High Current Output drivers

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gearhead
LQFP112 - Up with the play
Posts: 120
Joined: Sun Feb 03, 2008 9:30 pm
Location: Chicago, USA

High Current Output drivers

Post by gearhead »

I have been thinking about this for a while and thought I'd post a bit more about high current drivers. I was reviewing the Saab Trionic ECU (Moto 332 based) for algorithm type information and caught this. For all high current outputs, they use MTD3055VL http://www.fairchildsemi.com/ds/MT/MTD3055VL.pdf. Note that these are 12A devices. These are used for each injector as well as other solenoids, etc. My concern with smaller (5A) devices on the injectors is that low PW injector control may compromised. I know that higher current devices can be swapped in at any time, but thought I'd post this as a note. It'd be interesting to see if there is any difference in PW control accuracy with a stiffer FET driving the injector.

As a side note, it is interesting that they also use a LM1815 for the crank sensor conditioning. For the low level COP ignition outputs, they use a quad 700ma NPN driver (CA3262). all analog inputs go through a TLC546, 8 bit a/d converter and are fed in serial to the 332! This takes in: TPS, MAP, BattV, IAT, CLT, O2 and Knock. All 8 bit all serially in to the 20 MHz 332!!! This is for a very sophisticated adaptive engine management system.

Another note is the voltage regulator (2 channel):
Power supply
The board is power by the car battery. The input voltage (+11V - +14V) is transformed to a usable
voltage by the L4937 (Voltage regulator). The input voltage (from the battery) is supplied to
pin number 1 on the regulator. The output voltages (Vo1 and Vo2) are stabilized +5 volt which can
sink 50 mA and 500 mA respectively. The Vo1 is a standby voltage. If the ignition is turned off an RC
network will keep the power supply in operational state for a while and then lets it resume into
standby mode in which the power drain is significantly lower. The maximum standby current the
regulator can sink is 50 mA. Nice to know is that the regulator can supply its +5 volt output when the
input ranges from +6V up to +28V (with transient spikes up to +40V).
Also of interest is that Saab uses lambda adaptation to accommodate for altitude (baro correction)

Just thought I'd share.

Gearhead
thebigmacd
LQFP112 - Up with the play
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Joined: Thu Apr 10, 2008 5:51 pm

Re: High Current Output drivers

Post by thebigmacd »

Awesome info.
Keith MacDonald
Control Engineering (Systems) Technologist
Brian
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Joined: Sat Jul 26, 2008 1:08 pm

Re: High Current Output drivers

Post by Brian »

Good info Gearhead, thanks for posting.

I sometimes wonder why the IRF540 does not seem to be more favoured. It has a 44 milliohm on-resistance, high current capability (33A), ability to be driven straight from an in/out port, fast switching time, easy to source and cheap. It has no protection, but would have no problems blowing a fuse very quickly if something went wrong. I used a couple for a short period of time as hi-Z inj drivers and no probs.
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jharvey
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Joined: Tue Jun 10, 2008 5:17 pm

Re: High Current Output drivers

Post by jharvey »

Looks like this IFR is a fairly normal MOSFET, no extra bells and whistles. This can work for driving inductive loads, the problem with MOSFET's is the isolation voltage. If the snubber cap or diode fails, your inductive spike will propagate back through to the CPU, causing excessive damage. So many folks will use IGBT's for an increase isolation voltage and a decrease in damage in the event of a small failure.

I like the PIP because it's a MOSFET with bells and whistles. The over voltage protection and over temperature protection decreases the chances of getting a spike back to the CPU. These protections do it with much lower current and voltage spikes causing much less stress on the components. The additional benefit of the PIP is the linear off time. The linear off time decreases the tolerance brackets, increasing the ability to precisely control the inductive load.

Any how, both approaches work, and work fairly reliably. That's why freeEMS 1.0 includes provisions for either way. If you use a normal MOSFET, I'd recommend making sure you heat sink you snubber diode or oversize your snubber cap to prevent the chances of them failing.
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