Does that mean that I can't have a Schottky across the NMOS?RC snubber, zeners across the MOSFET and RC snubbers, schottkys across the injector.
You don't. I don't think I said that. I'm pretty sure I said schottky across the motor inductance or zener across the MOSFET.How can you put a Schottkey diode across the MOSFET? One way its blocking and serving no useful purpose. The other way it is shorting the MOSFET out.
First, about the PCB. All of my traces are shorter than 5 inches. Would that be good? If it helps, I will post my PCB design. Let me know. I did go with a Schotty across the MOSFET. I can make it a Zener (either parallel or anti-parallel, I forget), but I really do want to keep my board as small as possible. Adding another RC snubber would take a lot of space, but it would also make keeping all traces on one side harder. By the way, what do you mean by "mnimize loop widths"? I'm not sure what you mean by loop.
Does that mean that I can't have a Schottky across the NMOS?
Sorry if I don't seem to understand; this is my first experience with electrical design ever. But thanks for all the help!
Hard to say what is best since I don't know much about injectors. I don't deem shut off time too important so I tend towards the schottky diode across the motor more than the zener across the MOSFET (obviously Jag feels differently). RC snubbers just depends on how demanding things are. They have some pretty severe disadvantages in implementation. Mainly cost and difficulty to size, and they can be large. They carry pretty heavy inefficiency (ie. heat) and cost penalites for overdesigning them and if underdesigned they might not be effective enough. It's a lot easier if you have a diode (which is comparatively dead easy to select) elsewhere to clamp the voltage to a set level and just rely on the snubber to hold the fort until the diode has time to activate.Dknguyen, I posted (later/earlier, I'm not sure how it will appear) a question about what configuration you think would be best (and you too Jaguarjoe). I am thinking Zener across the NMOS, after reading more about it. The Zener I am looking at is (Digi-Key - 1N5368BGOS-ND (Manufacturer - 1N5368BG)). If I use a Schottky, it would be (Digi-Key - 1N5820-E3/54GICT-ND (Manufacturer - 1N5820-E3/54)).
And thanks for letting me know about the parallel vs. series stuff. I was concerned that there would be a voltage drop like that. Just wondering, because (I think) power is conserved despite the configuration, in a series circuit, if each consumes 1 amp and 6V, then would 4 amps be rushing through the circuit? I say that because in parallel, it would be 2 amps at 12V, so 24W. I'm not sure if resistance changes, but I think it does. It should, at least if I remember correctly from linear algebra.
Ok, so I am going with a Zener across the NMOS. Should my circuit be: +12V--->[+]injector[-]--->[drain]NMOS[source]--->Gnd with a Zener connected with the cathode on the drain side and the anode on the Gnd side? Jaguarjoe said to have the cathode toward +12V, which would indicate that it should have the cathode connected to the injector [-] terminal.
The zener diode in picture #2 is not correct. As it stands, it is acting as a regular diode across the coil like you would do with a relay coil. 0.7v drop.
It should be across the MOSFET with a rating less than the Vds of that MOSFET, like maybe 40 volts or so.
You want to eat the spike but not slow down the injector (assuming this is an IC engine application).
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