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  #11  
Old 06-27-2012, 06:10 AM
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Default Re: Evnetics Developing a DC/DC Converter

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Originally Posted by adeyo View Post
Sorry, newb question, but what exactly does a dc/dc converter do? ...& where does it fit in the EV system?
The wiki on this site (top right of page has lots of information such as this.

http://www.diyelectriccar.com/forums...ad.php?t=22818

Cheers,

Mike
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  #12  
Old 06-27-2012, 07:54 AM
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Default Re: Evnetics Developing a DC/DC Converter

Nah, no deposits because then I'd have to answer annoying "when will it be ready" emails...

60A at ~14V output is a bit more than I was planning on. I realize that some people really do need that much current - mainly because of electrohydraulic steering (e.g. - like used in the MR2) - but the majority of vehicles would do fine with a 40A dc/dc, and the vast majority - probably 80% - would do fine with 50A. In trying to balance all of the various requirements like cost, efficiency, using the case as a heatsink, input voltage range, etc., it looks like 14V/50A is a reasonable upper limit.

I was planning on making it possible to trim the output voltage over a narrow range using an external resistor which could be used to boost the voltage when the ignition is on much like the IOTA's.

There won't be a fan and the box I intend to use (Bud ANS-3809) is both waterproof and has EMI gasketing. It's not too pretty, but it gets the job done. There is a black powder-coated version for $10 more, IIRC.

I just need to make a bit more progress on the other two projects (one EV-related, one not) before resuming work on the dc/dc.
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  #13  
Old 06-27-2012, 11:29 AM
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Default Re: Evnetics Developing a DC/DC Converter

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Originally Posted by adeyo View Post
Sorry, newb question, but what exactly does a dc/dc converter do? ...& where does it fit in the EV system?
It takes your high voltage traction pack and gives you 12 volts so you don't need a 12V battery to power the car systems.

In point of fact it is a good idea to have a 12V battery and use the DC-DC converter as a charger for the battery. Think of it as an alternator replacement. You want a battery because if you have a catastrophic failure in the DC-DC you still want to have your lights and other safety equipment work until you get off the road. Using two independent DC-DC converters is a good compromise for not having a 12V battery because it is unlikely both converters will die at the same time. But a main traction pack issue will still take out both converters. This is why many people still recommend a battery. I am using an 8AH LiFePo4 pack and a 40amp DC-DC converter in my conversion.
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  #14  
Old 06-27-2012, 11:59 AM
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Default Re: Evnetics Developing a DC/DC Converter

I would very much recommend a small 12v battery to stabilize surge loads on the dc-dc (like intermittent wipers or vacuum pumps). I use 4 Headways to stay on the lithium side of things. Many people who have tried to use only a dc-dc without a 12v batt have them blow over and over.

It is not good practice.

Jack Rickard also has a theory that when your traction pack sags under heavy acceleration it can pull juice backwards from the dc-dc caps and cause them to blow. He recommends using a 1-way isolator on the dc-dc input to prevent this. Not a bad idea.

With these 2 precautions, most dc-dc failures would likely be avoided.
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  #15  
Old 06-27-2012, 12:13 PM
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Awesome. That link answered my question...thank you! I am already planning on running an alternator off the front shaft of my motor and using an auxiliary battery. I never knew this to be the purpose of a dc/dc converter. :thumbup:

Edit: thanks Dougingraham! That really further clarifies things. Helpful

Last edited by adeyo; 06-29-2012 at 07:17 AM.
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  #16  
Old 06-27-2012, 06:21 PM
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Lightbulb Re: Evnetics Developing a DC/DC Converter

If you can accept an input voltage range of 127 to 373 VDC, you can get 16 amp switching chargers for about $80 each:
http://www.mouser.com/ProductDetail/...%2f5DIEw%3d%3d

Mean-Well also makes a 1000W version for 12V, 24V, and 48V:
http://www.trcelectronics.com/Meanwe...r-pb1000.shtml

The 12V 60A version is $278:
http://www.trcelectronics.com/Meanwell/pb-1000-12.shtml

It might be worthwhile to have a look at one and see if it meets the need. For a special purpose OEM maybe they could build a version designed for higher input voltage, and specifically DC. I'm sure these are also for lead-acid batteries, but maybe they are still OK for running the accessories. You could keep the same battery as comes with the car for conversions.

I'm looking into a DC-DC converter design that boosts 12, 24, 36, or 48 VDC up to 160, 320, or 640 VDC. Having it work the other way is essentially the same, and could use the same transformer and control circuitry. The changes would be the higher voltage MOSFETs or IGBTs, and other components such as rectifiers and capacitors.

I plan to make a 2kVA and 5kVA model, although I may start with a 1kVA prototype because I already have some of the components, such as the transformer core and bobbin. I also found some EE55 ferrite kits on eBay for $13, probably good for at least 1kVA and possibly 2kVA. And they have E80 cores, good for 3-7kVA, for $14/pair. The bobbins are cheap and the wire is probably no more than $20-$30 or so. Everything else should be under $50, except for any special packaging.

Maybe I can collaborate with you on this design. I'm not looking to make a lot of money on this and I'm really not set up for manufacturing, so maybe we should talk about details via PM or email. My immediate needs are for small tractor conversions and step-up converters, and I'm not even sure I can or want to go into production, but I've gotten the interest of some people on the tractor forum so there may be a market.

Good luck!
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  #17  
Old 06-27-2012, 06:40 PM
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Default Re: Evnetics Developing a DC/DC Converter

Quote:
Originally Posted by Tesseract View Post
I was planning on making it possible to trim the output voltage over a narrow range using an external resistor which could be used to boost the voltage when the ignition is on much like the IOTA's.

There won't be a fan and the box I intend to use (Bud ANS-3809) is both waterproof and has EMI gasketing. It's not too pretty, but it gets the job done. There is a black powder-coated version for $10 more, IIRC.
.
not that you asked, but here's what I think. I like the voltage boost idea, my DC/DC is always at 13.5v, and my headlights are noticeably dim compared to my other cars. Of course my car is almost 20 years old so it could just be old wiring and dim lights, but I think some of it is the lower voltage.

I love the no fan and waterproof idea, but I think it would be a mistake to not make it cool looking. Lets be honest, the awesome looks of your controllers is a definite selling point. Look at how much work folks go through to make sure their Solitons are front and center in the motor compartment cause they look so awesome. I know you are not going after the bottom end market here, a few more bucks for a good looking custom box would move it appropriately upscale visually. Perhaps out of the question at DIY market quantities though?
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  #18  
Old 06-27-2012, 07:57 PM
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Lightbulb Re: Evnetics Developing a DC/DC Converter

These enclosures look nice. 6" x 6" x 2", green anodized extruded aluminum. On sale for $25:

http://www.newark.com/box-enclosures...ium/dp/26K9071

Or a larger one 8" x 6" x 2", blue, for $36:

http://www.newark.com/box-enclosures...ium/dp/26K9075

Or:

http://www.newark.com/hammond/1455zt...ium/dp/50H5773

Or if plastic is OK, only $14:
http://www.newark.com/jsp/search/pro...sp?SKU=10R1327
http://serpac.com/userprints/WM073i_UP.pdf
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  #19  
Old 06-27-2012, 08:03 PM
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Default Re: Evnetics Developing a DC/DC Converter

Quote:
Originally Posted by ruckus View Post
Jack Rickard also has a theory that when your traction pack sags under heavy acceleration it can pull juice backwards from the dc-dc caps and cause them to blow. He recommends using a 1-way isolator on the dc-dc input to prevent this. Not a bad idea.
This was just rehashed a few threads ago after Tesseract gave me a lesson a month before that. It isn't that the controller caps supply power because the input voltage sags, they would supply just a little in a smooth action as the sag happened. That won't blow any fuse and won't upset the DC>DC unless you go below its rated input voltage (some DC>DC converters will blow up just for that.)

The problem is that the controller is switching about 15,000 times each second. If the caps in the DC>DC try to follow that the input fuse will blow because it measures the current both ways and adds them up. If your DC>DC starts supplying and then receiving 30 amps back and forth constantly with the controller switching then the input fuse will likely blow. It should do that because heating caused by resistive losses goes both ways too. The diode will prevent the 30 amps from leaving the DC>DC but it is still using power off those caps (without the pack able to charge them for that tiny fraction of a second because the pack voltage is less than cap voltage) and when the controller power section switches off the current will rush into the caps. Depending on pack voltage swings, the impedance between the controller and DC>DC, and other factors (impedance and capacitance is everywhere) the blocking diode could solve the problem, or do almost nothing to help.

An inductor in the power line to the DC>DC can help, consistently. They are cheap and Mouser of Digikey, less than $10 for the part. If you are using any controller that might reflect to much ripple onto the system bus by all means use this simple fix for the DC>DC and avoid a half fix with a diode.
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  #20  
Old 06-28-2012, 04:54 PM
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Default Re: Evnetics Developing a DC/DC Converter

Quote:
Originally Posted by dladd View Post
... I like the voltage boost idea, my DC/DC is always at 13.5v...
13.5V is too low to properly charge a 12V lead-acid battery but it shouldn't result in noticeably dimmer headlights. I would check the voltage at the headlights to make sure you aren't losing too much in the wiring (which, I should note, is often intentionally sized too small to reduce the inrush current stress).

Quote:
Originally Posted by dladd View Post
...I think it would be a mistake to not make it cool looking....
We don't disagree in principle, but in practice this is a tougher argument to make because it is difficult to nail down the exact cost of a custom enclosure. For our controllers it's alright - not ideal, mind you, but alright - if we are off on our cost estimate by $100 or so, but that could be the difference between profit and loss for the dc/dc converter. Also, nickel plating is very expensive while anodizing is too inconsistent to use on aluminum castings. All in all my guesstimate is that a custom milled nickel-plated enclosure for the dc/dc would add $150 to the cost. That might make it too big a pill to swallow, particularly if it doesn't put out 60A or more.

Lately we've been kicking around the idea of combining the dc/dc and charger in one box... It seems like a good idea to me, and eliminates some of the cost concerns with a custom enclosure, etc., since the charger would require one anyway.
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