Master power switch for E-bike ideas

[Dannyx]

Re: Master power switch for E-bike ideas

I finally gave LTSpice a try and tried simulating my circuit. Here's the weird part: if I use a blank "nmos" model for the transistor, I get the curves in the first picture. This is clearly wrong, because the cap current (red) is WAY too small there - doesn't looks particularly realistic. The voltage at the gate (blue) IS acting more like what you'd expect though.

If I try replacing the "nmos" model with an ACTUAL transistor from the database, I get the second plot: the current just spikes when Vgs is around 3.6v then immediately drops back down. Even if I increase the R/C combo, it doesn't smooth out that spike any further... It obviously doesn't have my exact transistor, so I chose one which comes fairly close in terms of resistance and Vds. I have no idea how to use LTSpice yet - I just clicked on stuff, so I'm probably doing something wrong still

[Dannyx]

Re: Master power switch for E-bike ideas

Add an inductor in series with the line

This would open up a WHOLE other discussion

I doubt you really have to worry too much, the caps are probably insignificant compared to the inrush when the motor starts turning.

Yes, but at that point the FET is fully on, so dissipation is low, at least I hope so. Think I'll go with a 4.7k resitor, a 10uF cap and a 15v zener on the gate. The resistor will be dropping around 38v for a fully charged battery in this case, which peaks at 53v by my observations.

[eccerr0r]

Re: Master power switch for E-bike ideas

Add an inductor in series with the line
I doubt you really have to worry too much, the caps are probably insignificant compared to the inrush when the motor starts turning.

[Dannyx]

Re: Master power switch for E-bike ideas

Not sure about ESR, but I DO know there's two of them in there, each 470uF. I'm giving THIS a read.

[Dannyx]

Re: Master power switch for E-bike ideas

Vds is the maximum voltage between drain and source.

Correct. I was curious how this "operates" during turn-on/off and IN the actual on/off states, hence why I said I'm interested in the "practical" side of things, with my humble meter. A scope would probably be better suited for the job, but hey....whatcha gonna do ?

[eccerr0r]

Re: Master power switch for E-bike ideas

Depends on the ESR of those capacitors, should be able to calculate the inrush...

[stj]

Re: Master power switch for E-bike ideas

Vds is the maximum voltage between drain and source.
gate capacitance will relate to drive current(inrush) and maximum switching speed.

[Dannyx]

Re: Master power switch for E-bike ideas

Yeah, price is definitely a factor in this case: given it's still a goddamn N-channel FET at the end of the day, the price is RIDICULOUS, hence why I only have one shot at getting this right. Kill it and I'm crying for sure

My biggest concern is the startup inrush limit part of the circuit and what values those components should have, particularly the resistor and the capacitor, as I'm not sure what inrush curve I should expect from two 470uF caps in parallel inside my motor controller (so close to 1000uF total) at 50v+ for a fully charged battery.

[eccerr0r]

Re: Master power switch for E-bike ideas

Yeah pretty much your observations are correct. And you probably don't need to technically worry about most of the SOA graph, because it was meant for people PWMing. Same with capacitance - it's for PWM. If you're only using it as a bulk on/off switch you only switch every few minutes, you only need to deal with continuous (DC) ratings.

And yes these are real devices, there are always tradeoffs when you make MOSFETs. Sort of these four items: high voltage, high current, low capacitance, low price, PICK TWO. Not three, TWO.

(Technically capacitance and price should be directly correlated as an unfortunate side effect, so maybe you can take out one of these two. And then again higher voltage or current are also necessarily increases price.)

[Dannyx]

Re: Master power switch for E-bike ideas

Ok, I'm currently looking to grab one of those big FETs for this project along with some other stuff from a supplier I often order electronic stuffz from. I mean it's not like there's any urgency to it since it's friggin' winter and the bike is out of service for half a year...

I WAS gonna do that, but then I thought I should take a step back and analyze what it is that I really need....and it's also here I discovered I don't know a lot of things about FETs, or at least some things I THOUGHT were clear ended up getting confusing.

For instance this: I'm not sure what Vds means for my simple switch application. For the same price, I have a choice of two of those beefy boyz: THIS one which does 60v Vds and 400A or THIS one which does 100v Vds, but only 320A AND has a higher RdsOn. Realistically, I don't think I'll EVER draw that much current, unless maybe in very sharp pulses, so I aimed for the one with the lowest RdsOn. What does Vds mean in practical terms ? I tried reading about it as much as possible and I'm pretty sure it refers to the drop across D-S of the FET, regardless of whether it's on or off. In practice, which is what I care most about, I put my meter probes across D-S: if the FET is off (Vgs is 0), then Vds=VBat (?) because it's like an open switch ? If the FET is "on", Vds drops to nearly 0 because the "switch" is closed and conducting, having very little resistance of its own (RdsOn)?

This makes sense and I understand it, but I'm struggling to interpret the SOA graphs. Take my soft-start circuit I'm trying to create, does this mean that Vds starts off high at Vbat when the FET is off, no matter what, and slowly goes down as the transistor opens up more and the caps charge ? Based on the SOA graph, what happens AFTER the transistor is on ? Since Vds is now low, does this mean I can safely draw up to 100A through it ?

Then there's that high gate capacitance I need to address, but let's start with the basics. Feel free to enlighten me, as I feel ashamed for not being able to figure this out

[Dannyx]

Re: Master power switch for E-bike ideas

Let's kick it up a notch with one of THESE I was having trouble coming up with a feasible way of actually attaching whatever transistor I'd be using to the fairly fat wires of the bike, so I discovered these things which have mounting holes already on them. Trouble is: CRAZY expensive, considering it's still a goddamn FET inside ! If I blow it, I'll be crying....

[Dannyx]

Re: Master power switch for E-bike ideas

Looking back at my FET idea, mostly for learning stuff, I was looking at that SOA graph for the FET I proposed and tried to follow along with this tutorial here, though I'm not entirely sure I got it.

What does the horizontal axis, VDs Drain-Source voltage, represent in practice ? Does this mean the potential difference between D and S ? Taking my 48v battery as an example, with the FET "off", am I expected to see 48v if I measure across the FET ? Whereas with it "on" (saturated ?), there should be very little drop across it, since that's what you want a FET to do when fully on, so it dissipates as little power as possible, at least that's how I remember things.

That being said, when I slowly ramp up the gate voltage, I'm keeping the FET in its linear region where it dissipate a lot of power instead of being open...and eventually pops...

[diif]

Re: Master power switch for E-bike ideas

Not if you made it look like a Raleigh Chopper gear selector.
One here for reference. https://www.ebay.co.uk/itm/324856946839

[Dannyx]

Re: Master power switch for E-bike ideas

Can't you just make a mechanical knife switch out of some copper bar? Steampunk aesthetic and some cool sparks.

Too big to fit on there, plus it'd look kinda ugly for my build

[petehall347]

Re: Master power switch for E-bike ideas

Can't you just make a mechanical knife switch out of some copper bar? Steampunk aesthetic and some cool sparks.

that's the way .big and serviceable . my sort of electrickery

[diif]

Re: Master power switch for E-bike ideas

Can't you just make a mechanical knife switch out of some copper bar? Steampunk aesthetic and some cool sparks.

[petehall347]

Re: Master power switch for E-bike ideas

i still have a pretty old electric bike .so old the motor is in the front wheel .well i think it is .might be in the back . i wanted to use some sort of a mechanical speed controller as the electronic one is no good and likely missing now .

[petehall347]

Re: Master power switch for E-bike ideas

maybe a bit related . looks fun anyway .. https://www.youtube.com/watch?v=DyrG4yb_P5s

[Dannyx]

Re: Master power switch for E-bike ideas

wondering if you can get contactors for DC . bit more manly than relays . might use more juice to hold them in though .

They might be expensive/bulky. Already relays rated for high-ish DC are already outside my price range ! Also yes, there's the power consumption issue. I'm thinking to ditch the relay idea and have some sort of large mechanical switch with 3 positions: one "off" position, a second "inrush limit" position and one "ON" position. To get to the ON position, you'd have to go through the intermediary one first which charges the caps. Of course, there would be nothing preventing the average idiot from attempting to ride the bike on the actual inrush limit resistor if they don't turn the switch all the way......trying to figure out a way to mitigate this.

[petehall347]

Re: Master power switch for E-bike ideas

wondering if you can get contactors for DC . bit more manly than relays . might use more juice to hold them in though .