Blown Injector FETs

[Sprockets]

Well, at least the fets make an interesting key ring.......well, interesting to the people who read these forums at least :p

-Gavin

[jadzwin]

I  had customer who installed vems and had permanent fet failures (HighZ injectors). After second repair (I have replaced fets and drivers) i decide to modify vems to be "undestroyable".
I decide to use omnifets (VNP14N04). These FETS are very cool, have very good parameters, are thermal protected, short circut protected, ESD protected, and what is important they have integrated voltage protection so they can work without flyback wire (internal clamp at 42V). I have tested this feature and works perfect (i made some scope records) however in filnal configuration the vems works with flyback. The only disadvantage of vnp14N04 is that maximum input voltage (gate voltage in standard fet) is 18V. So the protection diode 1.5KE18 should be replaced by 1.5KE15.
I suggest this setup for HighZ injectors. It works great, can't be easy destroyed and VNP14N04 is quite chip (about 1Euro)

And what is most important the cars still works
Kuba

[lugnuts]

Why is it that every person here has a home-brew remedy that is more reliable than the FETs that are supplied with the vems ecu?

I had a Nissan KA24 install that ran fine for two weeks, then started losing a fet every week for 3 weeks in a row. I was told to "check the grounds" of course, did nothing to help. 

I did three things at the same time to fix it (had no other choice, I do not have any spare FETs left!) -
1) Replace Honda resistor box (with another one that was in a running Honda, the box from the car was put into the running Honda and it is still running.
2) Replaced 20awg flyback wire with an 18awg flyback wire. The 20awg flyback wire was previously tested with a battery and a string of light bulbs.
3) The Honda resistor box had no connector, instead used ECxx terminal pins, pushed onto the resistor box terminals and strain relieved. This time we found the correct connector and installed it.

I think #3 was the problem.

[lugnuts]

I spoke too soon, this morning I get a call that the car is broken again.

It is happening on Cylinder #2 everytime.

1) If it is the injector shorting out, the resistor's 6.8k ohms should be enough to protect the ecu?
2) If it is the wiring to the injector, same as above, even with a dead short across the inj wires, the resistor is still 6.8k.

I'm going to make a new injector harness just to say that I did it.

I don't know what can be causing this. In the previous 8 years and 100+ installs/harnesses, I've never had any other brand of ECU (Autronic,Electromotive)with a failed injector driver. This ecu has a nice features-to-dollar ratio but I am really wasting a lot of time with the failures, one failure is too many.

[GintsK]

Are you changed also FET driver chip?

I think damage comes from gate side. From power side everything seems strong enough.

Personaly I use other FETs than from webshop. STF20NF06.
I had very few problems with these. But was.

[PeepPaadam]

I have had only one FET failure and that was due to broken ground wire. Never since (around 30 VEMS installs).

When FET fails for some reason, it's usually replaced together with the driver as Gints suggested...

[lugnuts]

I haven't replaced any FET's, I simply use another available driver.
 
I don't (want to) work on the ecus, and I'm weary (and wary) of selling ecu's that have such a high failure rate.

I know the average customer wiring is often poorly done and can cause problems. But I still don't see the failures with self-installed other brand ecus like I do with these. And the other brand ecu's I install do not have these failures.

No person or product is perfect. You will have faIlures sometimes. That is not the problem.
How the company reacts (or doesnt react) to the failure, then it becomes a problem..
All you ever hear is to check the ground and flyback wires, and the injector ohms - if you even get a reply at all.
After that, nobody knows what to do.

I'm about ready to jump ship.

[rob@vems.co.uk]

From a commercial point of view I can see your problem.
Fact is that a FET won't blow if its protected by a fuse and the flyback is correct.
In reality people hack OEMs (painfully badly fused), or make their own with usual results - then you're in a world of pain with a disgruntled customer.

[lugnuts]

From a commercial point of view I can see your problem.
Fact is that a FET won't blow if its protected by a fuse and the flyback is correct.
In reality people hack OEMs (painfully badly fused), or make their own with usual results - then you're in a world of pain with a disgruntled customer.

Thanks for the reply, you are a voice of reason as usual Rob.

What amperage value will each FETs handle? 1 amp? 5 amps?
It's definitely not feasable to fuse each individual injector.

In what scenario would the fuse protect the FET?
1) If the flyback was broken?    or -
2) If there was another problem?

And if the flyback is OK, and the fuse protects the FET from (some other reason for) death, the customer is still left sitting with a dead cylinder.

I'm still trying to figure out the scenario that is causing injector #2 FET deaths on this Nissan.
I need a new ECU now because I'm out of drivers and have still not found the problem.

[rob@vems.co.uk]

Those FETs are good for about 10Amps.

Sure the customer is left with a dead cylinder, they run on 3,5 or 7 but they're not getting petrol pouring into their sump, but they get to see a dead fuse which they can replace and slowly it can dawn on them that they have a problem without the VEMS needing to be returned for resoldering of components.

Its then the wiring thats the fault, and not the VEMS...

[GintsK]

Interesting what is historical reason why VEMS don't use Shottky diodes in each FET flyback path, but usual "fast" diodes??? Shottky is way faster IMHO.
From other point of view FETs are self protected. There is something like TVS diode inside each FET. If FETs are avalanche rated.

It can't be that current killing FETS. Fets has much of safety margin for that. Failure should come from voltage. Question is from what side: gate or power side...

[lugnuts]

<<<Its then the wiring thats the fault, and not the VEMS...>>>

So can anyone explain exactly what would cause the fets to die like this? 

1) Dead short across injector? (resistor box still provides 6.8 ohms to ecu)
2) Short injector power to ground?
3) short injector signal to ground?
4) short injector signal to non-resistor power?

I'll be removing the car's harness for complete disassembly to find the problem, if there is one.

[SamSpade]

Here's the datasheet of the Fairchild FQPF20N06L.

It's good for 11A even at 100°C. The gate voltage maximum rating (V_GSS) is +/- 20V so I doubt that causes the failures.

Over-current condition:
- If an injector driver blows and you use another available driver location (i.e. different EC36 output pin) but the same thing happens then it's clearly a wiring problem, probably a short.

Over-voltage condition:
- If you replace the driver and it regularly blows (i.e. same EC36 output pin) even if wired to a different injector, I would suspect a high voltage flyback problem.  Check the 2 fast diodes from the driver output (ex. diodes D7 and D79 for INJ Q1) to the flyback pad.  One of them might be destroyed or the soldering might be faulty.



The flyback diodes are SS24, 2.0A 40V Schottky diodes.

[GintsK]

Again: V3 do not use Schottkys for some reason. What reason?
Instead ES2J is used. It is ultra fast p-n diode with recovery time 35-50ns (1000 times faster than 1N400x). But anyway Schottkys are faster. Or no?

P.S. here is a quote from Jörgen regrding my PWM peak and hold experiment:

I'm afraid that you simplify this too much, it took a long time for us to find the actual problem with the PWM solution and your powerflyback variant doesn't address the main problem at all. It's pretty much identical to the prototype I built a few years ago (also without knowing what caused the random problems we sometimes saw). As you are obviously willing to experiment and have access to a number of cars to test on you should do some modifications that will address the problem. You need to replace a few diodes in the ECU with faster units. As it is VERY possible that the diode modification can cause other problems we decided to use the safe and in many ways better resistor solution instead.

The current diodes we have are about as fast as a common diode will get, this "ultra fast" diode is however sluggish compared to a Schottky diode. The Schottky type diodes will not survive as much voltage as the standard diodes but they are a lot quicker and they will prevent the very high inrush current that is the cause of most of the noise and that kills the FET when you PWM them. You should start replacing these diodes on all boxes you plan to PWM: D7,D8,D10,D16,D17,D25,D28 and D29. For added safety you should leave the original diodes in the D79,D169,D259,D289,D299,D179 and D89 positions, that way the box will pretty much go back to normal if the schottky diode would fail. You will need to examine the Schottky diodes on the test cars periodically to see how well they survive the beating they get. The behavior of the car will not change if the Schottky fails but the FET's will start to take a severe beating again.

I must say that I'm suprised that you used a FET for this, even if the part you use is fairly rugged I find that most FET's are much more fragile then an IGBT or NPN.

Note that any tests done with the 1600cc Bosch injectors are not likely to test how rugged the box is after it's modified. The 1600cc injector hardly put any load on the electronics and it's always used without a resistor. - Jörgen Karlsson, Gothenburg,Sweden.

http://www.vems.hu/wiki/index.php?page=MembersPage%2FGintsK%2FFlybackForLowZ

Gints

[SamSpade]

All 8 injector drivers on my V3.5 board (purchased May 2009, #2362) have the dual SS24 Schottky diodes installed per driver.  They're DO-214AA package manufactured by Taiwan Semiconductor, with the S-shaped diode logo on them.