1.25kW (84V, 15A) Li-ion charger - Page 1
Forum section: electronics / projects

author: jonathan_nadar
author_rank: Contributor
timestamp: January 15, 2021, 01:10:58 pm
content: I am designing an off-line battery charger for a 60Ah Li-ion battery pack. I have used SG3525 as the PWM controller on the primary side and an ST micro-controller on the secondary side for sensing and feedback via an opto. It uses a half bridge configuration. The switching transformer was designed using excellentIT.I have experience designing and testing DC-DC converters and motor controllers, however, they were all under 250W. I have designed the power stage after referring a few power supply schematics available online. I have not added a PFC circuit to keep it simple as this is my first off-line converter design. I have attached the schematic and the PCB layout, kindly suggest me if there are any mistakes and improvements that can be done to the schematic and the PCB layout as well.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 15, 2021, 08:15:40 pm
content: You have no primary overcurrent protection, you wouldn't believe how easily your expensive mosfets blow!Had you considered current mode control in any case and perhaps a full bridge, seems your really stressing things there.I guess you have a source of Litz for 200Khz operation ?

author: jonathan_nadar
author_rank: Contributor
timestamp: January 16, 2021, 05:47:13 am
content: I have used a fuse for over-current protection on the primary side. SG3525 has a single error amplifier, so couldn't implement a current mode control. I stuck to the half-bridge topology as the worst-case operating voltage and current fall well inside the SOA. I have also added a snubber network and an antiparallel diode across the transistor. I am not using a litz wire, but I have considered the skin effect and selected the enamel coated wire accordingly.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2021, 10:21:08 am
content: Fuse does not protect from over-current.It may be able to prevent a fire, though, when the MOSFETs fail short, so keep it there. But add proper active current limitation.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 16, 2021, 10:39:59 am
content: Quote from: jonathan_nadar on January 16, 2021, 05:47:13 amA simple calculation for you, skin depth at 200Khz = 0.15mm, a 0.3mm conductor has an area of 0.07mm2, at the normal transformer current density of 5A/mm2 that conductor can carry 0.35A.So your design is not possible without either or both reducing the frequency or using Litz wire.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 16, 2021, 12:51:47 pm
content: Got it, I'll do the necessary modifications.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 16, 2021, 12:56:28 pm
content: I am going to bundle up individual strands of this enamel coated wire(element14 part no:1230977). I have used a current density of 7A/mm2 as there'll be a fan to cool it. As per ExcellentIT, 36 strands on the secondary and 19 on the primary. It would be a pita to handle so many wires, so I am getting it wound from a professional winder.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 16, 2021, 03:30:33 pm
content: So you suddenly changed your mind on Litz then!Maybe you should start with something smaller, the schematic looks to me like a PC psu clone and its not really suitable for that power level.

author: nctnico
author_rank: Super Contributor
timestamp: January 16, 2021, 05:58:00 pm
content: Quote from: jonathan_nadar on January 16, 2021, 12:56:28 pmYou will be better of buying litze wire. Do not try to wind several parallel windings using seperate wire. Even if you are 1 turn off you'll have a shorted winding. An alternative is to make the litze wire from several seperate wires but this takes knowing the length of your winding first.

author: poorchava
author_rank: Super Contributor
timestamp: January 16, 2021, 08:02:17 pm
content: 200kHz is really steep for off line switcher. I also think that not going with PFC is really gonna make things harder. You'll need massive input capacitors to have semi stable input voltage. And unless your feedback is done very well, you'll have considerable 50Hz ripple on the secondary side.PFC is actually quite easy to do. For under 1.5kW you can finds tons of tested reference designs from major players. From my experience those designs mostly work well (unless you screw up the layout or something) at least to the point of not blowing up right away when powered on.+1 on primary side current limit. You can get it done with an off the shelf current transformer and a few passives for like 1.5€ and it will save your mosfets countless times.As for frequency, i didn't look up the specs on those fets, but just from the power ratings and such i can see probably like 20W of switching losses per fet unless your primary side is ZVS/ZVS/resonant/etc.The choice of fet is not that good. Since you're hard switching, you want something that has fast reverse recovery and low reverse charge. In comparison to your fets Infineon's Coolmos CFD7 at similar rdson has about 7x less are and 3x less trr.I don't really understand the SG pwm controller. Its archaic, same as TL494, and you already have a uC in your design. I'd go for something that has HRPWM and ditch the 30y.o. chips. At 200kHz cycle by cycle control most likely isn't gonna happen, but you can do it every 2 or 3 cycles.Litz wire is a must at those power levels. Even at 50kHz and in topologies that don't have the input voltage/double current.Honestly, unless you have a stringent space requirements of like a good challenge, I'd go with easy lower frequency. Maybe around 40kHz maybe. I have never seen a successful kW range hard switched ac/dc anywhere over 200kHz. In resonant mode this may be doable.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 17, 2021, 03:40:05 am
content: Alternatively, if isolation is not needed (84V is well above the "safe to touch" threshold), you can do a buck PFC direct to the battery.

author: wraper
author_rank: Unknown
timestamp: January 17, 2021, 03:54:28 am
content: You screwed up creepage distance in transformer. You have 2 unused pins on the secondary side yet still placed V_sec2 winding on primary side close to other things. If you have no other choice, you could at least place only one pin on this side, with transformer pads removed around it.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 17, 2021, 07:40:53 am
content: Nice topic, can you share your Altium and excellentIT files? for excellentIT please take screen captures of the values that you have entered in the program.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 17, 2021, 10:47:19 am
content: Quote from: fourtytwo42 on January 16, 2021, 03:30:33 pmNo, I had considered using multiple wires in parallel in the design, that's the reason an ETD39 core. However, I didn't want to call it litz as it's an entirely different component altogether.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 17, 2021, 11:09:46 am
content: I'll learn more about PFC and implement. It looked very daunting to me but will give it a try. I'll implement primary side over-current protection as well. It looks relatively easy.Quote from: poorchava on January 16, 2021, 08:02:17 pmI have got both volume and weight constraints and need to deal with vibrations as well, so that's the reason I am using a high frequency. I have done some calculations and found out that the switching loss is about 8W and conduction losses at 3W at the worst case. I asked my friend to carry out a thermal simulation on ansys with a heatsink and some forced ventilation, and they were running cool.Quote from: poorchava on January 16, 2021, 08:02:17 pmThese work well, and numerous designs were available, so went with it.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 17, 2021, 11:10:45 am
content: Isolation is necessary, so had to go with a transformer-based topology.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 17, 2021, 11:14:03 am
content: Well right, I missed it. There's some board space available, I'll reroute it. I initially had something there which was removed from the design later, so had to bend the track.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 17, 2021, 11:26:42 am
content: Quote from: ali_asadzadeh on January 17, 2021, 07:40:53 amHere they are.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 17, 2021, 01:42:51 pm
content: Thanks

author: RoV
author_rank: Regular Contributor
timestamp: January 17, 2021, 05:33:41 pm
content: Hi,apart from the doubts already expressed on the controller IC, I would use a stronger driver, because the one used can only give + 0.29 A/ - 0.7 A and is also a bit slow for 200 kHz hard switching (200 ns ton/toff). I would avoid the 1N4148s on gates and put a lower valued resistor, like the 4.7 ohm suggested on the MOS datasheet.I am wondering if C4 is really necessary, since you are using a series of DC-link capacitors. It seems a weak element, I suppose a 1u 63 V part will get very hot, besides it will risk overvoltage even on the weakest unbalance.

author: poorchava
author_rank: Super Contributor
timestamp: January 17, 2021, 07:12:18 pm
content: Another matter: 15A is definitely in synchronous rectification territory. At 15A your schottky a have like 0.7V of drop, so about 10W in those 2 diodes together. If you drive SR mosfets correctly they are zero switching so no losses. You can reverse the output rectifier polarity so that the SR fets are ground referenced and don't need floating driver.Btw, as for trafo crrepage, you can safely specify removing certain pins from the bobbin. I have worked with numerous magnetics companies and none of them have seen it as anything out of ordinary (i.e. no additional costs).Litz wire is also standard. Winding with several strands of magnet wire also works in some situations. For example toroidal inductors, where you need something like 5...6 mm.sq. of cross section. Often using 3 separate thinner wires makes it cheaper to manufacture. This does not work for HF transformers though. For a trafo you want strands with diameter twice the skin depth. At 200kHz this may not be doable since this would mean <100um wires. Good call on ETD core though. Standard E cores with rectangular column are a b***** when it comes to winding litz into them.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 18, 2021, 10:48:18 am
content: The mosfet gate capacitance was quite low, that's the reason I didn't go for the powerful one's. I had considered IRS2110. but I felt it was an overkill. The 200ns number is the propagation delay and as per my knowledge, that just adds a small lag to the control loop.Quote from: RoV on January 17, 2021, 05:33:41 pmI have selected its value as 2.2uF (modified it after referring to switching power supply by Abraham pressman) and it's necessary to prevent flux walking. The voltage rating doesn't matter much as the voltage across it won't be high.

author: jonathan_nadar
author_rank: Contributor
timestamp: January 18, 2021, 11:13:18 am
content: Adding an SR started to increase the complexity as I had to match the timing with the primary side. Right now efficiency is not up on the list, so used the diode with a heatsink instead. The junction was running at about 80 degrees C as per the simulation. I'll upgrade it though on the next version.Quote from: poorchava on January 17, 2021, 07:12:18 pmThe charger would be facing a lot of vibrations and the transformer is the heaviest component on the board, so wanted to use the remaining pins to add some rigidity.

author: poorchava
author_rank: Super Contributor
timestamp: January 18, 2021, 05:01:35 pm
content: If you have vibrations to the extent where they are a serious problem, then 2 pins won't make any difference.You can shit some silicon or other similar adhesive underneath and around the trafo. You can also look for bobbins that have holes for screws. At least for ETD59 i know they exist, not sure about ETD59. You can also add some plastic fastenings (like zip ties, but wider). You can also add a "washer" made of PCB laminate or something, that would be screwed to the main PCB using threaded rods or metal standoffs.And SR is another reason why secondary side control is better. You can just drive the primary side fets via GDTs, which is