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Evnetics Developing a DC/DC Converter

42K views 88 replies 34 participants last post by  Tesseract 
#1 ·
I thought this deserved it's on thread...

:D

I finally caved on this and am designing a DC/DC converter. To keep costs down it is not going to have any programmable frills - not even adjustable output voltage - but it will be water-resistant, if not water-proof, accommodate a very wide input voltage range (probably 90-425V), be naturally capable of paralleling, short-circuit protected, reverse-input voltage protected (no way to protect against connecting pack voltage to the 14V output, though) and somewhere around 40-50A max output current - depends more on the thermal stuff than anything else.

Should have a prototype done in a month or so. Estimated cost of the production unit will be pricey - probably in the $400 range because, sorry, we make kick-ass stuff in tiny quantities and don't use slave labor, so... definitely a "get what you pay for" kind of thing.

I don't know about the rest of you but I'm pretty excited to have a purposed built EV DC/DC convert that's available to the DIY community. My IOTA is probably the weakest component in my current conversion and I'll be happy to replace it with something that's design to handle up to 425 VDC.


Shane
 
#4 ·
IOTA is the weak link in my conversion too, I've already had one fail. The voltage boost input (boosts output voltage from 13.8 to 14.3) has stopped working on my second one. The voltage boost is a great feature, saves the 12v battery from boiling when the ignition is off, and keeps the headlights bright when the ignition is on.

It would be great to have a fanless DC-DC, is it possible? IOTA is not designed to take the heat of being trapped in a hot car in the summer, the fan is always going 100%.
 
#12 ·
Nah, no deposits because then I'd have to answer annoying "when will it be ready" emails... :D

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.
 
#17 ·
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. :rolleyes: 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. :D 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?
 
#14 ·
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.
 
#19 ·
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.
 
#15 · (Edited)
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
 
#16 ·
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/Mean-Well/PB-230-12/?qs=AmzscSOM9ZYKHcxj/5DIEw==

Mean-Well also makes a 1000W version for 12V, 24V, and 48V:
http://www.trcelectronics.com/Meanwell/battery-charger-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! :)
 
#27 ·
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/Mean-Well/PB-230-12/?qs=AmzscSOM9ZYKHcxj/5DIEw==
Yes, most people are aware of the MeanWell switchers. We even use them as individual chargers for the 12V AGM batteries for our dyno. They do have a disturbing tendency to blow up every now and then, so not the greatest quality, but at $30 each (for the 150W version) it's a tolerable failure rate.

That said, we mounted them in a NEMA enclosure with multiple fans for cooling and inside said box they aren't subjected to vibration, impact, temperature extremes, or high humidity. They get none of these benefits when installed in a car.

It's admirable to try to find a less expensive product, but at some point you go too far down the price vs. quality vs. performance scale. The MeanWell switchers are well past that point in my professional opinion.

Maybe I can collaborate with you on this design. ...
Thanks for the offer, but the dc/dc converter won't be too much of a challenge for me. I've more or less designed it already, anyway, I just need to lay out the board, build a prototype, etc...
 
#22 ·
..
What kind of size charger are you thinking of (in kw)? Or will you have more than 1 option?

Will it be possible to parallel them?
...
I just started on the design so can't answer any questions yet. It will be isolated, it will be at least 8kW, and it will be programmable. That's about all I've nailed down right now. :D
 
#26 ·
Just to enhance my own understanding, I had thought that the DC/DC converter and charger, for 12V accessories, was a single unit, and would always be used with a regular lead-acid battery as may have been in the vehicle. And I had assumed that the charger would just keep that battery topped off from the main battery pack. Of course a separate AC-DC charger could be used to keep the accessory system charged up during extended storage.

Then there would need to be the main battery charger, which would need to be powered by AC mains of 120 or 240 VAC. And as long as that power was charging the main battery pack, the DC/DC charger would also be charging the accessory battery.

If the accessories battery was a LiPo type, then the charging profile would need to adjusted for that chemistry, but otherwise the design and construction would be about the same. And since well-designed battery chargers have an output current limit of perhaps 13.8 or 14.4V, they could also be used without a battery in case it fails and needs to be disconnected. In such cases it would be good to have a decent size capacitor in the charger to keep the voltage steady under momentary current surges, and to limit ripple and noise.

Is this correct, or am I missing something? :confused:
 
#37 ·
Throw a in a 120V AC plug for those of us that like to plug things in in the car or use the car as a mobile power source? ;)
Erf... you mean make the charger bidirectional, so it can invert the battery pack into 50/60Hz AC? That would be possible only if the charger had a single input and output voltage; once you make it so it can operate over the universal AC line range (100-265VAC) and deliver a variable output voltage/current the goal of bidirectional operation is a distant memory...
 
#36 ·
You plan to use bipolar transistors, mosfets, or IGBT's for the switching of the DC? Also are you going to use RC or RCD snubbers because I get tired of seeing such a cheap out way of taking care of back EMF from the transformer input side rather than use half bridge synchronous rectification to have a path for the back EMF. I say use half-bridge synchronous rectification. You are the quality over quantity type after all.

Finally, you do not plan to use a TL431 voltage reference as a bastardized OP amp in which they were really never meant to be used this way as so many manufacturers like to use them....:rolleyes:
 
#38 ·
For AC power you can just get a 12V automotive inverter on the accessory battery. If you want serious power from the main battery pack you can use a VF drive. I got my 2 HP drive for about $60 and there are others on eBay for around $100-$150. It runs on 200-400 VDC and puts out 60 Hz three phase. You can set it for 120VAC output if that's what you need.
 
#46 ·
Appeal to the masses?!? I estimate that around 1000 cars per year are converted into EVs... In the entire world. Appealing to the masses doesn't even factor into our design process. If it did, we wouldn't being making stuff for this market in the first place! :rolleyes:

I've previously mentioned that I am not a big fan of "all in one" boxes unless there is a very compelling benefit to doing so. In the case of the dc/dc and charger, the main benefit is that it could allow kicking up the output power of the dc/dc for "free" because the money that would be spent on the output filter inductor and enclosure could be redirected to the actual power conversion components.

Any other benefits - such as reducing the amount of wiring - are more subjective, since that may or may not be relevant to a particular installation. For example, the charger should be mounted as close to the battery pack as possible while the dc/dc should be mounted as close to the 12V battery as possible, and those two things may be on opposite ends of the car.

So I am wavering on this one...


...If you want serious power from the main battery pack you can use a VF drive....
This suggestion is creative, but bordering on irresponsible. Not every AC electronic device will appreciate getting hammered with a PWM approximation of a sine wave without first being LC filtered and most VFDs do not like to see unbalanced currents on each of the three phase legs. All in all, a recipe for disaster.


You plan to use bipolar transistors, mosfets, or IGBT's for the switching of the DC? Also are you going to use RC or RCD snubbers because I get tired of seeing such a cheap out way of taking care of back EMF from the transformer input side rather than use half bridge synchronous rectification to have a path for the back EMF. I say use half-bridge synchronous rectification. You are the quality over quantity type after all.
I use whichever device is most appropriate for the job!?! That said, I haven't used a bipolar transistor in a switcher since the early 90s. 99% of the time I use MOSFETs, and only very recently have I considered IGBTs, as their higher current vs. die area is rarely worth the more complicated gate drive circuit required at power levels below, say, 1-2kW.

I don't get your distaste for passive snubbers. They are a reliable way of both absorbing the energy stored in the leakage and stray inductances in the circuit, as well as damping the Q of the inevitable resonances between switch output capacitance and said inductance. Most means of recycling this energy - by using a quasi-resonant switch, for example - result in a less rugged design as a result of a disturbingly high tendency to fail at light or no load.

Finally, you do not plan to use a TL431 voltage reference as a bastardized OP amp in which they were really never meant to be used this way as so many manufacturers like to use them....:rolleyes:
It is true that using a TL431 to drive an optocoupler for the feedback loop in a switcher is a bit removed from it's intended purpose as a variable shunt regulator, but it is almost always the opto itself that limits dynamic range and transient response, not the TL431! Dr. Ridley wrote an excellent article on using the TL431 in a power supply some years ago which I highly recommend reading.
 
#40 ·
I personally am not a fan of the 'all in one box' idea. I'd rather have separate components. It allows more modularity in design, and easier/cheaper repair down the road when a component fails. Just swap out whatever 'box' failed instead of having to open up and repair it, or worse yet having to pull the whole power cube out to send back to a mfr for repair.

Sorta like the combo TV/VCR's they used to make. Cool idea, but you end up throwing out a perfectly good TV when the VCR craps out.
 
#41 ·
I completely agree with you. There are definitely downsides to everything. Hopefully, the cover would come off and each separate component would be easily removable with quick-disconnect wiring.

I am thinking like a Desktop computer. Lots of bits packed in there, but still easy to take apart and service. Imagine if your computer was littered all over your desk. Hard drives, disk drives, power supply, motherboard, etc.. What a mess! :( That is the current state of EV's. A bit of a rats nest.
 
#42 ·
I like the potential cost savings of the combo, but I have to agree that individual products make better business sense. The target market of one of these items is very small to start with, force the customer to buy both with no flexibility mixing and matching and the customer base gets even smaller.

If there was minimal cost difference it would be worthwhile but it's unlikely that you can "throw in" a good dc/dc converter for just over the cost of the charger.
IE Charger is $2500 or Charger WITH 50A dc/dc is $2600 attractive option
Taking the Charger from $2500 to $3000 just to include the dc/dc might not be attractive to some.

As mentioned by ruckus, simplification in wiring is always a plus though, and the combo would likely be more compact that two separate devices.

****prices made up just to compare points, I have no idea what these will sell for!!
 
#44 ·
The problem with that logic is you're preaching it to the DIY community. That all-in-one box with the Apple sticker on it, in the EV world is called Leaf, Spark, MiEV(sp?), etc. All sorted out and ready to go, in one pretty package - for a price.

The DIY community is more analogous to the guy who refuses to give Apple, HP, etc, their money and still orders all those components individually, or in grouped sets, and rolls his/her own. To those individuals, freedom of choice is vital.

However, I don't think grouping two components like a good charger and dc-dc in one package would be seen as controlling or restrictive.
 
#51 ·
Rereading your post, I thought you meant metal oxide varistors, rather than resistors. And I'm just not familiar with the topology shown in the link. I found a similar half-bridge circuit in the following app note, but it does not have the third capacitor, which I think may be superfluous:
http://www.st.com/internet/com/TECH...AL_LITERATURE/APPLICATION_NOTE/CD00003910.pdf

For my own purposes, I am planning to use a push-pull topology, for a step-up DC-DC converter. And I do not need regulation, so the PWM can be nearly 50% which makes almost pure DC after the FWB. The DC-DC converter being discussed here is used to convert the battery pack voltage of 144-300V or so, to 12VDC for the accessory battery. This is a new concept for me, so I don't have a good grasp of what is really needed and why these converters are designed as they are.
 
#52 ·
Maybe late but a vote against the integrated model. Mostly as mentioned these components are typically on opposite ends of the vehicle. I like the flexibility to locate them where it make sense, and with the integrated component, one function will almost always be less than ideal. Add $50 for a pretty enclosure and we'll just suck it up.

Have you given any thought to a water cooling option for the DC/DC? Since we go through the trouble to water cool the controller, why not an option to eliminate the fan entirely and cool with rad fans. Just a thought. Hell if we can string enough heat makers together in a cooling cycle, we can eliminate those dual element heater:D.
 
#55 ·
...Add $50 for a pretty enclosure and we'll just suck it up.
$50 is the cost of the plain-Jane die-cast enclosure I mentioned before; a custom casting that is then CNC machined and nickel plated is going to cost a lot more than $50... probably at least $150. That kind of cost can be absorbed in a more expensive product like a motor controller, or charger, but it's tough to squeeze it into an exceedingly unsexy product like a dc/dc converter. If I put the dc/dc into a nice enclosure but have to charge $650 for it instead of $500 then the vast majority of DIYers will pass it over after (rightfully) concluding that too much of the price is tied up in aesthetics, rather than performance. Still, knowing how Seb is we'll probably take a crack at a custom enclosure anyway...

Have you given any thought to a water cooling option for the DC/DC?...
My goal is to not need supplemental cooling at all - relying strictly on natural convection - but I won't know for sure until I build a prototype.
 
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