Peak Hold Injector Voltage Clamp
Peak Hold Injector Voltage Clamp
(OP)
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
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





RE: Peak Hold Injector Voltage Clamp
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RE: Peak Hold Injector Voltage Clamp
RE: Peak Hold Injector Voltage Clamp
If you turn on the high side FET whenever the low side is off (with shoot-through protection) the injector current can be easily adjusted.
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
RE: Peak Hold Injector Voltage Clamp
The way you appear to have it wired, you are burning either 48W, or at least 12W, just to get 32W, or 2W. With a decent converter, you'd be looking at around 90% efficiency.
TTFN
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RE: Peak Hold Injector Voltage Clamp
I am looking to update a design that works fine with only a couple of transistors and a coupe of resistors, the only issue is that the Darlington will be have about 2V across it when it is on (only used in saturated mode). I am looking to update the design by using a P-MOSFET based circuit to replace the Darlington based circuit. Your proposals don't seem to by me anything other than more parts. The Darlington circuit's function is to provide a re-circulation current path for the injector during the PWM "off" time - that's all. Note that the suggestions are appreciated.
dgallup,
While the data sheet may show the LM1949 being used in a PWM mode, it is not as efficient than more modern designs using power MOSFETs (the Darlington has a saturation voltage of about 2V). Plus the circuit shown will always clamp the flyback voltage at a high voltage, which decays current flow in the injector faster (great if you're trying to turn the injector off, not so great if you're trying to PWM it. Thanks
RE: Peak Hold Injector Voltage Clamp
Most modern PWM outputs from microcontrollers can do centre-synchronised PWM (useful for motor control). If you can do that too you don't need your recirc diode D9 either.
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
Perhaps I was too quick to discount your suggestion.
Can you please detail your suggestion a bit more so that I can "see the light" (one less transistor is always better, unless it costs me $$$ to get rid of it).
I would appreciate it if you could identify an application note (or schematic) to help me understand your suggestion and if you had a part recommendation, that would be great.
Thank you
RE: Peak Hold Injector Voltage Clamp
Why not dump the Q4 circuit completely and just rely on the D9 diode when Q1 is off. Use a Schottky diode to achieve a low voltage drop, the lower the better.
I don't see any point in using a traditional 1/2 bridge for this application because the diode across the upper FET will be carrying the current, not the FET itself.
RE: Peak Hold Injector Voltage Clamp
The reason for having Q4 (and the overall reason for this posting) is that I have a couple of conflicting requirements for driving the injector during the "injection" event.
During the PWM, 1A current "hold" time, I want the current to recirculate through the injector for as long as possible so that the PWM frequency and duty cycle can be minimized. Since the current decay time during recirculation is dependant on the voltage across the inductor (the smaller the voltage, the longer the decay), having a diode across the injector is great during current "hold" time.
Unfortunately, when I need to turn the injector "off", I need the voltage across the inductor to be large so that the current decays "now" as injector timing is critical. So I need to be able to turn off my "diode" and let the flyback voltage build up to a higher clamp voltage. I am doing this by using a "protected" Q1 mosfet, which has a built in voltage clamping function (I hope to use one that clamps at around 60V).
Good question, hope I provided a reasonable answer.
RE: Peak Hold Injector Voltage Clamp
Typing 'half bridge' into Google images provides 21,000,000 hits of which numbers 2 and 5 immediately show what I'm on about.
http://www
http://www.4qd.co.uk/fea/half.html
If you drive both MOSFET gates from your microcontroller you have eliminated Q3 and D9 from your schematic.
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
This is really not "basic" stuff" and I don't want you to hold my hand and lead me down the path of ignorance.
You have not read (or do not understand) what I am trying to accomplish. AGAIN! - I want to be able to turn on and off, a voltage clamp to BATT, with a minimal amount of parts, and in an elegant / efficient manner.
As LionelHutz already pointed out, an N-MOSFET has an intrinsic Source to Drain diode which means it will always be in circuit and clamp the inductive flyback to BATT - ALWAYS, which is what I don't want.
RE: Peak Hold Injector Voltage Clamp
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
murpia - The 4qd circuit description is crap. The diode across the MOSFET is carrying the current they are drawing.
RE: Peak Hold Injector Voltage Clamp
My symbol for Q1 shows a device with a PROT section connected to the D, G, S, of MOSFET. This was my attempt to show a "PROTected" part such as the NCV8405 that has a ~45V clamping function for inductive loads built in.
As for the p-mosfet, I am not a power guy and have never designed a circuit using a p-mosfet. I do have an idea as to how I would connect it, but my "idea" circuit still needs D9 because of the intrinsic S-D diode of the mosfet, and with no experience, considerations like making sure the G-S voltage limit isn't exceeded (during high voltage clamping) come late to me (I currently am considering clamping this voltage with a zener). So, I basically was requesting a complete robust, efficient, and elegant active clamp circuit design with my initial question. Thanks
RE: Peak Hold Injector Voltage Clamp
I'm satisfied the OP's requirement can be fulfilled with a half-bridge of bottom N-channel MOSFET (avalanche rated) and a top P-channel IGBT (avalanche rated), driven by complementary PWM.
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
murpia - I'm just saying the 4qd page is crap. It shows 2 n-channel devices and then claims the current will go backwards through the upper device.
RE: Peak Hold Injector Voltage Clamp
The 4QD circuit would need a charge-pump voltage booster to generate the gate voltage to turn on the top N-channel device. Dedicated gate driver chips are available to do that job (and others, like provide shoot-through protection). For this application a P-channel device might be a better bet.
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
RE: Peak Hold Injector Voltage Clamp
http://www.maxim-ic.com/app-notes/index.mvp/id/652
See figure 4.
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
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
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RE: Peak Hold Injector Voltage Clamp
Did you ever take a decision on trying any new circuit ideas?
Regards, Ian
RE: Peak Hold Injector Voltage Clamp
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