MCU 20A 450VDC fuse investigation

[kiev]

i was hoping to obtain several for comparison, e.g. 20,30,40A rated, in the ZXISO style, but only have an OEM PEC 20 and a Littlefuse 30 at this point due to lack of stock in the US distributors.

On the PEC drawing there is a call-out for item 7, a small dot or blob of a tin alloy (solder?), which can be seen below. It is also seen on the Littlefuse version and in the vapor trail of the blown fuse, so this must be some secret sauce for making fuses.

Maybe it adds some thermal mass to extend or delay the breaking current, or it adds a concentrated mass point to control where the fusing will start (like a score line on a piece of glass to control where it will break when stressed), or ...?

Top view, L to R: 30A, 0.075" wide reference, new 20A, blown 20A

Side view, same order as above except reference is 0.036" width

rotation to get good side view of 30A fuse element

 

[WReed82]

Kenny,

How did you get those very cool photos?!

One guess on the alloy blob is something that would heat up, release smoke, and
discolor the inside of the fuse so it is obvious the fuse is blown.
Alternately, if it has a relatively low melt point, it could tell a tech that the
fuse has been near its thermal limits, but didn't actually exceed it.

Thanks and good health, Weogo

 

[coulomb]

On the PEC drawing there is a call-out for item 7, a small dot or blob of a tin alloy (solder?), which can be seen below. ... so this must be some secret sauce for making fuses.

From the end of this page:

"Another slow blow fuse is the "M" type (named for the inventor). This has a small blob of dissimilar metal "soldered" onto the fuse wire. As the "blob" heats up and melts it diffuses (blends or alloys) with the main wire to change composition and melting temperature of the wire, at which time the wire melts and breaks the circuit. This action provides a time delay function at slightly excess current, while the main wire will blow instantly at significantly higher excess current."

But that's strange, since this fuse seems to blow quickly compared to other fuses that are in series with it.

 

[kiev]

That's a good finding on that reference and the history of the thermal blob.

i've been looking at the Laef OBC and it appears that they use a 420VDC 30A SHV27 fuse on the Positive side output. This is classed as an inrush-withstanding type of fuse. The 2012 FSM wiring diagram for the VBC shows it as a 25A fuse? This same type is used in the Mits OBC output, but is only rated 20A.

Another note on the LOBC, there are no snubber caps across the POA and NOA terminals such as is found in the mits, even though they have the same pinout of the waffle plate.

 

[kiev]

Moving back over here from some discussion about failure mode of the OBC (snubber caps or fuse first?) on the troubleshooting and repair thread page 47,

i ran a simulation with a nominal 10 Amps flowing thru a 40R load until a fuse blew, then opened the switch to turn off the "Boost" and let the energy ring out from the coils and caps. The scope time-history displays the peak voltage at the 1nF snubber cap when the fuse blows on the left side, and the latest time step value on the right.

[EDIT: added more circuit details and repeated the sim, peak voltage across snubber is a bit higher]

 

[DBMandrake]

2.4kV ?

So the original 2kV caps would probably suffer a failure while my 6kV caps would survive in this scenario ?

 

[kiev]

Here are the I-t fuse curves for the MCU and OBC 20A devices. The MCU version is physically larger.

 

[kiev]

2.4kV ?

So the original 2kV caps would probably suffer a failure while my 6kV caps would survive in this scenario ?

If they could survive a 354 kV impulse--the peak of the spike.

i haven't measured the miev coil inductances but the laef coils were about 200 mH. Any simulation, of course is GIGO, so this is just trying to illustrate how if a fuse lets go, then it generates a spike that could take out the snubbers.

The scope chart has the peak on the left side; at the right side is the data for the last time step when it was stopped. Sorry for any confusion.

 

[Don]

I'd be up for preemptively replacing the fuse(s) if we can come up with a group buy on a suitable fuse. My fuses are probably less stressed than most as our cars have 30K and 35K on them and have almost never been charged 'full tilt' on L2 as I've set my EVSE's to a max of 12 amps and probably 50% of the time or more, I use L1 at either 8 or 12 amps - I have 4 EVSE's plugged in all the time, so choosing how fast I need to charge is quick and easy . . . . just plug in with the applicable one. One EVSE is set to 16 amps L2, but I never use that one on either of the iMiEV's

I do kinda like the idea of mounting a pair of clips on the tabs removed from the old fuse so a clip in replacement can be used - I'd imagine there is a much greater assortment of clip in fuses than sourcing only those with tabs on them, right?

Don

 

[kiev]

i ran the Paul Falstad (Physics site) sim with a switch on the snubber so i could short it out or go open circuit. i let the fuse get to 50% melted before hitting the switch, then opened the Boost switch and stopped the scope.

Opening the Cap: i also ran this case a long sim time (1.1 sec at 20usec steps) until the fuse cooled back down to 50% melt

Shorting the Cap: also ran the fuse cool down sim for this case (391msec at 20usec)

Accidentally double clicked and bounced the switch during a Shorting run which produced some interesting results:

 

[kiev]

Basically anything that abnormally interrupts a charging session has the potential to cause OBC damage, for example:

1. Fatigued MCU Fuse
2. Weak or Leaky Snubber Capacitors
3. AC input power: interrupt, voltage fluctuation, etc
4. 12V Aux battery: weak, old or worn out, dropping voltage to ECUs, dropping relay contacts, etc
5. ? placeholder, Did i leave anything out?

 

[JoeS]

Kenny, thank you for running those simulations. Interesting to me is how design tools have evolved over the years.

Yes, count me in for a group buy (two plus one spare), although I'll probably hold off installing until after my warranty runs out as I'd like a new-design OBC from Mitsu.

Re: clipped-in fuses vs. screwed-down pigtails means four contact surfaces instead of two. Since the ECU is a sealed compartment I would expect no corrosion-inducing moisture that might affect the clip/fuse contact surface, so perhaps it shouldn't matter. Hey, could put a bag of desiccant in there to make sure. :roll:

Re: Chargeus Interruptus can't think of any more reasons off the top of my head. In my case I had been running mechanical timers on the input to my i-MiEV EVSEs for about five years so my fuses must be about ready to pop.

Is there a known sequence inside the car that gracefully shuts down the OBC when the J1772 handle button is pushed?

 

[coulomb]

i found these at Mouser, HEV030.ZXISO.

[ Edit: Disregard this post. It was based on the incorrect assumption that the OEM fuse was 7.2 mm diameter. It's 10.3 mm diameter, so no bending of tabs is needed. ]

I'm thinking the 15 A part with the "bolt down (axial)" termination style would work out slightly better (there is also a 30 A bolt-down (axial) part if you are worried about it blowing too quickly). So that's the 0HEV015.ZXBD part (link is to Mouser USA with price in US$).

My reasoning is that you have to bend the tabs to make the fuse fit with the bolts at 45 mm apart, and the axial mounting versions have slightly longer tabs. You'd be bending like this:

I made that diagram from the drawing for the ISO (non axial) termination versions, but the principle would be the same. I suspect it's still going to be a bit of a fiddle to get the bolts in, depending on the bolt head size. It might even be worth using M4 bolts (I assume that's what they are) with smaller heads.

 

[coulomb]

Is there a known sequence inside the car that gracefully shuts down the OBC when the J1772 handle button is pushed?

I was wrong when I initially thought the LEAF ignored the J1772 plug's push-button. It does indeed immediately and electronically (i.e. safely) ramp the charge current down to zero when the button is pressed. It's just that there is absolutely no indication that anything has changed; the blue LEDs stay the same, there is no beep or anything else to suggest that the current has stopped. As soon as you let go of the button, the current ramps back up to normal.

I actually find this handy when changing current on my adjustable current limit EVSE. If you want to terminate the charge, you therefore have to press the button then remove the J1772 plug.

 

[kiev]

i had to put a leading zero on that part number to get mouser's search to find the part, 0HEV0rr.ZXISO where rr is the amp rating, e.g. 15,20,30 etc.

[edit] the OEM datasheet is on post#3 on page 1 of this thread.

 

[nuggetgalore]

G'day.
thanks for investigating the fuse quandaries.
But I am still con-fused (pun) how to go from here. Should I aim to get the same amperage or as suggested by some to go to 30 A? I a m less concerned with the body type, I can make it fit.
It all then depends on what I can get in a reasonable time and how few I can buy ( PEC may only sell boxes of 200) I don't know?

 

[coulomb]

Should I aim to get the same amperage or as suggested by some to go to 30 A? I a m less concerned with the body type, I can make it fit.

We've been ignoring the possibility of some sort of fault that ends up pushing say 25 A through the wires to the fuse. It's unlikely, but it could happen, and if it does, it could start a fire. The main job of the fuse is to protect the wire.

The wire on one side looks like it's barely 2.5 mm², if so safe to about 22 A. So I'm going to say that from a safety perspective, we really should go to the trouble of finding a 20 A rated fuse.

I note that the Eaton fuse I found is rated at 20 A (they don't make these PV fuses with a rating higher than 20 A). But they are out of stock at present (other suppliers might have that part number in stock). The manufacturer says it's specifically designed as a photovoltaic fuse, which also has me slightly worried. I don't know much about fuse selection, other than the need to have a DC rating. It's also about 50% more expensive, possibly because it's rated for 1000 VDC.

Is there a cross sectional area marking on the thinner of the two wires, or an AWG designation?

 

[kiev]

That was an issue i found-- the minimum order quantity from PEC.

i needed something quick to get back on the road so i ordered what was available from Mouser, which was the 30A since they were out of stock in the 20A version and it had a 12-week lead time.

Fuses are intended to protect wire, in this case the OBC-to-MCU wire, from an overcurrent event which could melt it and start a fire, and my guess was that any abnormal situation which could develop enough energy to blow a 20A fuse would likely also blow a 30A.

But in this case there are also 20A fuses soldered to the OBC and the DC/DC converter boards inside the OBC box to protect this wire. They are a smaller form factor, but in all the OBC failures nobody has reported blowing either one or burning the wire. They are not an easy bolt-in replacement such as the MCU fuse, but there are a total of 3 fuses for that one wire.

i can't think of any reason to have an MCU fuse except in the case of a car wreck while driving in which the rear end gets crushed forward to pinch and cut the wire and cause a short. If that happens you would want a fuse there.

 

[DBMandrake]

That was an issue i found-- the minimum order quantity from PEC.

i needed something quick to get back on the road so i ordered what was available from Mouser, which was the 30A since they were out of stock in the 20A version and it had a 12-week lead time.

With all this talk of finding alternative fuses I'd like to point out that the original Mitsubishi fuse is still readily available from 3rd parties and at a reasonable price, so there's not really any need to look for half baked alternatives that aren't a good match. I ordered mine from here:

https://www.amayama.com/en/part/mitsubishi/9499a656

On examination after receiving it the fuse is the genuine Mitsubishi branded part.

The seller is in Japan and it took a little bit under 2 weeks to arrive in the UK including customs delays. Minimum order is one fuse, although I ordered two just to have a spare on hand if the first one went splat due to overlooking any damaged components so I wouldn't have an additional delay waiting for a second fuse. (As it was, the repair was successful the first time, so I still have one spare new fuse on hand, along with spare capacitors )

My only gripe is that the package incurred customs fees and a resultant customs delay as ordering two fuses pushed the total value over the threshold where customs charges apply in the UK. :evil: (Tip to anyone ordering in the UK - if you're not in a hurry to complete the repair order one fuse and avoid the customs fees hassle...)

 

[misterbleepy]

My only gripe is that the package incurred customs fees and a resultant customs delay as ordering two fuses pushed the total value over the threshold where customs charges apply in the UK. :evil: (Tip to anyone ordering in the UK - if you're not in a hurry to complete the repair order one fuse and avoid the customs fees hassle...)

That happened to me too - but luckily still way way cheaper than a new OBC