Peak Hold Injector Voltage Clamp 1

[CaptainCarp]

I am designing a peak-hold fuel injector driver and need some suggestions about modernizing a sub-circuit.

I need to have a means of "clamping" the injector to BATT during PWM of the "hold" current (a controlled "clamp"). I incorporated a "clamp" circuit that I found in an old injector driver datasheet but I am not satisfied that I couldn't do better with a power MOSFET topology. I don't like the amount of power being dissipated in Q4 during pwm (1A x ~2V) and was hoping that someone here could suggest a differnt topology. Thanks

 

[LionelHutz]

Sure, conducting through the diode, not the device...

 

[murpia]

Er, no...

http://www.maxim-ic.com/app-notes/index.mvp/id/652

See figure 4.

Regards, Ian

 

[IRstuff]

Not exactly: "Figure 4. If you use the complementary gate-drive approach and the output is loaded lightly, the inductor current reverses during the synchronous rectifier's on time and the next half cycle begins with current flowing backward through the high-side MOSFET (MOSFETs are bidirectional). [bold][blue]During the switching dead time, current flows through the parasitic diode.[/blue][/bold]"

Current can flow through BOTH. However, the substrate is usually quite lightly doped, and therefore presents a relatively high series resistance for any current through the substrate. There would also be some additional parasitics, depending on the number of substrate contacts and where they're located.

The channel, while rather thin, is usually inverted to a much higher conduction state than the substrate, and for a power transistor, the contact and other parasitic series resistances are intentionally minimized. Therefore, when the transistor is biased ON, the channel will present the least resistance path for current flow. When the transistor is OFF, the substrate will carry the current, albeit at a much higher voltage drop.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[murpia]

CaptainCarp:

Did you ever take a decision on trying any new circuit ideas?

Regards, Ian

 

[CaptainCarp]

Ian,

I have got a little further with how I think I want to proceed. I have selected the main components (I think) but still have to select some resistor values. I have installed LTspice and will try to simulate the circuit, but it may take a while as although I know how to "run" the simulation, creating / including components that are not part of the supplied libraries is new to me so I have to learn how to do this.

Thanks