Some time back I was start find out how to drive Low-Z injectors directly from VEMS in peak-hold mode.
There was two reasons for it:
I found that Power Flyback board gives good injector opening but closing time is too long and AFR at low loads is hard to control even with small ~350cc Low-Z injectors. Hihgh voltage flyback gives good control over injector
Second: I had a car with clean wiring harness and I don't want cut it for installing inline resistors.
In fact flyback circuit is simple voltage limiter. It must cut every voltage spike at level safe to FETs which drive injector. Higher cutting voltage gives faster closing of injector.
Using PWM-ing there is two main problems: heat and noise.
Heat. I*U - Higher voltage gives more heat. But at higher voltage current falls too. My tests shows ~15W with 6x LowZ.
Noise is main factor why VEMS developers do not recomend use PWM-ing. It can cause board resets, false sensor readings (especialy Rpm sensors), garbage on LCD and so on. Really hard problems!
If High-Z used, VEMS use voltage limiter too. It is 30V TVS diode. It can easily survive with this kind of injectors - current is low.
Solution was simple - external voltage limiter with heatsink. And 4 capacitors for noise reduction.
Here is schematic:
Transistor IRFZ46N or even IRFIZ48N (more robust) both cheap.
need mica insulator kit
Zener is up to 42V (because 18V protection diode together with this device gives 60V - FETs limit)
Capacitors - two is mounted inside ECU between flyback pad and power ground.
Other pair is on the flyback board.
Resistor - 1kOhm
Note: device is connected in same way as common 30V TVS Zener diode.
I try this board on 1600cc for 2L 16V engine. For first was 30V diode and resistors used, but i do not like idle quality. With PWM-ing and this flyback device car pass emissions test! (E85)
Also device survive in race environment where 5 injectors commonly run higher than 60% duty cycle.
Note. Use this on your own risk. device is not officially supported. If you find a way to improve it, please post your experience.