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  #1  
Old 08-14-2015, 03:58 AM
Tony Bogs Tony Bogs is offline
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Lightbulb SiC LLC modular charger design

Inspired by PStechPaul's idea of a modular charger and the availability of SiC and LLC devices,
I've decided to go ahead with a SiC / LLC based modular charger design / DIY homebrew build.

Design choices:
  • A SiC PFC CCM boost input stage, 390V DC out.
  • 3350 W out, (230V, 16A AC in) per module.
  • LLC transformer topology for isolation and additional boost up to 580V (for HV battery packs). FIXED FREQUENCY. Boost: by transformer turns ratio only.
  • CC mode only, buck hysteresis current control
The measurement of the LLC inductance (36.9uH) is an important step in the design proces.
Attached Images
File Type: jpg measure_LLC_inductance.jpg (184.2 KB, 90 views)
File Type: png SiC_LLC_CHARGER_PWR.png (90.9 KB, 167 views)

Last edited by Tony Bogs; 08-15-2015 at 11:18 AM. Reason: Added picture of LLC inductance measurement setup and schematic mod
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  #2  
Old 08-16-2015, 12:57 PM
Tony Bogs Tony Bogs is offline
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Default Re: SiC LLC modular charger design

The LLC CONVERTER PWR PCB layout for the double insulated charger.
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File Type: png LLC_CONVERTER_PWR.png (43.3 KB, 54 views)
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  #3  
Old 08-16-2015, 01:10 PM
bigmotherwhale bigmotherwhale is offline
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Default Re: SiC LLC modular charger design

very nice layout
have you ever tried reflex charging?
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  #4  
Old 08-16-2015, 01:58 PM
Tony Bogs Tony Bogs is offline
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Default Re: SiC LLC modular charger design

Thanks and no.
First layout corrections: low loss caps and gate zeners. First test probably end of this week.
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File Type: png LLC_CONVERTER_PWR.png (45.1 KB, 36 views)

Last edited by Tony Bogs; 08-16-2015 at 04:10 PM. Reason: PCB layout corrections
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  #5  
Old 08-18-2015, 05:09 AM
Tony Bogs Tony Bogs is offline
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Default Re: SiC LLC modular charger design

I've had a quick look at reflex charging.
First impresssion: great for NiMH (memory effect) and lead-acid (loosen phosphate deposits).
I'm designing a charger for LFP batteries (obviously the best DIY choice ).

The high LLC resonance (transformer) leakage inductance
(cause: large distance between primary and secondary for isolation)
means that the Q factor will be extremely high
when a battery is directly connected to the LLC output.
This presents a major challenge for the frequency setting. Not practical.
Solution: a hysteresis controlled SiC buck stage between output and LLC.
Results in a high Rac load for the LLC. (Q=SQRT(L/C)/Rac)

The Q factor still remains high for LLC (~7 at max load).
Allowing ZCS doubles the bandwidth for frequency setting (gain>0.95).
To prevent high losses, it's going to be an all SiC design.

LLC frequency: 135 kHz

Last edited by Tony Bogs; 08-18-2015 at 05:11 AM. Reason: typos
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Old 08-18-2015, 06:12 AM
bigmotherwhale bigmotherwhale is offline
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Default Re: SiC LLC modular charger design

Yeh i have seen and used reflex charging for lead a NiCad, but i have only seen a hand full of articles on using it on Li chemistry, one used a ZVS inverter and the reports were quite positive so i think its something that needs more research into, someone should make a small one for testing 18650s and compare it to cccv over a few hundred cycles to see what happens.
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Old 08-20-2015, 01:57 AM
Tony Bogs Tony Bogs is offline
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Default Re: SiC LLC modular charger design

And at least a few hundred batteries for a statistically significant result.
The papers I've read on LFP charging indicate that CV charging is not a good idea, especially at high voltages.
The charging strategy I am going to use is almost reflex, CC mode. With one big difference: no discharge cycle (can't see any positive effects for LFP).
The rest cycle is very short, only there for measuring the float voltage to determine the end of charging (eleminating internal resistance related errors).
At what voltage? Again, statistics. To determine the voltage at which a high SoC can be reached with only a very little chance (the statistics part) of overcharging a cell.
Then, for that tiny chance of a failing cell, the xS imbalance detection method can be determined, weighing cost, ease of use, mass etc.

SiC in ZCS mode. I've done the math.
Based on 2.5A pk at turnon, ~10% of the maximum tank current , since the ZCS mode stays close to resonance where the tank is mainly resistive.
Cree SiC C2M0080120D mosfet: approximately 10W switching loss per mosfet, IXYS Si IXFH44N50 more than 40W.
Both at 135kHz with SiC Cree diode C3D20060D antiparallel.
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  #8  
Old 08-21-2015, 03:15 PM
Tony Bogs Tony Bogs is offline
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Default Re: SiC LLC modular charger design

And a layout for some corrective measures to keep V/I in phase. Assembly starts tomorrow.
Attached Images
File Type: png PFC_PWR_SW_PCB.png (53.0 KB, 35 views)
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  #9  
Old 08-22-2015, 06:18 AM
Tony Bogs Tony Bogs is offline
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Default Re: SiC LLC modular charger design

I've found the name for the charging method I'm going to use. It's: step current.

This paper describes the method and the underlying runtime model:

Min Chen and G.A. Rincon-Mora, "Accurate electrical battery model
capable of predicting runtime and I-V performance", in IEEE
Transactions on Energy Conversion, vol. 21, no. 2, pp. 504-511, 2006

users.ece.gatech.edu/rincon-mora/publicat/jrnls/tec05_batt_mdl.pdf

First time I saw "step current" was in this paper (Danish Technical Uni):

http://orbit.dtu.dk/files/9852018/Paper_EV_charge.pdf

Figure 3 on page 3 shows the pulse step charge / discharge curves.
The increase in internal charge resistance is very clear @ 85%.
That's about where the charging should stop.
The exact point for a multicell pack depends on LFP performance statistics (distribution).
Haven't found a reliable source for it yet.

Last edited by Tony Bogs; 08-22-2015 at 06:21 AM. Reason: Page number3 was missing
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  #10  
Old 08-22-2015, 09:41 AM
Tomdb Tomdb is offline
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Default Re: SiC LLC modular charger design

The last layout you posted, is this just a buck stage for regulation?

Love to see some update schematics of your complete test setup, or has that no changed?
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