Thanks for the comment. This gives me the opportunity to explain my rational behind the design. Very long answer unfortunately.
The reason for peak + hold in my opinion is to get a high electromagnetic force to open the injector quickly (Peak). The hold current is just lower to not overheat the injectors but still enough keep them open. So the exact wave form of the hold current is of lower interest to me. As long as the energy in the injector coil resistance is reduced enough. The difference for an injector driver peak and hold compared to an coil driver peak and hold is the last part where in the injector case you want to get rid of your coil energy as quickly as you can (quick close). That means to get as high a fly back voltage as you can without destroying any components during the closing time.
The idea for the design is a simple lowish complexity PWM peak and hold injector driver. But with a fast injector closing time by using a high flyback voltage ONLY during closing of the injector. (during PWM mode a diode is employed to keep the system efficient and reduce heat in on PCB components). The following steps are created using the logic gates:
Step 1 (Peak): Open the injector when injector pulse is starting. Ramp up the current until peak value sensing that with the comparator and current sense resistor. When peak is reached set the flipflop to let the system remember for this injector pulse the peak has been reached.
Step 2: (Hold) Now the hold current needs to be set. This is done using a fixed PWM duty cycle where the duty cycle chosen such that it meets Ihold ~= (Vbat * Dutycycle)/Rinjector. This is only true for steady state but simplifies the system greatly! (only one fixed duty PWM signal needed for 4 injectors). During PWM the high side driver switches the flyback diode on. So no high inductive voltage during PWM. Essentially making it a badly filtered DC/DC converter during this time.
Step 3: (close) To close the injector quick, release the diode with the high side driver and let the inductance get rid of this energy thru the avalange or clamping of the low side mosfet. Also some zener to the gate of the lowside driver is places to make the clamping voltage lower if desired or when a non repetitive avalanche rated low side driver is employed.
Now for your questions:
“That type of PWM peak-hold injector is not usually used with injectors, the use of a window-comaparator for the hold current is more conventional.”
Can you explain a bit more about the window comparator in use case in this situation? I guess that could be used during the hold phase to get some defined hold current with hysteresis? Or is this to get the peak current and then reduce the setpoint to the hold current?
“The PWM driver has a very slow decay from the peak, as you can see in your waveform captures, that gives non-linear closing times at short pulse widths. “
Yes the decay is slow. Especially because the fixed PWM duty cycle start directly after the peak. The high flyback voltage during closing gets the energy out as quickly as possible but 2 times the initial current gives you 2 times as long to get rid of the inductor energy. The whole energy dissipation is expected to take 0.14 for 1.1A or 0,22ms for 1.6A and a 40V inductive peak voltage. That is easily accounted for in the VE table I think (need to try indeed). A second note: These LPG injectors are not ideal for short pulse widths anyway. The opening time is 1.8ms the closing time 1.2ms so anything less than 3ms total is really bad anyways. So better use them with full sequential and longer pulsewidths.
“have you measured the voltage across the current sense resistor during avalanche clamping?”
Yes this is always 0 or maximum 0.4V if 4A peak is reached. One side of the resistor is to ground other is to the mosfet so the inductive energy (and voltage) is put into the mosfet. Not the sensing resistor.
“I have some concern regarding inductive spikes, you have no filtering or protection on the comparator input and no hysteresis, without any amplification your switching threshold is quite close to ground. You also have no filter on Vref.”
For the design it would have been better to separate the signal ground for comparator and such from the power ground so these currents to not “lift” the ground to skew the measurements , And yes you are right I should make some filter on the comparator inputs/Vref. But at the moment I have seen no issues. Good input for the next iteration. To keep the design simple no amplification is employed. The peak voltage is ~0.4V so also not extremely low.
“You have an 18V varistor, that is going to clamp before any other electronics in the vehicle so your board will become the central clamping point for any load dump pulse - it is not going to survive that.”
Hmm good point should have put that one behind the fuse, If your car has enough energy in spikes >20V to break the voltage protection something must be really really wrong already. Next iteration I put it behind the fuse so that it blows with over voltage.
“What is maximum inrush current for the 1mF caps? I would expect to see some current limiting. As a filter they don't have any series impedance, an inductor in the supply line would make a big”
Inrush is indeed big (obviously). The wires connecting the board in the car have some series resistance to help. Inductor in series is an option next iteration but what value? It must be small enough not to cause inductive spikes. I didn’t see it as a big problem this inrush current.
“I can't make out what the top transistors are supposed to be, most of the peak-hold circuits that I've worked with have been high side/low side pairs but thats not you've got, you have a transistor in series with a recirc diode, are you trying to turn off the rectifier diode and then clamp the flyback energy through the OR gate? “
Yes, see my explanation this is to close the injectors quickly with high flyback voltage clamp over the low side mosfet but to make PWM possible with a rectifier diode so not to get all that coil energy in the lowside mosfet constantly.
“why do you have two transistors in parallel? “
Because these are in a sot23 casing making way to much heat with the 100mOhm on resistance. So i used two in parallel to reduce the heat per transistor by a factor of 4. (half the energy because it is shared + half the restive energy dissipation because R/2 in total). Also note the high side driver is reverse to what you would normally do. This is to keep the body diode of the high side driver blocking during inductive voltage spikes when closing the injector.
“What are your injector characteristics?”
For the LTspice model I used 3.5mH and 2 Ohm. That happens to be the LPG injectors I have around here. Advertised as 3.5A peak and 1.5A hold (max). But these get really hot with 1A hold already. So I keep them to 1A hold 3.5A peak. The pictures in the measurement folder are of some other LPG injectors with unknown inductance and 3.1Ohm resistance.