12v relatively high current (~20A) buck/boost DC-DC converter - does it exist?

pmrobert

I've been searching for the unicorn in the subject - does such a unicorn exist? I'm certain that it does but my Digi, Mouser and Google-fu is evidently rather weak. The device/module I'm in search of would have standard motor vehicle +12 in and a constant +12 (or up to +15v, that'll work as well) out but must have at least 20A continuous current capacity. The key part is the output voltage needs to be constant and sag free despite the ever-present vehicle bus variability from starting, lighting loads, charging requirements, etc. Anyone have any ideas?

-Mike

feng

www.victronenergy.com

User Name

I haven't done this professionally, but with certain personal projects I've had good luck paralleling two or more switching power supplies. On each output I've added a power Schottky diode (typically robbed from an old ATX supply) and a low-value ballast resistor.

Each supply is adjusted to approximately the same output voltage and the ballast resistors are sized to give each supply a couple hundred mV to work with. So if you had four supplies rated at 5A continuous, the ballast resistors might each be around 50 milliohms (in practical terms, two 0.1 Ohm, 1 W resistors connected in parallel).

The higher the resistance value, the safer this scheme is. Of course the losses also increase. BTW, the diodes may not be necessary in every case. I add them to ease my mind.

evanh

Sounds like a 12v battery and charger will suffice. Or even just the battery with a diode.

pmrobert

Thanks for the input, gentlemen. Upon rereading my initial post I'm sure I misused the term "constant"! The goal is to provide vehicular fuel injectors with a steady, constant +12 (or +13.8,etc., the exact value is not important) at all times. The reason is that, being solenoids, their opening time varies directly with the voltage available to them. Said voltage can vary from 8 volts with a nearly flat battery to 14 or so in real life, depending on battery state, headlight state, etc. The varying voltage requires a lookup table in the ECU whose result is used to calculate the actual opening time of the injectors which is inversely proportional to the voltage. The actual values vary dramatically between injector models, categories, etc., is nonlinear at low overall commanded pulse durations and can only be very accurately determined by physical testing on a test bench. By providing a constant, stable, nonvarying voltage to the injectors the opening time is only needed to be determined for that voltage thus obviating the need for the time consuming and rather pricey testing required to quantify the opening time vs voltage data. For most applications, it doesn't really matter but I have a few customers that have rather expensive competition vehicles which are running right on the safe edge of engine parameters where a running a bit lean due to a voltage sag,etc., may mean engine damage. A stable injector power supply with sufficient current management margin would eliminate a variable in the fueling equation. One of my injector reports is attached.

-Mike

tonyp12

Buck-only will be easier to find, how about always using 9V out?
Buck that 15-10v to a steady 9V and time the solenoid on that.

And is 20A just in short spurts that a 10F cap will help?

pmrobert

Hmm, excellent food for thought. I'll run a couple of injectors on the test rig at 9V and see what the opening time looks like. The low Z injectors are ~2 DC ohm (though I have a Mazda rotary injectors where they're 1.2 ohms) and are typically given DC for the opening time then PWM (20-40% duty cycle at 20kHz usually works fine) to hold them open for the rest of the injection event. The low Z injectors are typically used in higher RPM applications specifically because they generally open significantly faster than the high Z - which are typically 10-14 DC ohm and are not PWM driven. I am not personally familiar with the super caps but discounted them due to the rather high internal resistance as compared to the load they'll be dumping into. The opening time is generally less than a millisecond for the low Z, I guess I'd have to get a sample cap or two and just see - the caps are cheap enough to play with. Comments/questions gladly accepted!

-Mike

jmg

pmrobert wrote: »

Thanks for the input, gentlemen. Upon rereading my initial post I'm sure I misused the term "constant"! The goal is to provide vehicular fuel injectors with a steady, constant +12 (or +13.8,etc., the exact value is not important) at all times. The reason is that, being solenoids, their opening time varies directly with the voltage available to them.

Yes, but being solenoids the force is actually related to Amp-turns, thus maybe you should look at current driving these.

Current Drive also removes thermal effects from copper heating, that pure voltage regulation does not solve.
With current drive, you can also start with a higher voltage, which gives a faster dI/dT current ramp, and thus faster action. Some designs step the current after the movement, as the holding current is commonly much lower.

The Energy saved has to help here too...

pmrobert

Yes, I understand that. I guess it is being pseudo-current driven in that PWM is used to simulate/effect an effective lower holding current during the hold phase. I use LM1949 and believe it opens the solenoid with DC at whatever bus voltage is available then detects saturation (via external sense resistor) and drops to 25% of the initial current value for the hold event duration. Consistency of the opening time is what I'm aiming for here - I do know I can run them on the bench @ 16v (using the LM1949 and a lab power supply) at high duty cycle for many hours without thermal issue with any of the components. Your higher voltage suggestion has merit - perhaps I just need an appropriately rated DC/DC 16v converter to create the consistent energy environment I'm looking for from the mess that is the +12 on most vehicles. Thanks for the thought provoking comments!