# Vicor DC-DC build help needed...



## Ziggythewiz (May 16, 2010)

I don't think the heat sinks will do much in a plastic box.


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## favguy (May 2, 2008)

Eh??? lol! the box is heavy die cast aluminium!


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## Ziggythewiz (May 16, 2010)

Oh, sorry. Looked like a typical plastic project box.


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## dougingraham (Jul 26, 2011)

You can just trim the outputs of each module and then parallel them for this application. The output voltage drops when they reach the current limit and when paralleled they will naturally load share although not evenly until all three are current limiting. However a car 12V power system doesn't need great load regulation. I am using three 5V 40A modules in series trimmed down to give 13.6 volts (4.53 volts per module) and then floating an 8AH battery. For a while I had three slave modules paralleled giving 80 amps capability (a little over 1000 watts) but the charging surge is probably not good for the battery when it is low and gets a 10C charging current. I like the idea of a small battery because the car systems will continue to work even if the main fuse on the traction pack opens or the DC-DC fails.

I am not sure what is best for input filtering. I am using a megapac and so far it has worked well. The Megapac has PFC so it boosts the input voltage to around 370 on an internal buss and then the modules buck that down. Yours are going to see the input sag so you may want to decouple the input from the traction pack somewhat so the modules don't see all the battery sag when you mash the throttle to the carpet.


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## dtbaker (Jan 5, 2008)

looks like you are committed to DIY w/ the vicors, and I certainly will not get another chennic (had two die in less than 12 months service), I have been very happy w the 600 watt ACME for $200 from EvolveElectrics.com so far....

its an open chassis design though, so needs protection from splashes.


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## favguy (May 2, 2008)

Thanks for the feedback so far.

Yes I'm pretty much commited to the DIY Vicors at this point! I've just about worked out what I'm doing with both the input filtering and outputs now, the parts are on the way for me to build it up.

I am still concerned about weather or not to worry about the Y capacitors between input and output leads to baseplate, I suspect I can run without them for the EV use planned, input anyone???


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## rwaudio (May 22, 2008)

favguy said:


> Thanks for the feedback so far.
> 
> Yes I'm pretty much commited to the DIY Vicors at this point! I've just about worked out what I'm doing with both the input filtering and outputs now, the parts are on the way for me to build it up.
> 
> I am still concerned about weather or not to worry about the Y capacitors between input and output leads to baseplate, I suspect I can run without them for the EV use planned, input anyone???


I've done a lot of other stuff with dc/dc converters, typically those caps are for noise, in an EV that won't matter much. If they were critical for stability they would simply incorporate them into the design and they'd be on the inside.


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## Joey (Oct 12, 2007)

Hi Favguy. I think your concept for the DC-DC is intresting and I'm researching doing something similar. I don't like the reliability and reported problems with the available converters out there, and now that the Evnetics DC-DC project appears to be on hold, the options out there are limited.

I'm running 192 V nominal, so I can't run the exact modules you have. The VI-2*7** modules run a wider voltage input (100 to 375 V) but they only output 100 watts each ($155 for primary and $135 per booster).

Was there another reason besides availability that you decided on three master modules instead of using a primary and booster modules? I'm reading the application manual now and will be hitting up my EE friends for help and advice soon. 

Please keep us up to date with what you discover and how you pick component values. Any luck with figuring R1 and R2 in your drawing?


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## swoozle (Nov 13, 2011)

favguy said:


> Thanks for the feedback so far.
> I am still concerned about weather or not to worry about the Y capacitors between input and output leads to baseplate, I suspect I can run without them for the EV use planned, input anyone???


I've been running a Vicor primary and 4 boosters for about 1500 miles now with no caps and no problems. I do have a diode and inductor on the input.

I like that I can have the boosters automatically switch off (primaries can be done this way as well) when the key is at ignition off, leaving just one energized to handle accessory power. It pulls about a tenth of an amp in standby. I can live with that.


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## Tomaj (Oct 3, 2011)

I am also building my own DC-DC using Vicor modules.

I would need some help.
I have 2 VI modules with 48V input and 15V output. 
System nominal voltage is 78V, max 96V.

Can I use 2 modules, run them in series on input and parallel on output?


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## frodus (Apr 12, 2008)

I'd just run each off half the pack and parallel the outputs. They're isolated, so it's fine to do that. But realize, that the input currents may not match between modules, and the output current may not share equally. Nothing wrong really, but since you're using 2 modules and not using the current sharing PCB they offer, they may drain each half of the pack unequally, but this is usually in the order of a few mA.


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## dougingraham (Jul 26, 2011)

Tomaj said:


> I am also building my own DC-DC using Vicor modules.
> 
> I would need some help.
> I have 2 VI modules with 48V input and 15V output.
> ...


What is the exact model number? They have a wide input range. If it is the correct model you might be able to just hook them up. I just checked the datasheet and what you probably have is a VI-232 which accepts 42-60 volts. This would not be a good fit for your pack. If you split pack them the minimim you need is 82 volts and max of 120. The modules you need for this would be VI-243. That one would accept a range of 55-100 volts. It accepts a transient range of 45-110 volts.

I would suggest you trim the output down to 13.6 (for a LiFePO4 or maybe as much as 14.4 for Pb) and add in a buffer battery of some kind. The real reason to use a buffer battery is in case the DC-DC drops out. If it drops out then a lot of stuff quits working like your motor controller and that would be an uncomfortable few seconds while it reboots.


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## Tomaj (Oct 3, 2011)

Modules are VI-2N2-CW. Input voltage 36-76V. I can not use aux. battery because there is just not enough space for it on my motorcycle.

For now, I will just wire them in series, I think it have to work...am I right?

Later I will try to add aux battery.


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## rwaudio (May 22, 2008)

Tomaj said:


> I am also building my own DC-DC using Vicor modules.
> 
> I would need some help.
> I have 2 VI modules with 48V input and 15V output.
> ...





Tomaj said:


> Modules are VI-2N2-CW. Input voltage 36-76V. I can not use aux. battery because there is just not enough space for it on my motorcycle.
> 
> For now, I will just wire them in series, I think it have to work...am I right?
> 
> Later I will try to add aux battery.


Wrong... they will not work in series, you will have to wire each one up to half the pack.


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## dougingraham (Jul 26, 2011)

Tomaj said:


> Modules are VI-2N2-CW. Input voltage 36-76V. I can not use aux. battery because there is just not enough space for it on my motorcycle.
> 
> For now, I will just wire them in series, I think it have to work...am I right?
> 
> Later I will try to add aux battery.


It will barely work if you wire one unit to the upper half of the pack and the other unit to the lower half of the pack and parallel the outputs. If your traction pack sags below 72 volts (and it will) there is a good chance that the 15 volts will go away. And if the 15 volts goes away anything that is powered by it will go away. Since this is a motorcycle and your biggest load is going to be the headlight you can easily get away with a really tiny buffer battery. And the buffer battery will hold up your systems while the DC-DC drops out when the traction pack sags out. At least for a while. Your buffer battery can be made from something like

LiFePO4 cells

Get 4 of them and put in series. Top balance and trim the Vicor outputs to 13.6 max and go. This will be about 1 inch thick by 4.25 inches by 2.7 inches. I made a 4S4P pack of these and floated them at 13.6 for about 8 months now. Your 4S1P arrangement is a pretty small battery but can source 70 amps. It can be charged at 23 amps but you would only see those kinds of currents if the pack was dead and then only briefly. Since in the best case your two modules paralleled will only put out about 15 amps this should work out well. But remember that this is only a bandaid. It would be better to use the correct modules for your traction pack because this setup will unbalance your traction pack.

Best Wishes!


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## frodus (Apr 12, 2008)

Tomaj,

What is your traction pack setup? how many cells in series? Sounds like maybe 26 cells? Are they lifepo4?

If I'm right, that's 13 cells for each half pack. Fresh off the charger they'd be around 48V, and if you go down to ~2.8V or so, you're around the bottom limit of 36V. all of these modules though, have another lower limit, but their output power is 75% of nominal..... so you MAY be able to use them, even if your batteries say below 72V on the pack.

Best way to check, is try to test varying voltages on the input to see where the output drops out. If you have an adjustable power supply, start at 36VDC and start adjusting down. If not, then try to hook the DC-DC up to 12-cells, and then start decreasing by 1 cell..... 11-cells, 10-cells, etc.


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## Tomaj (Oct 3, 2011)

Traction pack is 24S1P LIFePO4 (A123 20Ah cells). Nominal voltage 76,8V, max. voltage off the charger 87,6V. Low. voltage (2,5V per cell) 60V.
I have 3S 5Ah Lipo battery and can use it as buffer battery. But that is only 12,6V. And probably I have to use balance board for them.


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## frodus (Apr 12, 2008)

24s1p may to be too low for those DC-DC's to operate over the entire range because you'll be sagging well below 3V... likely down into the 2.7-2.8V region. Do what I said above and see when they cut out.

3s lipo fully charged to 4.2V is 12.6V... and you can trim the output of the Vicors to that voltage...but no higher. I doubt you'll need to balance often because it's going to be more of a buffer and not charged/discharged a ton. It should be fine, but YOU HAVE TO TRIM THOSE 15V VICORS DOWN LOWER (Below 12.6V preferably). Most of your onboard "12V" stuff should be fine right around 11-12V.

The Vicor issue aside, that pack is really small for a motorcycle. I hope it's a very small motorcycle and you don't expect to go very fast or very far. A123 may take high C-rates, but at higher C-rates, you will still sag.


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## Tomaj (Oct 3, 2011)

Thank you for your help.

I am going to use those 2 48V modules on my other motorcycle. It has 36S3P LiFePO4 battery pack, so perfect voltage for those 2 modules.

Now I need to find somewhere Vicor for 72V, not so easy task

24S2P pack is in enduro (Husqvarna 125ccm) bike powered by Motenergy ME4201 motor, 250A max. Batteries should handle this. Goal is 90km/h top speed and 35km range at normal riding.


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## frodus (Apr 12, 2008)

Why does it have to be Vicor? I know there's Sevcon and Surepower DC-DC's that are sealed and good fits for motorcycles.

And BTW, there are several VI-241 modules on ebay right now.... only 75W output, but I don't know what you're using it for.... so that may be low.

You said above that this was a 24s1p pack.... which is it? Sag at 250A with 2p is around 6.25C. Sag at 250A with 1p is 12.5C. That makes a huge difference in sag.... so they might actually work just fine.


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## Tomaj (Oct 3, 2011)

I can get Vicor for very reasonable price-50$ and it is very small. Sevcon is 220$, Chennic 80$ (to big).

Yes 75W ia a little bit low. Headlight 55W (high beam 60W), rear light 5W, turn signals 5W...dashboard, BMS, controller...150W should handle this easily.

Sorry my mistake, it is 24S2P.


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## frodus (Apr 12, 2008)

Look for "vicor DC-DC VI-241-12" on ebay. It's 45V-160Vin and 13.8Vout 75Wout and the guy has 5 available. $39USD + $19 shipping + $6 each additional one, total for 2 would be right around $103USD and has a 14 day return policy.

Do you have another source for Vicor DC-DC's?


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## Tomaj (Oct 3, 2011)

I have seen those modules.
When I add tax, customs fees, shipping cost...150$ And they are already used.

I get those 48V Vicors for 30$/piece

Is there another isolated DC-DC converter for around 100$?


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## frodus (Apr 12, 2008)

Not really, not one that is sealed.

Make hime an offer maybe.


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