New EV Charger Design - Modular

43516 Views 68 Replies 14 Participants Last post by PStechPaul, Aug 22, 2019

PStechPaul

Discussion Starter · Nov 8, 2014 (Edited)

After delving deeply into the EMW 12 kW DIY charger, I think it would be best to make a new design. My concept is to make modules, each of which can be used on 120 or 240 VAC single or three phase, or up to 300 VDC. The modules would be 1.2 kVA for 120 VAC, and 2.4 kVA for 240 VAC or 300 VDC. They will be capable of being connected in parallel to obtain higher power. I think these modules could be built for a parts cost of less than $150 each.

The IGBT I show here is an ultra-fast 35A 600V device that is designed for switching applications up to 100 kHz, so I think the inductor and capacitor size and cost may be greatly reduced. And this part is only about $1.50.

Here is a "first shot" at this design. It has been done using Mentor Graphics PADS 2004 and most of the parts are fully characterized with part numbers and approximate cost, as well as PCB decals so that a board can be made directly from the schematic. A BOM in Excel format can also be produced easily with a VBA script.

Here is a PDF which is a little easier to read:
http://enginuitysystems.com/files/EMW/EV_Charger_PCB.pdf

And the BOM (preliminary) showing total parts cost less than $150:
http://enginuitysystems.com/files/EMW/EV_Charger_PCB_BOM_20141108.ods

This version is not PFC and non-isolated. It also has only a single pushbutton for start/stop, does not have a BMS connection, and has no display. But it has a serial port which can be connected to a Bluetooth module for viewing and logging data, and for commands. I am using only a 14 pin PIC16F1825 but it will probably need a 28 pin processor to provide the additional I/O needed. I have an Arduino UNO and I will try to add the connections to match its pinouts. I might also see if I can adapt the EMW control board with its display and function keys.

I am putting together an order to Mouser for some of the parts I will need for this design, and I am also going to get an AVR Dragon which is a $53 emulator/debugger/programmer which is really the very minimum needed for development of any serious design with the Atmel series microcontrollers. I plan to use this for analyzing the EMW charger (when I get a complete unit or the boards and parts needed), and hopefully be able to provide recommended modifications to the hardware and firmware to improve the reliability and performance. I think it may require a complete new set of boards, but many of the expensive components may be able to be re-used.

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PStechPaul

Discussion Starter · #21 · Feb 14, 2015

I will be using this thread to solicit ideas and feedback on my proposed charger project. At this point I envision a single control board and up to eight 2 kW power units. This will make for a low-cost "entry level" charger with 2 kW (120V 20A) for around $400 and then about $100/kW for the additional modules (about $200 each).

Hopefully I will be able to make these chargers isolated, which may add some cost. It will involve incorporating a high frequency transformer, which may allow the use of high speed MOSFETs and a ferrite E-core transformer. An isolated design is generally about twice the volume of a comparable non-isolated version, but if it can operate at 50 kHz rather than 12-20 as presently used, it can be proportionately smaller and perhaps cheaper.

More to follow...

dcb

· #22 · Feb 14, 2015 (Edited)

one of the challenges is to have the right buck inductor(number of turns and gauge) for a given output voltage (something I dont see in the EMW) so that you can deliver the "advertised" 1.5-2kw for various battery voltages.

There may be some optimizations in boost inductor selection for a specific battery voltage too, but it isn't critical.

PStechPaul

Discussion Starter · #23 · Feb 14, 2015

One advantage of using a transformer is that the secondary can be wound specifically for the target voltage. The primary can stay the same, for a nominal 350 VDC from the PFC boost front end. Another possibility is to use dual windings for both primary and secondary. Thus for a 120V input the primaries can be in parallel, and switched to series for 240V. Similarly, the secondary could be parallel for up to 180V at 10A, or series for 360V at 5A.

A toroid transformer is fairly easy to wind, especially for lower voltages. I would expect a core capable of 2 kW would have about 5 volts/turn at 50 kHz, so 180V would be just 36 turns. It may even be best to use 6 output windings of 60V each that can be connected for 60, 120, 180, or 360V. This would cover most battery packs from 48V to 320V, and thus a 2 kW charger could provide 33 amps from an ordinary 120V outlet. The output voltage selection can be done with a matrix of slip-on terminals on the PCB. It might be necessary also to switch the filter capacitors, rectifiers, and chokes, but having six small circuits may be more cost-effective than one with larger components. It is also easier to make a PCB with multiple traces for 5 amps than one for 30 amps.

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dcb

· #24 · Feb 14, 2015 (Edited)

Problem is the buck inductor (and switch/diode) is what needs to be flexible. if you are thinking transformer for isolation, typically that is part of the pfc side (and introduces extra magnetic components and rectifiers/switches)
i.e.

I assume you want smooth power to the battery, i.e. some sort of buck converter feeding a battery instead of just a resistor there, and want pfc so you can get the most out of a 20A breaker. There may be a way to switch the transformer for good pfc without an additional inductor, haven't worried about isolation too much so I haven't thought about it really.

So a multi-winding buck inductor is probably more appropriate than a transformer, unless you have a different pfc/regulation scheme in mind. But it is still an interesting idea, some jumpers for different output voltages.

(and the advantage here is that you can use a high frequency transformer for isolation, as opposed to a 50hz transformer on the input, if you want isolation, but still a buck stage is an appropriate addition for a battery charger)

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PStechPaul

Discussion Starter · #25 · Feb 14, 2015 (Edited)

4

Once you have smooth DC from the input rectifiers, capacitors, and PFC circuitry, very little output smoothing will be required. The isolation transformer creates a square wave which, when rectified, has only some small high frequency components during the switching. Here is the rectified output of my DC-DC converter at 10 kHz with a 48W resistive load:

This is the prototype. It uses a ferrite E-core transformer with 1:10 ratio for a 10x voltage boost. The trace above is for 24 VDC input.

This design is probably good for at least 1000 watts, but I may need to use a higher frequency, and more or larger MOSFETs. Here is the basic circuit:

A simulation:

http://enginuitysystems.com/pix/Half_Bridge_24V-320V.asc

Note that the output inductor is only 20 uH. It may need to be larger for a buck current regulator. This is really a DC-DC voltage converter. But it will probably not need a very large inductor if the output voltage is fairly close to the raw DC, which can be matched pretty closely with the series/parallel arrangement of the transformer secondaries.

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dcb

· #26 · Feb 15, 2015 (Edited)

It's looking smooth zoomed out, I find that a battery with a low esr is a lot more challenging than a resistor though, minor voltage fluctuations result in large current swings in the load. Post the asc if you would please, I would like to experiment with it a bit.

edit, disregard, got the link

edit2, looking pretty smooth with a 312.58v battery @ .2 ohms, and ferrites are cheap and don't like dc offset anyway.. will have to chew on it a bit.

PStechPaul

Discussion Starter · #27 · Feb 15, 2015

What you really need is the adjustable buck current charger, which I have also built and simulated:

http://enginuitysystems.com/pix/Buck_Current_Charger_36-24V.asc

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onegreenev

· #28 · Feb 15, 2015

What's going to be the hardest to implement, bucking down to match a voltage or boosting to match a voltage? My Synkromotive will boost to pack voltage and works best when the source is closest to the minimum voltage of the discharged pack. In order to use the wall voltage I must have a pack with minimum of 58 cells so the lowest voltage the pack will see is above 120 volts. I skirted around that by using my old transformer welder to drop the 240 volts to just under 70 volts and charged my pack by boosting back up. This way I was able utilize the higher amperage available from that 60amp circuit. I pulled a solid 45 charging amps into the pack.

PStechPaul

Discussion Starter · #29 · Feb 15, 2015 (Edited)

Using a transformer, the primary can be matched to the PFC-processed line voltage, 180 VDC for a 120 VAC line, and 360 VDC for a 240 VAC line. It can use 200V capacitors in parallel or series for optimum utilization. A three phase source does not really need PFC and the capacitors can be much smaller. Voltages as high as 600-800 VDC might be handled, but then the protective components (fuses, circuit breakers, and relays) become more critical and expensive, and other components such as MOSFETs and even resistors will become critical. And wire insulation and PCB trace spacing will be an issue.

The secondary can then be designed to produce close to the needed maximum battery voltage, and the wire size will be able to handle the maximum current according to the power specification. A buck switching regulator with a single winding on a choke, with a working voltage of 350 VDC, needs to be able to handle 34 amps for 12 kW, and if you want to charge a 180V pack at 12 kW you need 67 amps. So the single winding inductor design limits the charging power for lower voltage packs.

[edit] I tightened up the hysteresis and now this circuit runs at 20 kHz (rather than 10) and has better current regulation. It just involved changing R11 from 50k to 10k:

The maximum here is 486 watts output and 558 watts input for 87% efficiency, which could likely be improved greatly by some optimization.

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arber333

· #30 · Feb 15, 2015

I tried 3phase charging and i saw cca 780uF input caps are enough to smooth out the rail voltage. That being 580VDC still, and requires higher rating IGBTs and caps...

On the other hand, if we used three isolated 2kW modules with central control unit (or even 6) i could connect modules from L1 to N and L2 to N etc... that would be a star connection with 6kW output while having only 3x 230VAC for input! Much cheaper construction vs full 400VAC phase to phase! Now that would be possible since input would be isolated.
Before i could only connect 3 diodes in 3phase bridge and - directly to N. That caused for terrible N line heating, but i could charge with 230VAC up to 8kW if the EVSE was strong enough.
Now i use 3phase 580VDC input but can only charge up to 40A. That means 6kW max for 150V battery. Limits of single IGBT...

I believe separate module control is key. Make it resillient enough to drive 6x modules from 3 different phases and be able to disconnect (in aviation we say jettison) faulty modules and thus reducing output in case of trouble (or if there is no input present) on one line while still preserving function.
There is also possible to run all modules from same phase and would be easier to implement.
I am just saying dont forget unfortunate us that are limited to 16A phase input.

Good work!

A

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dcb

· #31 · Feb 15, 2015

I would say that three phase is not really the target market for a 1.5-2kw charger. The common denomiator for @home charging has to live with 100 amp service @ 240v. The next thing to look at is all the j1772 evse's out there, where the common denomiator is 240v and 30 amp as far as I know.

So probably 240v/30A or 7.2kw or 4 x 1.8kw will get you %98 market coverage, with "emergency" 120v capability.

3 phase is nice to have, but a little elitist at this point in time.

arber333

· #32 · Feb 15, 2015 (Edited)

dcb said:
3 phase is nice to have, but a little elitist at this point in time.

Well i didnt mean full 3 phase phase-phase 400VAC! That IS asking a lot from 2kW units. Specially isolation standards are tough there...
Rather i meant RST towards N connection. Each isolated 2kW unit could have R-N, S-N, or T-N connection that would be 230VAC. That way you could have threephase distributed load without having to worry about 600VDC isolation.
We have a lot of Mennekes 22kW EVSE here in EU while powerful single phase are rare and if i could use at least 10kW from 3phase i would travel without worry.

A

PStechPaul

Discussion Starter · #33 · Feb 15, 2015

I hadn't really thought much about three phase, but having multiple isolated modules is a good way to make that feasible. Three phase in the US is rare outside of industrial and large commercial distribution systems, and is usually 120/208 and 277/480. As long as there is a neutral, these can be accommodated, as well as the 220/380 in the EU and elsewhere. It might be possible to create a "phantom" neutral" and connect the charger modules in star much like a three phase motor has an internal winding connection that is nominally at zero voltage if the delta phases are balanced. However, some three phase delta sources have a ground connection on a phase (high leg delta) or on a center tap between two phases, and that would create problems. This can be dealt with by testing all three phases to earth ground and allowing power to be connected to the charger only if the voltages are appropriate.

arber333

· #34 · Feb 15, 2015

PStechPaul said:
I hadn't really thought much about three phase, but having multiple isolated modules is a good way to make that feasible. Three phase in the US is rare outside of industrial and large commercial distribution systems, and is usually 120/208 and 277/480. As long as there is a neutral, these can be accommodated, as well as the 220/380 in the EU and elsewhere. It might be possible to create a "phantom" neutral" and connect the charger modules in star much like a three phase motor has an internal winding connection that is nominally at zero voltage if the delta phases are balanced. However, some three phase delta sources have a ground connection on a phase (high leg delta) or on a center tap between two phases, and that would create problems. This can be dealt with by testing all three phases to earth ground and allowing power to be connected to the charger only if the voltages are appropriate.

Well here we normally use 3x 230VAC single phase L1,L2,L3 and are fused at 16A towards N. In our house i measured current and it is NOT distributed equally. One phase carries like 3/5 and other two each gives 1/5 of current. System doesnt care though... I loaded the poor phase with additional 1,5KW heater (cca 3,2kW phase sum) and lights didnt even go dim. I guess main safeties have some more margin still...

A friend connected 6x 1kW chargers the way i described earlier and he measured 2kW load on N line! His chargers take whole trunk though...
I guess the current from each leg subtracts from each other and in the end result is a single load. Strange...
We could use that truth though.

A

dcb

· #35 · Feb 15, 2015

arber333 said:
Well here we normally use 3x 230VAC single phase L1,L2,L3 and are fused at 16A towards N.

That adds up to 11kw, you will have a hard time getting 10k out of that without pfc per phase. but 6x 1.8kw pfc chargers would be a good fit if they can play nicely together.

PStechPaul

Discussion Starter · #36 · Feb 15, 2015 (Edited)

I had forgotten about the advantage of using three phase rectification to reduce the need for capacitance and PFC. So although three single phase charger power modules could be connected with inputs to each of three phases, they would need the usual capacitance and PFC. A three phase bridge rectifier will produce a DC voltage equal to the peak voltage of the L-L input, so a 3-phase 380 VAC source with 220V to neutral will produce about 536 VDC. This is dangerously close to the 600 VDC rating of common MOSFETs, IGBTs, and other components.

Here is a simulation, and it also shows what can happen when two capacitors of the same value but different parallel (leakage) resistances are used to obtain higher voltage rating. After one minute, the capacitor with 500k sees more than 80% of the full output voltage. In practice, the leakage may increase with voltage and thus balance the voltages, but it is much better to use balancing/bleeder resistors across each capacitor. This is IMHO another "potentially" serious problem with the new V14 EMW charger.

http://enginuitysystems.com/pix/3_phase_380_L-L_FWB.asc

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arber333

· #37 · Feb 15, 2015

You could wire 2kW units 2S3P so each would see only 1/3 of the 600VDC input. That and the absence of need of PF correction could lighten the price/weight. However those series would be more prone to failure, since if one module goes whole line colapses.

A

PStechPaul

Discussion Starter · #38 · Feb 16, 2015

There is no way to wire three phase units in series to lower the voltage. A three phase bridge requires all three phases to be connected. It would be possible to wire two single phase units in series on each phase, but you need to make sure the input voltages balance. I think it will require a different front-end design for 380 VAC or 480 VAC three phase. Then a transformer can be used to obtain isolation and conversion to a voltage suitable for the battery pack with a buck current regulator.

PStechPaul

Discussion Starter · #39 · Feb 21, 2015

This project is still in early planning stages, and in its simplest form would consist of a control unit with one or more user interfaces, and a power unit of about 2 kW that can accept 120/240 VAC (and perhaps other sources), and connect to a battery pack. Options are still open for details such as the processor. I am most familiar with earlier Microchip devices such as the PIC18F series and more recently the PIC12F18xx and PIC16F18xx which have some advanced features, are inexpensive, and available in DIY-friendly low pin count DIP and SOIC packages and choice of 3.5V L versions and 5.5V.

The PIC24 series has 20 pin and 28 pin versions and cost only a little more than the PIC16, so I may consider them for a final version. But for now I'd like to stick with what I'm familiar with, and I have some tested and debugged code that may or may not be compatible with other devices. I have discovered that there is a lot of inconsistency in names for SFRs and their bit definitions, and the Microchip libraries often fail to resolve these. It may be that the development team is concentrating effort on the rapidly expanding new families and cutting back on support for "legacy" devices.

If you have the skills to design parts of the hardware and software for this project, I would welcome your participation. I intend for this to be a collaborative open source effort to develop a relatively simple, robust, reliable, and inexpensive product.

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dcb

· #40 · Feb 21, 2015

PStechPaul said:
I intend for this to be a collaborative open source effort to develop a relatively simple, robust, reliable, and inexpensive product.

Check out kicad, open source schematic/pcb tool (good for collaborating), runs on linux (open source operating system) and windows/crapple. Makes baby food files directly (gerber).

http://www.kicad-pcb.org/display/KICAD/KiCad+EDA+Software+Suite

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