Any way to safely test if an AC-DC converter can accept DC input?

[FyKnight]

If you're really still interested in CCS, you can get some documentation/information packs from CharIN (the association in charge of CCS) on request for free https://www.charin.global/ The info pack has pretty complete information on the CCS protocol and processes includes some detailed slides on the charging process as well as safe guards. Not quite 100% of what you need for a from scratch implementation due to needing some cited standards but pretty good none-the-less.

Edit: If you look for "Design_Guide_Combined_Charging_System" there's an older version floating around on the internet but the contents looks the same as the version I have. Both are dated 2015-06-02.

Thanks again sandalcandal, if it's the one with the Euro car manufacturer logos at the top of every page then yes, I found that document a couple of months ago when I started this journey, it's a really good overview. Is there more detail in the info pack? i.e. actual standards? Funnily enough I was on their site again today downloading a few things I'd missed.

I'm not against buying the standards either (especially in Estonian Euros ) but if this direct DC or square wave idea works out then I won't have to.

I really appreciate your replies as well as all the other responders to this thread. What a great community!

[mikeselectricstuff]

I just had a look at the board from an old OBC I have here - I think it's from a BMW PHEV.
I can't see anything that would preclude it running on DC. It doesn't have a current transformer, only a sense resistor after the input rectifier. This makes sense as it can derive AC input current from this, and it's smaller and cheaper than a CT.

I suspect the most likely obstacle is a software-controlled PFC wanting to see AC, though I can't see why it would care, unless it's using something like synchronous rectification for efficiency. Should be fairly obvious from the board if this is the case.

[sandalcandal]

Some notes on compatibility with square wave inverters with respect to standards. J1772 clause 4.6.1.4 stipulates harmonic distortion immunity specified in UL 2231-2 (can be view free here: https://standardscatalog.ul.com/ProductDetail.aspx?productId=UL2231-2) J2894-1 "Power Quality Requirements for Plug-In Electric Vehicle Chargers" also recommends THD of 10% [max] but practically speaking, apart from possible but unlikely noise [and PFC control loop] issues I don't see any reason they shouldn't work.

However, if efficiency is the concern, current state of the art gets over 99% efficiency with "pure sine" output using WBG (wide band gap) semiconductors and a totem pole topology e.g. https://training.ti.com/demo-900-v-5kw-bidirectional-ac-dc-converter-gan Obviously there's a bit of a cost premium but if you're making something new, might as well.

[sandalcandal]

Thanks again sandalcandal, if it's the one with the Euro car manufacturer logos at the top of every page then yes, I found that document a couple of months ago when I started this journey, it's a really good overview. Is there more detail in the info pack? i.e. actual standards? Funnily enough I was on their site again today downloading a few things I'd missed.

I'm not against buying the standards either (especially in Estonian Euros ) but if this direct DC or square wave idea works out then I won't have to.

I really appreciate your replies as well as all the other responders to this thread. What a great community!

Yep, ~130 page long slide deck with some EU car groups at the top of every page. That's really the main one with all the juicy info, the other documents are mostly formalising documents citing standards and branding guidelines. The fine engineering details e.g. numerical values, timing and other parameters are all in the pay walled standards unfortunately.

Good you know about the Estonians (I hope it doesn't get shut down).

[NiHaoMike]

Again with any DC charging, you'll need a current regulating DC-DC converter of sufficient power capability. You're unlikely to find anything off the shelf for cheaper than an equivalent power inverter. You then also get the benefits over being able to run other mains appliances off the inverter.

Easily available for about $100 in the form of the buck/boost part of a Prius inverter, good for 10-20kW continuous. You'll also get two 3 phase inverter outputs and a 12V DC/DC converter.

[sandalcandal]

Again with any DC charging, you'll need a current regulating DC-DC converter of sufficient power capability. You're unlikely to find anything off the shelf for cheaper than an equivalent power inverter. You then also get the benefits over being able to run other mains appliances off the inverter.

Easily available for about $100 in the form of the buck/boost part of a Prius inverter, good for 10-20kW continuous. You'll also get two 3 phase inverter outputs and a 12V DC/DC converter.

That sounds like a nice idea. I didn't think about using a salvaged inverter. Are there any good resources on getting a Prius inverter working outside the car?

[NiHaoMike]

https://openinverter.org/forum/viewforum.php?f=14
Note that although the low side cap limits the working voltage to 270V or so, you can get around it by connecting the input between the high side and low side. You'll also have to change out the board with the control ASICs for one that uses more generic control logic.

[FyKnight]

I just had a look at the board from an old OBC I have here - I think it's from a BMW PHEV.
I can't see anything that would preclude it running on DC. It doesn't have a current transformer, only a sense resistor after the input rectifier. This makes sense as it can derive AC input current from this, and it's smaller and cheaper than a CT.

Oh awesome thanks very much! I'll try to open mine up and have a look this arvo.

I suspect the most likely obstacle is a software-controlled PFC wanting to see AC, though I can't see why it would care, unless it's using something like synchronous rectification for efficiency. Should be fairly obvious from the board if this is the case.

So if I see a regular diode FULL BRIDGE rectifier (oh wrong channel, sorry!) then it should be right?

[FyKnight]

Some notes on compatibility with square wave inverters with respect to standards. J1772 clause 4.6.1.4 stipulates harmonic distortion immunity specified in UL 2231-2 (can be view free here: https://standardscatalog.ul.com/ProductDetail.aspx?productId=UL2231-2) J2894-1 "Power Quality Requirements for Plug-In Electric Vehicle Chargers" also recommends THD of 10% [max] but practically speaking, apart from possible but unlikely noise [and PFC control loop] issues I don't see any reason they shouldn't work.

OK brilliant! Since it's a stand-alone system I'm not worried about regular noise affecting other devices, although I'm a little concerned about spikes when switching.

However, if efficiency is the concern, current state of the art gets over 99% efficiency with "pure sine" output using WBG (wide band gap) semiconductors and a totem pole topology e.g. https://training.ti.com/demo-900-v-5kw-bidirectional-ac-dc-converter-gan Obviously there's a bit of a cost premium but if you're making something new, might as well.

The thing is, apart from the cost and weight, I have to factor in the OBC inefficiency. And the battery efficiency (intra cycle hopefully), then the MPPT efficiency. It doesn't just add up, it multiplies out to a pretty bad deal.

Yes I looked into using a batteryless inverter but they seem to support well less than their regular rated load (and cut out) when no battery is present, and initial capacitor charging current knocks them over too. It also isn't clear if they do MPPT in batteryless mode, and finally I don't know how to determine the used vs available power in order to ramp the current limit (request) up and down. So I discarded that option to focus on direct DC, and in the meantime went with a regular inverter + a couple of MPPT charge controllers and a high C battery to get something working (like everyone else). This was also informed by doing a couple of measurements of my OBC efficiency which came out to a total loss of 300 - 400 W (just comparing SoC after a time vs input energy). I can't see how that can true though after learning more about OBC design so I'll revisit it. Maybe the 12 V DC-DC charger was running at that time...

[FyKnight]

https://openinverter.org/forum/viewforum.php?f=14
Note that although the low side cap limits the working voltage to 270V or so, you can get around it by connecting the input between the high side and low side. You'll also have to change out the board with the control ASICs for one that uses more generic control logic.

That is a pretty cool option hardware-wise, but implementing the control logic could be a bit hit and miss and take a good while to get right. I'd probably be better off spending the time on doing something new with CCS.

[sandalcandal]

So if I see a regular diode FULL BRIDGE rectifier (oh wrong channel, sorry!) then it should be right?

A bang-bang square wave inverter with no 0V dwell time might (or might not) cause some issues for a synchronous rectifier but a passive diode rectifier would also likely see some increased loss due to hard switching caused fast reversal of voltage but unlikely to cause malfunction.

Some teardowns of other OBCs for reference:

(weird semi-active bridge?)


(diode rectifier in Tesla Model 3)

Another suggestion, maybe see if there are any scrap/salvage OBCs for your EV you can get before you go trying to tear into your own working car.

However, if efficiency is the concern, current state of the art gets over 99% efficiency with "pure sine" output using WBG (wide band gap) semiconductors and a totem pole topology e.g. https://training.ti.com/demo-900-v-5kw-bidirectional-ac-dc-converter-gan Obviously there's a bit of a cost premium but if you're making something new, might as well.

The thing is, apart from the cost and weight, I have to factor in the OBC inefficiency. And the battery efficiency (intra cycle hopefully), then the MPPT efficiency. It doesn't just add up, it multiplies out to a pretty bad deal.

Yeah, you could get better solar battery to EV battery efficiency if you get a DC-DC system working and bypass the OBC I guess. Not sure the cost and effort would be justified without seeing your actual numbers however.  I'd expect an OBC to have >90% efficiency and a well implemented DC-DC to have ~95% [98% on the high end]. 5% improvement might not really justify the costs. ~10% in a "best case" might also not be worth it if the costs involved are too high.

[FyKnight]

A bang-bang square wave inverter with no 0V dwell time might (or might not) cause some issues for a synchronous rectifier but a passive diode rectifier would also likely see some increased loss due to hard switching caused fast reversal of voltage but unlikely to cause malfunction.

I'd worried about this and will probably put in some very small zero time because I'll be scared to have both paths on at the same time with differing on- and off-transition times. Doubt it'd make it much better for the rectifier though. But then again unless there's some other capacitance in the panels (probably insufficient given the speed of MPPT scanning algos) I'll lose efficiency whenever I'm not passing that current through. And now I want to add some capacitor or inductor haha... but no, I really don't want to end up designing most of a regular inverter here! Let's see how I go with the dumb approach first.

Another suggestion, maybe see if there are any scrap/salvage OBCs for your EV you can get before you go trying to tear into your own working car.

Hahah yes, and if I can get a spare battery with it that'd be even better! You read my mind and I was looking on Pickles last night. No-one has crashed their shiny new MG EV recently though.

Yeah, you could get better solar battery to EV battery efficiency if you get a DC-DC system working and bypass the OBC I guess. Not sure the cost and effort would be justified without seeing your actual numbers however.  I'd expect an OBC to have >90% efficiency and a well implemented DC-DC to have ~95% [98% on the high end]. 5% improvement might not really justify the costs. ~10% in a "best case" might also not be worth it if the costs involved are too high.

You're definitely right, though in my EV driving experience, 10% extra would be really nice sometimes! If you are limited in solar array space then it's even more important. And if you also build in the MPPT controller to avoid extra conversion stages and battery losses then you're even further ahead.

I wasn't sure if I could link YT videos from other channels here but I see you did (and thank you for those teardowns!). Here's the MPPT + DC-DC converter board that is on its way to me now, not powerful enough for my application, but let's see how it performs:

Edit1: timecode in link didn't work: go to 21:15 to see the board
Edit2: OMG I just realised the source of one of those videos. I have so much to learn. Thanks again everyone!

[sandalcandal]

I'd worried about this and will probably put in some very small zero time because I'll be scared to have both paths on at the same time with differing on- and off-transition times. Doubt it'd make it much better for the rectifier though. But then again unless there's some other capacitance in the panels (probably insufficient given the speed of MPPT scanning algos) I'll lose efficiency whenever I'm not passing that current through. And now I want to add some capacitor or inductor haha... but no, I really don't want to end up designing most of a regular inverter here! Let's see how I go with the dumb approach first.

"Dead time" is the technical word used in power electronics for this period when all the switches are turned off. You definitely need some sort of buffering (DC-link) for smoothing power off the panels. Photovoltaics don't care about the average power, they need the instantaneous power draw to be at their maximum power point. A capacitor would generally be required to get the high-frequency smoothing required but I don't have the expertise in solar power systems to know more niche methods.

You're definitely right, though in my EV driving experience, 10% extra would be really nice sometimes! If you are limited in solar array space then it's even more important. And if you also build in the MPPT controller to avoid extra conversion stages and battery losses then you're even further ahead.

Yes, if you are limited in space for your solar array then squeezing the most out of the available solar area is important, you can't just redirect lowered power electronics costs to buying more (or to an extent better) panels in the limited solar array case.

[FyKnight]

I finally took the plunge and undid the excessive number of screws to open the OBC.



Here's my interpretation (any of this could be wrong!) The top left is the Type 2 AC cable coming in. The top right had better be the battery — didn't actually test that though.

The grey pads are some heat transfer material, and luckily half of it stuck to the lid so I could see what was under them better There's a nice pair of inductors which I expected. Some absolutely massive FETs pinned against the wall. A capacitor bank under the black plastic at the bottom right, more boxy capacitors dotted around.

In the middle of the board (actually poking up through a hole in a mezzanine board) are what looks to be two good size transformers. Maybe some sense transformers above them too.

So yeah, given the proliferation of probable transformers, I'm going to guess direct DC into it is a bad idea. Please correct me if I'm wrong though. I have a higher resolution photo (and videos) too if helpful.

[f4eru]

DC can make a PFC go mad...
DC can get an arc over a relay contact keep burning and not extinguish.
DC can mess up active X cap discharge circuits.

I personally would not take the risk for an OBC, but I would for a smaller PSU.

[uer166]

The inductors on bottom left are the PFC boost, and you can see the capacitor bank on bottom right, it then loops back around to upper right through transformers. It's probably some sort of LLC/PSFB/DAB converter. There is nothing fundamentally preventing the use of DC input here, but as was mentioned, you may explode the input contractor that's not rated for DC (fatal), or confuse the PFC controller. I'd recommend trying it just so we see the result for funsies, assuming it wouldn't be a huge loss! 

[FyKnight]

There is nothing fundamentally preventing the use of DC input here, but as was mentioned, you may explode the input contractor that's not rated for DC (fatal), or confuse the PFC controller. I'd recommend trying it just so we see the result for funsies, assuming it wouldn't be a huge loss!

Excellent, maybe I can replace that contactor with one that is rated for DC then. I'm willing to risk confusing the PFC controller. Technically, those pins could expect to see DC in some (never implemented AFAIK) charging systems so... it's not inconceivable that the OBC would be safe to receive it even if it didn't charge with it.


Yeah breaking this OBC would be a big loss, being the only way to charge my regular car. I reckon I'll try it with square wave input first, and wait to pick up a wreck to try actual DC.

Thanks very much for your replies folks!

[Yorch]

Great thing! Since years i have this idea to try this but did not dare yet.

Any news from your side?

Actually i have a Hyundai Ioniq Facelift. At the beginning it had a failure in the DCDC converter, causing the 12V Battery to go empty. Because the DCDC is withing the OBC, they changed the complete unit, but send it back to Korea, so no chance for me to teardown. On the Unit its written Input Voltage 85...285V, 20...100Hz.
These 20...100Hz makes me belive that frequency in fact does not matter at all....


Your point that it has to be "DC-Mid" save is worth thinking. The question is, when designing a OBC, do i have to consider it will be connected to failed DC-Mid DC-Charger? Since now only Tesla uses this at its old superchargers, but they will turn on the voltage only if a Tesla has identified by the CAN-Bus comminication over the pilot pins...

[f4eru]

There might be a misconception here.
The DC-MID mode is not passing the power through a converter. it is contacted directly to the battery voltage, and the converter is in the charge point.

[FyKnight]

Any news from your side?

I bought all the equipment to do it but didn't get around to trying it yet, mainly due to being distracted with another project (building my new workshop!)

However, I've since heard separately from a contact who has tried it with 160V DC from batteries directly into the AC pins, and not only did it not break the OBC, it successfully charged his Volt.

There might be a misconception here.
The DC-MID mode is not passing the power through a converter. it is contacted directly to the battery voltage, and the converter is in the charge point.

The point is not to use the mode (which is rarely supported), but that its existence means the OBC could be designed to safely accept DC on those pins, and not melt a coil or lock a contactor. So it might be safe to test it even if it didn't work to charge.

[Seekonk]

I have a solar powered summer home and am thinking about getting an EV for that location.  Opposite reason from most, I want to bring power home so I can run an AC.  I do a lot of direct power from the panels and have a very minimal battery. A shunt regulator to a water heater keeps the array voltage stable. I run a LG washer right from the panels using an old MSW inverter with no battery. Just feed HV panel DC right into the H bridge and some 12V to run the control electronics.  The panels will supply the current limit.  The H bridge of the inverter will keep running regardless of the voltage and not cause any faults.

[f4eru]

The point is not to use the mode (which is rarely supported), but that its existence means the OBC could be designed to safely accept DC on those pins, and not melt a coil or lock a contactor. So it might be safe to test it even if it didn't work to charge.

it's not. DC is only present if negociated sucessfully.
DC will probably charge your car. But it remains basically unsafe.

[TimNJ]

Just generally speaking, if there is a fault which should cause a fuse to blow, the AC rated fuses may not be able to interrupt DC. DC is pretty hard to interrupt by comparison, so you should always check the specification of the included fuse(s) and check if they have a DC rating (with ample breaking capacity). If not, you should consider adding a proper DC rated fuse, with enough breaking capacity for the prospective fault current of the installation.

[FyKnight]

I run a LG washer right from the panels using an old MSW inverter with no battery. Just feed HV panel DC right into the H bridge and some 12V to run the control electronics.  The panels will supply the current limit.  The H bridge of the inverter will keep running regardless of the voltage and not cause any faults.

Nice work, that's a great idea! I'll do the same for my test now (already have the MSW inverter) instead of building my own.... thanks!

it's not. DC is only present if negociated sucessfully.

So is AC but I doubt it would physically damage it to have AC present outside the negotiated state.

DC will probably charge your car. But it remains basically unsafe.

For sure, yeah look I hear you and I realise this is optimistic thinking. But if I break something then it'll be a fun project trying to fix it

Just generally speaking, if there is a fault which should cause a fuse to blow, the AC rated fuses may not be able to interrupt DC. DC is pretty hard to interrupt by comparison, so you should always check the specification of the included fuse(s) and check if they have a DC rating (with ample breaking capacity). If not, you should consider adding a proper DC rated fuse, with enough breaking capacity for the prospective fault current of the installation.

Thanks for the suggestion! Thinking about it, this should only be an issue on the input side right? After that it'll all be the same inside the OBC. However coming from PV panels their Isc is only about 10% higher than Imp, so not much for a fuse to do there. OK well sure, if I had a bigger array than the OBC could use (over ~7 kW / 32 A Isc) then this would be possible... that'd be a nice problem to have

[richard.cs]

it's not. DC is only present if negociated sucessfully.

So is AC but I doubt it would physically damage it to have AC present outside the negotiated state.

Kind of a yes but, I am aware of at least one AC implementation that is damaged by a corner case. On a Renault Zoe single phase charging is achieved by connecting L3 to N with a contactor because the charger only has inputs for L1 L2 L3. The trigger for this was (or used to be) that there was no voltage on the DC bus, but L3-N voltage was not checked before closing the contactor. Fine most of the time, but consider if L3 does have volts relative to N, but L1 and L2 are open circuit or misconnected to the same phase as L3. Then there is no DC bus voltage, but closing the contactor shorts L3 and can result in it welding closed. I think it has been fixed in newer models.

Another one, on cars where 3 phase charging is 3 separate single phase chargers it is quite possible that connecting all three phases to the same phase may result in triple neutral current and a very unhappy cable.

Not all cases that might reasonably be expected seem to have been considered by designers.