1.25kW Charger: Snubber resistance overheating - Page 1
Forum section: electronics / projects

author: jonathan_nadar
author_rank: Contributor
timestamp: October 26, 2021, 10:26:13 am
content: I have been working on a battery charger, link to a previous post of the same herehttps://www.eevblog.com/forum/projects/1-25kw-(84v-15a)-li-ion-charger/msg3415722/#msg3415722.Design specifications: Half-bridge isolated converter, Peak power 1.25kW, Continuous power 1kW, Switching frequency 200kHz, primary-side PWM control using sg3525, secondary-side feedback using microcontrollerThe output voltage regulation works well and currently, I am testing it on light loads, about 100W. I have added the voltage waveforms across the output rectifier with and without snubber. The snubber circuit improves the waveforms, however, the resistors are extremely hot after a minute of operation. I have selected the snubber values after simulating various combinations on LTspice with as many parasitics I could think of and the 560, 1nF worked the best.Please let me know what could be the reason behind this and how I can work around it.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 26, 2021, 11:49:27 am
content: At this switching frequency the impedance of the 1n cap is about 500 ohms! A current of Vopeak/Xc will flow via the 1n when the diode is reverse biased leading to a peak power disspation of several Watts in 560R!seehttps://www.daycounter.com/Calculators/Snubbers/Snubber-Design-Calculator.phtmlandhttps://incompliancemag.com/article/snubbers-to-kill-parasitic-resonances/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2021, 12:26:35 pm
content: Huh, surprised I missed the first thread.L1 is way, way too small, and only rated 50V. Expect to need much more in general for the line filter.The waveform indicates saturation due to asymmetrical pulses. I don't know why. Perhaps there's too much ripple into the SG3525.The ringing is due to leakage inductance, and can be estimated for certain transformer winding geometries. The geometries that aren't so easy to calculate, are also so much worse that they're not useful here, so this is convenient.First priority is interleaving the secondary half(ves), then interleaving the primary with that. With an output impedance around 5 ohms, the low impedance of layered foil windings will likely be beneficial. You might consider a 16:12 ratio with a transmission line transformer type design, which would need, let's see, a stack of foilsCan also dump the aux winding, move it to the TNY perhaps. This might require a custom wind (though I guess it is anyway, so, eh?), you'll not usually see significant isolation between secondaries of an off-the-shelf inverter transformer. Or could use two smaller ones, primaries in parallel, nothing wrong with that. I mean, the aux winding is kinda neither here nor there, it's rough on C18 but mainly only on startup?WHOA, IC2 is...Wait, you've got iso GNDs right beside each other schematically. But they're pinned correctly.Oh, IC1 is the same way, but at least the iso barrier is drawn there. Sheesh.Might also help to label 12 and 3.3 by side. Could rename 325VDC_GND to LINK_N say, then 12VDC to LINK12, and 84VDC_GND to just regular GND (or 0 or whatever).The verbose names are hard to remember:Fan_speedB+_senseCharger_+_senseCharge_relay_onTemperatureFixed_contactor_open84VDC_Charge_onDetachable_contactor_openSC_bank_charge_onAnd it's not nice ping-ponging between sheets trying to follow them, the MCU could easily fit on the SG3525 sheet along with the opto (which doesn't connect to ANYTHING on sheet 2!), and more connections can be wired vs. net named.And, why are there two buffers after the secondary? Oh, there's three pin numbers there, that must be a diode, D1? Ohh, it's the dual diode, yep. Two missing lines makes so much difference.Anyway, before I get too drawn into schematic layout and style, the stinker here is the lack of current-mode control. This was mentioned in the earlier thread, but I will be much more firm about it.Simple to add: use the SG3525 error amp to set PWM based on feedback current, sensed with an isolated (Hall effect) sensor on L2. At this current level, HE is perfectly acceptable; a 15 or 20A range device will do, I suppose.The SG3525 is then a transconductance amplifier, taking in a reference voltage (from the MCU, I assume that's a DAC output) and putting out a current. The MCU then runs a PID loop to maintain output voltage.The inner loop is designed so that, under absolutely any condition -- startup, short circuit, open, whatever -- the output current is limited, and safe with respect to inverter capacity.This avoids so many stupid nuisance failures of the switches, and makes limiting output current almost trivial (fully trivial, if the current limit needn't be variable). Current mode control is an absolute priority.Can even go to the length of adding a fault latch and desat protection (disables output if a transistor shows significant voltage drop while conducting), though I wouldn't find that a priority at this power level. I do recommend it for industrial scale (3kW+) supplies. So, it's close, could certainly give it a try, but it's also not too terribly annoying to fix, if/when it blows up.You do need a better analog isolator. IC1 will not do. Its CTR and offset vary wildly with temperature and age (worse than +/-50%). A more likely option here is a PWMDAC, using PWM straight from the MCU, a digital isolator, and an RC (or better) filter. A 5V-compatible isolator can run directly fromVref, no worries.Do check your compensation network. C33 is completely out of place. Did you mean to nameCC_CV_emitterasINV_in? As is, I think C35-R35 dead-ends. Who knows, I can't follow net labels everywhere, but I don't see another search result at least. Also as is, the opto is probably a constant current, but it will have significant AC impedance, affecting compensation itself. Probably better to use it as a reference (toNONINV_in?). And also as shown, gain is very high (assuming the above interpretation is the correct intent), then R35 into R47 is massive gain, exacerbating the offset and gain errors of IC1. Or if it was intended to be sensing output voltage, then R35 should be in series with C35 to get good output regulation; but anyway, that seems to be the MCU's responsibility, so it shouldn't have capacitors at all (or large ones, at least), and a software PID loop or whatever should be serving that purpose instead.Mind that, if you do change to PWMDAC, the PWM filter will likely be the dominant pole in the system, much slower than the modulator itself (i.e., 3525 input setpoint to L2 output current). For a charger, this is irrelevant, regulation only needs to be met at quite low frequencies (fractional Hz?), and anything higher is gravy. If you intend to also use this as a general purpose supply, you may find it attractive to add an SPI or S-D DAC instead. Or isolate the gate signals, and move the 3525 to the secondary side.Or drive it entirely by the MCU itself (full digital), but I certainly wouldn't recommend that as a starting project! At least, not without more protective logic (fast watchdog timer and shoot-thru fault, optional desat fault).Tim

author: sandalcandal
author_rank: Unknown
timestamp: October 26, 2021, 01:15:41 pm
content: Quote from: jonathan_nadar on October 26, 2021, 10:26:13 amLTspice can show you power dissipation values too, does the circuit not match what you were expecting there? Is the voltage across the snubber resistor the same as simulations?Designing and simulating a snubber requires knowing parasitic values of the circuit and components, can you share those numbers?1nF is a bit high from gut feeling. Normally the snubber cap value will be pretty close to the parasitic capacitance of the diodes.

author: jonpaul
author_rank: Super Contributor
timestamp: October 26, 2021, 03:52:01 pm
content: Bonjour:The snubber and overall design is impossible to comment on without a complete schematic and transformer winding sheet, core, gap, etc.The ringing is due to energy stored in the transformer leakage inductance per cycle. = 1/2 L Ilkg (exp 2)Normally snubbers are placed at the transformer primary.Excess snubber power is evidence of poor design.it is possible to reduce the power by recovering the energy.Bon chanceJon

author: jonathan_nadar
author_rank: Contributor
timestamp: October 27, 2021, 04:54:27 am
content: The peak power is several watts, however, the average power dissipated by the resistor is about 750mW.

author: jonpaul
author_rank: Super Contributor
timestamp: October 27, 2021, 05:50:02 am
content: Hello: 750 mw is fine in a 1kW supply.Even at 100 W load iti si less than 1%. The heating of the resistor is only related to RMS power.Jon

author: jonathan_nadar
author_rank: Contributor
timestamp: October 27, 2021, 05:55:41 am
content: Quote from: T3sl4co1l on October 26, 2021, 12:26:35 pmI think I used the schematic symbol of the particular inductor and forgot to remove the comment. The CM choke with the element14 PN-1864500 is used.Quote from: T3sl4co1l on October 26, 2021, 12:26:35 pmRegarding this, I am not fully convinced with my method of providing feedback from the microcontroller. I have connected the CC_CV_emitter and INV_in nets on the PCB using a 0-ohm jumper as I messed up the nets during design. Currently, I am using the shutdown pin of sg3525 to provide feedback via the opto as feedback to the inverting input pin didn't work properly.I am using an AC-DC converter board (E14 PN: 3584011) instead of the TNY circuit as I couldn't wind the transformer properly and the IC goes into shutdown as soon as the switching starts.I am planning to implement current mode control in the next version. My colleague has designed and tested a PFC based on UCC28019 and would be including it in the next version which would take care of overcurrent as well.Regarding the transformer winding, I used enamel coated wires as they were easily available (during Jan when the pandemic was on)and the transformer winder was ready to wind it as well.Once the current charger works well, I am planning to work on a PSFB topology using LTC3722 as it solves a majority of control problems and implement a better feedback mechanism with limited involvement of the microcontroller.I'll definitely use your suggestions regarding schematic layout in my future designs.

author: jonathan_nadar
author_rank: Contributor
timestamp: October 27, 2021, 06:07:02 am
content: I have used 50nH stray inductance(1nH per mm rule) and 1nF parallel stray capacitance. I reduced the capacitance and increased resistance while testing and the waveforms got worse.

author: jonathan_nadar
author_rank: Contributor
timestamp: October 27, 2021, 06:10:09 am
content: Here's the transformer winding details.

author: sandalcandal
author_rank: Unknown
timestamp: October 27, 2021, 10:47:43 am
content: Quote from: jonathan_nadar on October 27, 2021, 04:54:27 am750mW is far too low for a snubber in a 1kW system. I assume this is the simulation value and not the actual measured power dissipation causing you overheating because that's too low to cause overheating. I would then deduce you have not used realistic values in your simulation.Quote from: jonathan_nadar on October 27, 2021, 06:07:02 am50nH is far too low. 1nH per mm is for a bond wire in open air, not for turns on a core. If you want to get an idea of leakage inductance of a transformer, a better estimate is 5% of the winding inductance (with other windings open circuit).From the transformer stats you posted, you have 698uH primary inductance and a 16:10.5 winding ratio so secondary inductance will be \$L_s=L_p \left(\frac{N_s}{N_p}\right)^2=698*\frac{10.5^2}{16^2}\approx 300 \mu H\$ The leakage will then be something like 5%, so around 15uH. Though the leakage could be between 10% to 2% depending on the winding strategy and quality of winding.Edit: Corrected inductance calculation.

author: Circlotron
author_rank: Super Contributor
timestamp: October 27, 2021, 12:20:47 pm
content: Quote from: T3sl4co1l on October 26, 2021, 12:26:35 pmI had trouble with an SG3525 giving asymmetrical pulses a while back. Turned out the oscillator cap discharge resistor was zero ohms, which the thing didn't like despite the datasheet saying zero ohms was okay. I see yours is a fairly low 5.6 ohms.->https://www.eevblog.com/forum/projects/sg3525-oscillator-asymmetry-when-error-amp-is-maxed-out/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2021, 02:05:26 pm
content: Quote from: jonathan_nadar on October 27, 2021, 05:55:41 amHmm, that doesn't exist? But OK, parts may differ.But then, what parts should I assume, or not?QuoteYou show the transformer calculation, but how was it actually wound? Is it literally just, 16 turns of the bundle indicated, then 11 and 11 turns of the other bundle, etc.? Yeah, that's going to have terrifically high leakage.As I was... GOING to say, and forgot to return to that thought-- foil windings might be good. Though with the turns fairly high and the ratio being so oddball (11 and 16 are relatively prime) it's probably fine to stick with strands.Make pairs of the strands, primary and secondary! You'll need TIW to do this as such, or you can wind whole layers, in, however many strands fit on a layer (minus some space at the ends for creepage distance) let's say, and wind as many layers needed to reach the total strand count. Alternate layers, so the leakage inductances per layer act in parallel, reducing about in proportion to the number of layers. Also, this can be easier (or more compact) to wind, using multifilar (strands laying flat, in sequence, rather than twisted up).Tim

author: jonpaul
author_rank: Super Contributor
timestamp: October 28, 2021, 05:30:34 am
content: Rebonjour, a few points please:Our winding sheet is much more detailed and includes ever detail needed to wind the part.Wire guages, insulation type, interlayer insulation,Turns per layer, type of wind (eg random, perfect lay, universal)Termination of each windingInterconnection of wdgShields eg Cu foilAt 200 kHz the skin and prox effect may require litz or bunched wire.We would interleave P and S to reduce Llkg.We never did these theoretical guestimate spice calc, and used Magnetics Designer SW.Hope this is of use,Kind Regards,JonPS: We have designed and manufactured transformers and magnetics since 1977. We have several million parts in product and in use.