High Current MOSEFT Circuit Resistors

[shortbus=]

Have you looked into any of the parts on IRF's automotive site? Haven't followed this thread from the start but, there may be something that will do what you need in a single chip.

International Rectifier - Automotive

 

[Jaguarjoe]

I didn't look at them all but this IR part will also drive a pair of MOSFETS:

https://www.electro-tech-online.com/custompdfs/2010/11/auirs4427s.pdf

I don't know anything about price tho.

TC or IR part wires up just like you drew. TC part can invert or not invert depending on part #.


Optos either have photodiodes or phototransistors in them. Diodes are faster but can't handle as much current as xisitors. I haven't seen every opto isolator out there but they are just like the one you drew. If the LED is on so is the diode or xistor. You can non-invert by placing the xistor on the "high side" like you did, or invert by placing it on the "low side" .
Your opto output circuit needs a resistor to gnd. When the photoxistor is off the TC part's input is just hanging, can't have that. Resistor value is determined by supply voltage and photoxisistor specs for max current. LED side of photoxistor also needs a resistor to limit LED current to ~10-20ma. Value depends on: (uP output voltage minus the LED's Vf)/ma.

0.7 volts is the Vbe of a silicon xistor. The B-E junction of a silicon BJT is a diode. It needs 0.7v just to begin working.

I think BMW varies cam phasing (like everyone else) and intake valve lift through some arrangement with the rocker arms. I'm pretty sure it goes from 0 to 10mm intake valve lift. The only Miller cycle engine I ever saw was in a Mazda years ago.

**broken link removed**

 

[Silntknight]

First question, do I need to have the opto if I have a MOSFET driver? I figure it is good because I can isolate my uP circuit completely. Otherwise, I'd need the uP grounded to the car battery (bad short circuits...).
For inversion, I am pretty sure I want inversion because optos invert (I think). Does this mean I have to "flip" my opto so the PWM input goes in the collector and the emitter is grounded? On the DigiKey page, it says that it is inverting and "low-side." Can you see the next circuit I draw to check it again? I calculated the LED resistor to be 21 Ohms. (3.3-1.6)/100 = 21. What should the pull-down resistor value be? 20k is what I've seen with Arduino-style applications. I will go with that for now.

Thanks for the link, by the way. That was actually really interesting. Now, I'm considering throwing in an EGR, if I can get a good O2 sensor to help manage fuel control. I might even consider the amount of bypass for the EGR as a means of throttle. Of course, that would mean I'd have to modify my piston head, but I have no problem with that. I have a few extras lying around. My engine will be a combination of GDI, Valvetronic, Miller, Lean Burn (only 15:1 AFR), and IDE (I place the two spark plugs near the injector). This is actually a project for my high school's Supermileage Team. We compete with other high schools for the highest fuel economy. I am retrofitting a 3.5 HP Briggs & Stratton L-Head engine to have OHV, fuel injection, and spark control. I'll be playing around with it as soon as I get it built so that I can actually mess with things like spark timing.

What do you think is better for direct inject, to inject right after the intake closes, or to inject so that there is about .5ms between the end of the injection and the spark ignition? I programmed the uP for the former, but I could re-write the code for the latter easily enough. Also, I am adding a powered intercooler (running off of Peltier coolers coupled to thermocouples running off the engine waste heat) to the air intake instead of pre-injection because of how hard that would be to program. What about fuel heating though? I have heard of teams doing it, so it might help. I'm not sure.

Finally, this is a little esoteric but what about regulating fuel pressure using another MOSFET with PWM (like everything else I guess). The Gnd line from the fuel pump would be regulated to provide the right amount of fuel for a given pulse width. For example, the pulsewidth would be a constant 1.7ms (or something of that nature). Because 1ms is used in opening the injector- and it's assumed that no fuel is injected in that time- 0.7ms is active injection time. Then, by varying the fuel pressure, the amount of fuel being injected is regulated. This eliminates problems with timing the fuel injector to the Hall sensor (and synchronizing it with the valves and spark). Maybe, if I do a LOT of math, I could find a way to use all throttling methods, except a throttle plate, at once. Anyway, let me know what you think. Thanks!

 

[shortbus=]

I think you need to inject fuel after intake closing, to give it time to mix with air in cylinder. If injected right before ignition it wouldn't disperse enough to completely burn.

What are you using to get fuel into cylinder? I have been thinking about this on my own (just as a mind exercise) a hypodermic needle pressed into a hole in cylinder head is what I came with. they are stainless steel, heat resistant and cheap. The ones used for farm veterinary would perfect.

The use of the automotive mosfet driver would eliminate the need for the opto. There are a lot of different mosfet drivers that will work and eliminate the opto.

 

[Jaguarjoe]

I don't follow your calc for the LED resistor. 3.3-1.6=1.7volts. For 20ma, 1.7/.02=85 ohms. This is not a standard value so I'd bump it up to 91 ohms to save a little bit of current and heat.
GDI runs at an insane injector pressure. In the neighborhood of 1,000 psi so it must inject fuel just prior to ignition. It works so fast I don't think a PWM'd pump could keep up with varying pressure demand..
GDI can run at equally insane AFR's- like 30:1.
A Miller cycle GDI engine would be cool!
I think a 1000psi direct injector is going to consume more than 1 amp.

 

[Silntknight]

The datasheet spec's a max of 120mA for the LED. I chose a safe 100mA, but I'll bump it to 50 or so anyway. Does that make sense? Maybe I'd go with 91. Decision for later. Optos and resistors are pretty cheap.

Ahh, so you've come across my problem. Do you remember that I wanted to run two "injectors/solenoids" off of one line? Well, the reason is that there is a valve covering a normal fuel injector. The cylinder pressure isn't higher than the fuel pressure during injection, so it will inject, albeit at a lower rate (thanks for helping me realize that!) The valve, by the way, is the second solenoid/fuel injector connected to the same circuit.

I was planning on contacting Siemens about their piezo injectors. They consume fairly little current. As long as it is less than 11.5 amps, I have a MOSFET that can handle it. The only question is the trace width. Still, fuel pressure then becomes a problem. Unless I have unreasonably fast PWMs (10us or so), I'd be dumping WOT amounts of fuel at idle. I'd be forced to run a direct inject injector at a much lower pressure. If it will operate on 3 bar, I could run it at that easily. We'll see. Also, why "must" a GDI inject right before ignition? I guess PWM-ing the pump for a true GDI wouldn't be a good idea then. I do have 10us resolution, though, so if the pulse widths were just long enough, I could do it.

Shortbus=, you should see the link Jaguarjoe posted. It explains that, with proper geometry, having it inject right before the pulse can make lean burns possible. In fact, I could go to 30:1 AFR, like a GDI can, if I make them close temporally (and spatially). It would also help to have the right piston head shape.

Back to the main points, is the circuit correct now for the opto/gate driver? How can I tell if the opto inverts??? The datasheet shows Ic increase with If. Wouldn't that mean it doesn't invert? I should contact Vishay... Also, the anode does go to the pin, right? I know that's pretty basic, and I am pretty sure it does, but...

 

[Silntknight]

Also, should I be fine without resistors between the opto emitter and the Vin of the driver? The driver handles up to 1.5A, and I doubt the opto can source that much current.

Back to the whole anode resistor thing, is it [V(uP)-V(F)]/I(F)? All of those voltages should be at their max and the current should be around 20mA?

 

[Jaguarjoe]

Please post the part # for the opto. 120ma is a huge amount of current for an opto's LED, I don't believe it. The uP is good for 25ma max which is far short of 120ma. Almost all "normal" LEDS run fine at 20ma, that's where 91 ohms comes from. You could probably run them at 10ma and be OK, the uP would appreciate that.

The gate driver is just like a high current, very fast SPDT switch. You supply its input with a "high" signal at 1ua of current and the driver will allow up to 1.5 amps of current to flow into the MOSFET gate to turn it on quickly. When you supply the driver input with a "low" signal, it connects the gate to gnd allowing up to 1.5 amps to flow out of the gate which turns the MOSFET off rapidly. So only 1ua of driver input current switches 1.5 amps of MOSFET gate current. The MOSFET gate has capacitance which must be dealt with. When turning on, the MOSFET sucks in current to charge the cap. When turning off, the MOSFET must discharge that cap. This is a very small capacitance so the 1.5 amps only flows for usecs. I=CdV/dT.

 

[Silntknight]

Vishay TCET4100. I realized I listed the maximum "safe" rating. The "absolute" maximum is 60mA. Sorry about the confusion. Anyway, I am going with 91 Ohm resistors. I changed the part I was using from the Vishay to another one that cost less (high school budget...)

Is 20K enough for the pull-down resistor connected to the driver input? I figure it has to be higher than the input resistance, which 20K should be. Is the circuit alright?

So do I need a resistor between the opto emitter and the driver's input?

Also, why are there no small 5K resistors? I need one and I'd rather not string 5 1K resistors in a row.

 

[Jaguarjoe]

20k should be fine. That gives 600ua of photoxistor current. The higher the current the slower it works.
No resistor is needed in series with the driver input. The input is very high resistance.
You can go with 150 ohm LED resistance if you want. This lowers the current the uP has to put out which lowers the power dissipation of the uP which is a good thing.
5k is not a standard 5% value. 4.99k is a standard 1% value though. Google "E96 series"

 

[Silntknight]

If only 1uA is needed, I might just throw in a larger pull-down. 100k maybe?

As long as I have enough current to power the LED, then 150 ohm is fine.

Thanks for this. Because I want a tightly regulated 1mA of current (from a 5V source), I need something close to 5K. I did the math quickly and 4.99K is extremely close to 1mA, so thanks!

 

[Jaguarjoe]

The pull down resistor serves 2 purposes. It determines the current flowing thu the photoxistor when it is on and also pulls down the driver's input when the photoxistor is off. Too large of a value will not allow enough current to flow through the photoxistor. The photoxisitor current is the sum of the current flowing into the driver (1ua) plus the current flowing through the pull down resistor. I would leave the pull down at 20k.

EFI's correct for battery voltage. You may need to tie the solenoid power gnd to the uP gnd.

 

[Silntknight]

I was actually hoping to avoid tying the grounds together. I wanted to use the opto to create two fully isolated systems. I'll test this a few times and see if tying them makes it run, if it doesn't isolated. If it turns out that I have to tie the grounds together, I might as well remove the opto and use non-inverting drivers. The less delay time I have to compensate for, the better.

 

[Jaguarjoe]

I don't know how much influence Vbatt has on the injector pulse. Isolating the uP from the power circuitry is a nice feature.

 

[shortbus=]

GDI runs at an insane injector pressure. In the neighborhood of 1,000 psi so it must inject fuel just prior to ignition. /QUOTE]

On doing a little research I found that my thinking about when fuel is injected was right. Check out this on GM's direct injection engine; 2008 GM 3.6-Liter VVT Engine With Direct Injection

 

[Jaguarjoe]

There's something missing in the second paragraph of that article. It jumps from the intake stroke to the end of the compression stroke. If the fuel is directly injected during the intake stroke then there is no high pressure to overcome in the cylinder. Therefore there is no need for a high pressure injection scheme. GM didn't go through all the trouble of going to DI and then wasting it on the intake stroke.

 

[Silntknight]

I also noticed that. They did jump from one stroke to another. It would be strange for a GDI to inject during the intake (completely defeats the point, actually). I really hope that it is just a small oversight and not how the GM engineers actually designed it.

Siemens AG - Piezo Technology
**broken link removed**
**broken link removed**

 

[Jaguarjoe]

Piezo technology is cool. I just wonder how much voltage is required to drive them though.
My dad was quite successful at the opposite end of the spectrum making PZT pressure transducers and skin friction gauges.

Some GDI engines achieve a 65:1 AFR for short periods. How do you measure that?

 

[shortbus=]

You might get something from this (for one and two cylinder engines) https://www.electro-tech-online.com/custompdfs/2010/11/PA102.pdf

Or this https://www.electro-tech-online.com/custompdfs/2010/11/ThesisPDF.pdf

The first one puts a lot of what your trying to do into a single chip!

 

[shortbus=]

There's something missing in the second paragraph of that article. It jumps from the intake stroke to the end of the compression stroke. <snip> Therefore there is no need for a high pressure injection scheme. GM didn't go through all the trouble of going to DI and then wasting it on the intake stroke.

What stroke of the Otto/four stroke system is missing? I was taught it was - 1. Intake 2. compression 3. power 4. exhaust.

The high pressure is to more completely vaporize the fuel and cause the cylinder temp to lower allowing higher compression.

Are you guy's old enough to remember the "Smokey Yunick hot vapor engine"? The man was a genius and way beyond the times in his ideas. smokey yunick hot vapor engine - Google Search