Automotive circuit design headaches

I wrote an article for Electronic Design magazine about Bob Pease and his solenoid driver circuit. Former National Semiconductor employee Myles H. Kitchen was nice enough to drop me an encouraging note.

“Thanks for your great article on Bob Pease and the solenoid drivers. Having worked with Bob in the late 1970s and early 1980s at National Semiconductor, I came to appreciate his wisdom and simplicity for addressing issues that seemed simple, but were really quite involved. As someone who’s worked on automotive electronics my entire career, an issue such as a solenoid driver is critical. I recall when testing early automotive product designs at one company, we would put the module under test in a car, and then turn on the 4-way flashers to see if operation was affected, or if it stopped working completely. The combination of multiple inductive and high-current resistive loads operating on and off at several hertz would play havoc with the power supply, and immediately point out design deficiencies in module power supplies, regulation, protection, and noise immunity…. some of which could be traced to poor relay or solenoid driver circuits.  Surviving the 4-way flasher test was only a quick way to see how robust the new design might be, but it was a quick indicator if we had things right up to that point. I miss Bob and his ramblings in ED, but hope to see more of your work in the future.  Loved it.”

Well, having been an automotive engineer at both GM and Ford before moving out to Silicon Valley, Myles’s note sparked a flood of memories. His four-way flasher story was prophetic. When I was in college at GMI (General Motors Institute) one of my pals worked at Delco. They were just coming out with the integrated electronic voltage regulator in the back of the alternator, circa 1973. So all the executives were standing around at a demo and after they ohhhh and ahhhh, and congratulate themselves, my buddy gets in the car, and knowing what Myles knows, he cycles the air conditioning switch a few times. The “Charge” light promptly came on.

I asked my fellow student if he was in trouble or if they hated him for causing the failure, and to GM’s credit, he told me “No, they were actually glad I found it before it went into production.” It must have been some serious egg on some faces, though. After that, survival after repeated AC clutch cycling became part of the spec for the voltage regulator. I bet four-way flashers are included as well.

I later worked on anti-lock brakes for GMC heavy duty trucks. This was way before anti-lock brakes on cars, about 1975. We dutifully shielded all the wires to the sensors with expensive braided cable. When we pulled the truck out on the road, the brakes started modulating, with the truck just sitting there. We realized that the entire 24V power system was a pretty nice antenna and that noise can get into a module from the power side as easy as from the sensors. We begged the government to give us more time, and they did. Indeed, I don’t know if they ever put in antilock brakes on heavy trucks. Let me check, yeah, wow, it’s still called MVSS 121 (motor vehicle safely standard) and it finally went into effect in 1997. That was at least a 20-year delay in getting it working.

I told Bob Reay over at Linear Tech that automotive design was the toughest, because you had a military temperature and vibration, but consumer cost. He added another factor, the chips for automotive have to yield well, since you need to ship millions. What a crazy challenge.

When I thanked Myles Kitchen for his kind words and told him the above stories, he responded with a great story about load dump. The phenomena called load dump is usually caused by a mechanic who is troubleshooting the battery and charging system of a car. You get the car running, rev it up a bit, and yank off the battery cable. If the car keeps running, that means the alternator and regulator are OK, it is just a bad battery. Thing is, the alternator is often putting full output into this bad battery. And when you yank the cable off the battery, the voltage regulator controlling the alternator cannot react instantly. So there is this huge overvoltage spike as all the stored energy in the alternators magnetic field has to dissipate into whatever loads are still connected, like your radio. A load dump can put over 100 volts on electrical system. And it is not a fast spike; it can last for hundreds of milliseconds. Smart mechanics just leave the battery cable on and hook up a voltmeter to see if the alternator is putting 13.75 to 14.2 volts on the battery. So Myles recounts:

“Thanks for your email.  Yes, sounds like we’ve run up against many of the common automotive issues in our time.  I’ll add one brief anecdote here.  When I worked at Motorola’s automotive division, I certainly learned all about what a load dump is, but I’d never really heard of anyone experiencing one first-hand and what it could do.  One day, our admin complained that her 70’s vintage Plymouth Duster wasn’t running right, and that her headlamps and radio quit working.  She had been driving it the night before when something went wrong.  We brought it into the garage at Motorola, and found that she had a very discharged battery with very loose battery connections. You could just lift them off with your hand.  As a result, her battery was discharged, and when she hit a Chicago pothole it all went bad.  The resulting load dump had blown out every light bulb filament in the car, along with the radio.  Only the alternator/regulator had survived.  The ignition was still a points and condenser system, or that would have probably died as well.  A new battery, tight connections, and a bunch of replacement bulbs got her back on the road again.  And, I’ve never doubted the need for a load-dump-tolerant design since!”

Those are wise words from someone who has been there and seen it first-hand. And I wonder if the voltage regular in that old Duster was a mechanical points type. In the early days we automotive engineers would try to protect each individual component for load dump. The radio would have a Zener diode clamp, so would the cruise control module. Then manufactures put a big Zener clamp right in the voltage regulator that clamps the voltage on the whole car. Maybe that was too low an impedance to clamp, because now I see there are a lot of smaller distributed TVS (transient voltage suppressor) clamps that you use to protect the circuitry of your module.

There are two other approaches. One, you can just disconnect your circuit with a high-voltage FET when the load dump happens:

I used this circuit to keep automotive overvoltage from destroying an LT1513 chip I used as a battery charger. When the DC Bus voltage exceeds the 24V Zener plus the base-emitter drop of Q10, it turns Q10 on and that turns Q12 off and protects downstream circuitry from overvoltage.

Alternative two, you can put a high-voltage regulator in front of your circuit that will maintain power to your circuit through the load dump, at the risk that the pass transistor will overheat since it is dropping a lot of voltage while passing current during the load dump. Linear Tech makes such a part.

There is one more tip for every engineer regarding automotive electronics. Remember that there are laws that make auto manufacturers offer service parts for 10 or 15 years. So no matter what your application, you might consider using an automotive part like Atmel’s line of MCUs, memory, CAN/LIN bus, and RF remote controls. We state that we will be making many of these parts for over a decade. If you design them into your industrial, medical or scientific application (ISM) you can have some assurance you can still get the part for years, or at least a pin-for-pin compatible part. That means no board spins. On top of that assurance, most of the parts have extended temperature range, which might help in your application as well. Since we make the parts for high-volume automotive customers, they are usually priced very reasonably.