recommend checking the temperature of the BIP373s while slowly increasing dwell.  Maybe start at 1.2-1.5ms and increase it .2ms at a time while idling the car - if they become too warm to the touch, back the dwell off a bit until they stabilize at ambient temp or barely above.  Check it again after running the car at different RPMs to verify the same results.  They shouldn't run hot all the time...

Bosch does not have a data sheet for the BIP373 posted online but we were able to dig it up, here it is if you'd like to take a look, you can also find out more about their BIP product line at this section of the official Bosch website.

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