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Terrible Idea Boost Converter To Be Tested On Gocart
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Zero999:

--- Quote from: MagicSmoker on December 25, 2019, 11:17:30 am ---Terrible idea for a number of reasons, with perhaps the worst being that a standard boost converter can't deliver an output voltage any lower than the battery voltage because there is only a diode in the path between source and load. This also means there isn't a way to limit, much less regulate, load current, and that is highly desirable both for protection and usability in a traction application. (NB - a synchronous boost can turn off the output, but can't regulate it below the source voltage except in the reverse direction, because then it becomes a synchronous buck)
--- End quote ---
I don't see at as an issue. Use the boost converter to get a regulated 180V and add the standard motor + MOSFET and freewheeling diode to the output. Now the motor's speed can be PWM'd from 0 to full speed.


--- Quote ---Third is that lead-acid batteries deliver less and less actual capacity as current draw approaches/exceeds the 1C mark; ie - a drawing 100A from a 100Ah battery might reduce usable capacity to 45-55Ah (see the Peukert Effect).

Fourth is that treadmill motors are mostly rated on their peak power and if from a consumer grade device you can bet they won't withstand even running at 50% of their claimed power rating for more than a few minutes. The continuous power rating of a motor - much like with mains transformers - is rather inextricably linked to weight and size, after all.
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Yes those are far more important reasons why this is a bad idea. and the one about the lead acid reducing its capacity, at higher loads though is true irrespective of whether many small batteries are connected in series or a boost converter and larger battery is used.
i_am_fubar:
So far, I've hit a few snags, some real fails (like my floating current monitor IC back to front), some micro controller niggles (such as ADC cross coupling between inputs due to bad timings) and some legitimate gotchas (like noise on encoder screwing up speed readings. However, I've now hit....

PROBLEM THAT I NEED HELP WITH #1
My main boost switching element is a 80A, 250V MOSFET IXFH80N25X3. The gate is driven from a FAN3100TSX 3A gate driver running at 12v. Circuit as follows


Unfortunately, I am getting some bad ringing as the gate voltage reduces past VGSon. Will get the scope traces when I get home.

While the circuit mostly works, with the initial setup (gate resistor of 11 ohms) the drive chip was dying, ringing started and just didn't stop until failure, presumably through large negative voltage spikes or very high rate of on/off switching.

Up rated the resistor to 100r and no more dying driver, but very slow on-off speed. Not what I want for a SMPS. Dropped down to 20r to hit speed spec, but ringing back.

The layout is reasonably tight and as far as I can tell has limited inductance.


Any thoughts or ideas on how to optimise this gate drive?

Cheers All :)
mzzj:

--- Quote from: MagicSmoker on December 25, 2019, 11:17:30 am ---Terrible idea for a number of reasons, with perhaps the worst being that a standard boost converter can't deliver an output voltage any lower than the battery voltage because there is only a diode in the path between source and load. This also means there isn't a way to limit, much less regulate, load current, and that is highly desirable both for protection and usability in a traction application. (NB - a synchronous boost can turn off the output, but can't regulate it below the source voltage except in the reverse direction, because then it becomes a synchronous buck)


--- End quote ---
Not problem really if you use 180v threadmill motor and 12v battery.
Go-kart would probably barely if at all move with 12v.
MagicSmoker:

--- Quote from: mzzj on January 02, 2020, 10:04:39 am ---Not problem really if you use 180v threadmill motor and 12v battery.
Go-kart would probably barely if at all move with 12v.

--- End quote ---

So letting the motor sit there stalled while the 12V battery continues to dump current into it through the boost diode isn't a problem?

fcb:
In the mid-90's I built small bost converters that went from 12V to 26V at 10A output for the film industry.  We ended up with a custom planar magnetic inductor which was really expensive but dense. Going to 15x with a boost converter is pretty out-there, you'll need to be able to shut that FET off FAST, that's a heck of di/dt on the gate. Just doubling the number of lead-acids is going to halve your head-ache.

And THE biggest problem (by far) was scoping out the gate drive - all the high currents and fast edges meant you chased ghosts all day long.  Have you got access to an IsoVu?

Good luck - you'll learn alot from the project. Look forward to the next installment.
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