1kW 200/400V dc/dc converter design tip - Page 1
Forum section: electronics / projects

author: webgiorgio
author_rank: Regular Contributor
timestamp: January 25, 2019, 12:44:49 pm
content: Hello,I would like to make a DC/DC converter to use it in my Photovoltaic plant, where I need to boost 200 Vdc 5A to 400 Vdc 2.5A, so that I can parallel a string of 6 PV panels with other strings of 12 panels. 1200W.Vmpp for one panel is 33.3V (200V/6).Is a simple boost converter with 50% fixed duty cycle enough? (inductor, N-mos to ground, diode)Can you recommend any more energy efficient topology for this voltage and power level?Shall I implement a check of Vi and Vo? (actually, Vo is imposed by the strings with 12 panels and the inverter).I only need to make one, so I am not too concerned about the cost of it, it would be nice to make it efficient.

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 25, 2019, 12:52:04 pm
content: It is not a big deal, as you have low currentYou can use SiC Diode and transistor if you want high efficiency / high frequencyNot a big dealJust simple boost converter

author: Benta
author_rank: Super Contributor
timestamp: January 25, 2019, 04:37:05 pm
content: At 1 kW I'd choose a full-bridge converter with transformer coupling. The inductors in boost (this includes flyback) converters of that size tend to be unmanageable large, as they have to store the full energy.A forward converter could also be an option.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: January 25, 2019, 04:54:53 pm
content: Quote from: Benta on January 25, 2019, 04:37:05 pmYup, I like that thought also, you could do it also center-taped style, push-pull. Which is pretty much a variation of the forward converter.

author: coppercone2
author_rank: Super Contributor
timestamp: January 25, 2019, 05:06:28 pm
content: Someone should write a treatise on their experiance with magnetic elements in such smpsu of their own design. Its always been the x factor that makes me not want to venture into this area. I heard bad things about big magnetics even from seasoned university professors.

author: Benta
author_rank: Super Contributor
timestamp: January 25, 2019, 05:07:44 pm
content: Quote from: TrickyNekro on January 25, 2019, 04:54:53 pmAt 1 kW, it's not a good option. The problem is, only one half of the primary is active at a time, meaning increased copper loss. Also, the transistors need to be rated at 2 x VINplus leakage inductance, meaning at least 800 V types.

author: Benta
author_rank: Super Contributor
timestamp: January 25, 2019, 05:16:56 pm
content: Quote from: coppercone2 on January 25, 2019, 05:06:28 pmYou're absolutely right, I've done my own magnetics and it's a steep learning curve. The issue is, that there is no "closed loop" design process, it's purely iterative. Good data books, like the old Siemens "Soft Ferrites", are big help, but not easy to source these days.

author: coppercone2
author_rank: Super Contributor
timestamp: January 25, 2019, 05:31:20 pm
content: Yes its incredibly inconveniant to get into that field.

author: Mazo
author_rank: Regular Contributor
timestamp: January 25, 2019, 08:50:24 pm
content: Standard boost topology but spread out to 2,3,4 or more phases looks like a compromise between custom magnetics and unmanageable inductor size?

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 25, 2019, 09:04:03 pm
content: Of the shelf inductor 1000uH; 8A; 155mΩ (0.3A ripple)Simple 100kHz boostOutput capacitor rms current 2.5ATransistor rms current 3.6A peak 5.5AIt is not a big deal for parts, no need for something more complicated in no need for isolationEven can lower frequency as ripple is lowJust take care of board layout

author: jbb
author_rank: Super Contributor
timestamp: January 25, 2019, 09:27:51 pm
content: I'd like to muddy the watersAs you're doing this for a PV installation, a fixed gain of 2 is probably sufficient. After all, the PV inverter will have Maximum Power Point Tracking (MPPT), which could interact with the controller for a boost converter in strange ways.So how about a switched capacitor voltage doubler? That offers a fixed voltage ratio and doesn't need fancy control or large inductors. (Texas Instrumentshas an introductory post...) Switched capacitors are quite efficient when you want an exact voltage ratio and can be made in kW sizes.Note: some small EMI filter inductors are likely required on the input and output to prevent stray radio interference.In terms of magnetic converters:Quote from: Benta on January 25, 2019, 04:37:05 pmI like isolation as much as the next person, but wouldn't designing the transformer (+ filter inductor for forward converter) be more difficult than designing an (admittedly large) boost inductor?Here's what I would consider for a boost converter:Tight PCB layout is critical.As Mazo suggested, interleaving is helpful. Currents aren't that high, and duty cycle will be 50% (ish!), so I'm thinking 2 phases. This will help a lot with capacitor selection.Using a SiC MOSFET (or Gallium Nitride, GaN) + SiC Schottky diode to get high frequency; frequency around 100 kHzIf efficiency is critical, replace SiC Schottky with another SiC MOSFET for improved efficiency (extra for experts)This yields inductor values in the order of 500 - 800 uH, which you might be able to hit with something from the WE-HCF range (Wuerth Electronics)

author: Benta
author_rank: Super Contributor
timestamp: January 25, 2019, 10:13:38 pm
content: Quote from: jbb on January 25, 2019, 09:27:51 pmIn my opinion: no.The transformer design for full (and half) bridge circuits is straightforward. The same goes for forward converters. The filter inductor can be bought off-shelf.

author: webgiorgio
author_rank: Regular Contributor
timestamp: January 25, 2019, 11:44:07 pm
content: I looked up the voltage doubler, the idea was good at first sight, but then I saw it takes too many switches.To my little knowledge (I am electrical engineer, not electronic) a boost converter is totally fine with a voltage ratio equal 2, which can be done without use of transformers. For this voltage ratio, any topology with a transformer will be less efficient than a single inductor because there is twice the copper and not all the flux is concatenating with the secondary winding.About the inductor for a boost converter, I looked on Mouser and it is possible to buy for about 10-15 €. Ok.Out of curiosity I measured one from a computer power supply. About 500 uH. Wire is 1.32 mm2. So, I would make the prototype with two of those inductors in series. The core is green, with blue side. I am not sure how much flux can take before saturation, but if the wire is fine with the current, I guess that the core is fine with the flux.For higher efficiency, the use of a transistor instead of a mosfet came like a surprise to me. Why is it more efficient? Converters use mosfets most of the time...

author: Amper
author_rank: Frequent Contributor
timestamp: January 26, 2019, 03:22:07 pm
content: Just for comparison so you see that the problem ist not very difficult here is a picture of a 1975ish laser power supply. Its a Buck converter using only bipolar transistors, two or three OPs and one NE555 for generation of the switching signal. It will generate 200V 30A from 400V three phase. Frequency was around 30khz through an iron core inductor.

author: coppercone2
author_rank: Super Contributor
timestamp: January 26, 2019, 04:04:20 pm
content: what efficiency does it have?The zip-tie density is interesting. I thought that would be too much for terrestrial use. I woulda thought 1/4 of that would be good and 1/3 more then enough.

author: Amper
author_rank: Frequent Contributor
timestamp: January 26, 2019, 04:16:59 pm
content: The ziptie use is a trademark of equipment from this period, archeologists of the future will use this fact to date back machinery they found in the ruins of the old world.I dont know the exact efficiency but the black heat sink you can see is just a bent piece of 4mm sheet metal, no fins attached anywhere so the loss cant be to great or it would overheat.

author: coppercone2
author_rank: Super Contributor
timestamp: January 26, 2019, 07:01:36 pm
content: i wonder why though?I HATE repairing stuff like that. The only thing worse is wire nylon lashing with wire that is soldered to the PCB. IMO its like avionics grade.https://en.wikipedia.org/wiki/Cable_lacingI don't even know why. I saw a HP supply roll down a hill on a steep road bouncing up and down (extreme abuse due to flying off a car roof). The only weak point was the card edge connectors and the thinner metal on the transformer bracket. It must be related to weird vibrational frequencies. That won't help with gross impact IMO. If you made the transformer bracket 1mm thicker and put some screws on the cards (rather then just press fit into the connectors) it would be perfectly fine. I always wondered if you put some steel zip ties around the card edge connectors if they would hold it without popping the ends off.Even if you had the wires all super lashed your gonna probobly resolder it anyway if it takes a really stiff beating.Does this have something to do with automotive/turbine vibration ?Laser labs typically have seismic provisions..Does anyone know how the vibration changes when its lashed together that tight?

author: Amper
author_rank: Frequent Contributor
timestamp: January 26, 2019, 07:25:11 pm
content: I guess it was just easier to put more than to think where to place them. Its a pretty dense cable tree in there with thick wiring that is usually never replaced or modified. They may even have pre manufactured the cable tree externally on a fixture and kept in shape by ziptying the living crap out of it. Some times its even useful when you take out some assembly, take off all the cables and for reassembly you just need o connect them in the place they point to because they are fixed in position that nicely.

author: Benta
author_rank: Super Contributor
timestamp: January 26, 2019, 07:37:39 pm
content: Yep, those wiring harnesses were made on a fixture and mounted as a complete assembly at the end. Standard practice in the 70s...80s

author: Amper
author_rank: Frequent Contributor
timestamp: January 26, 2019, 07:55:21 pm
content: Yaaay, guessed right

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2019, 02:37:07 am
content: I would not use a switching converter at all unless I had to have a regulated output. Simpler would be a high frequency inverter to just double the input voltage.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2019, 02:46:26 am
content: Quote from: webgiorgio on January 25, 2019, 12:44:49 pmI don't think this will work. You'll need to mimic the maximum power point tracking (mppt) as well. I'd create 3 strings of 6 panels each. Preferably each on their own inverter DC input channel to maximise mppt effectiveness.

author: webgiorgio
author_rank: Regular Contributor
timestamp: January 27, 2019, 03:54:31 pm
content: A string of 6 panels would not have enough voltage in relation to the input range of the inverter (333V-500V).I think that the inverter does a good job to make the 12 panels operate at the voltage that corresponds to the maximum power point.If my DC-DC keeps Vin=Vout/2, this Vin corresponds to the MPP of a series of 6 panels.I neglected the losses, as I expect about 5W out of 1000W, so 0.5%.

author: Benta
author_rank: Super Contributor
timestamp: January 27, 2019, 05:33:17 pm
content: Quote from: webgiorgio on January 27, 2019, 03:54:31 pmAre you talking about the losses in the converter?If so, that's totally unrealistic, it corresponds to an efficiency of 99.5%.Expect a power loss of 50...100 W.

author: webgiorgio
author_rank: Regular Contributor
timestamp: January 27, 2019, 08:46:10 pm
content: I meant losses in my dc-dc converter.Mhh, you are right, 0,5% losses sounds too optimistic.Using this Sic mosfet C3M0280090Dhttps://www.wolfspeed.com/media/downloads/825/C3M0280090D.pdfRdson=0,28 ohm, switch losses are 50 uJ per period.At 2.5A average current:Conduction losses= 0,28*2,5²=1,75WSwitching losses= 50*0.100=5W (from graph 23)I hanven't look up a transistor, which was suggested as more efficient than a mosfet. Can it be?This diode STPSC1006Dhttps://www.mouser.com/datasheet/2/389/stpsc1006-956763.pdfwould have 3,56W losses at 2.5AIf the inductor has 150 mOhm, dissipates 1W.13,3 W so far. So, 1.33% losses. I still have to quantify the losses in the gate driver and capacitors.I could also reduce the switching frequency and increase the inductor to reduce the switching losses.The whole 7 kW PV inverter has 2-3% losses to do MPPT and AC conversion (according to the datasheet, it is a SMA SMC 7000 TL)I hope that my simple boost converter can have around 2% losses.Can I reduce the switching frequency at partial load?

author: Benta
author_rank: Super Contributor
timestamp: January 27, 2019, 10:03:18 pm
content: Which inductor have you chosen? This is a key question.

author: Amper
author_rank: Frequent Contributor
timestamp: January 28, 2019, 12:18:29 am
content: Even though his is a different kind of inverter and the power is multiple times larger i like to see some mildly similar things for comparison:

author: coppercone2
author_rank: Super Contributor
timestamp: January 28, 2019, 01:24:52 am
content: hmm, I am just curious. Would tantalum caps work, or maybe wet tantalums, to get size down while increasing reliability close to film caps? (if they are properly over rated). ?like hermatic ones.

author: David Hess
author_rank: Super Contributor
timestamp: January 28, 2019, 03:52:09 am
content: Quote from: coppercone2 on January 28, 2019, 01:24:52 amTantalum parts could be used but would be awfully expensive and they are only available up to about 50 to 100 volts for hermetic packages. You might find them in a military, aerospace, or extreme industrial application. Polymer electrolytics would be better for lower ESR and higher ripple current rating but they are only available in even lower voltages. That leaves aluminum electrolytics, film, and ceramic.

author: jbb
author_rank: Super Contributor
timestamp: January 28, 2019, 04:28:41 am
content: Well, a SiC MOSFET + Schottky diode is almost certainly the right tool for this job. A Gallium Nitride (GaN) device might be a bit better. PCB layout will be absolutely critical to get the best performance out of your devices.You could reduce the diode conduction losses by using a second MOSFET as a synchronous rectifier.You mention using a ‘transistor’ instead. MOSFETs, IGBTs and BJTs are all types of transistor.In terms of changing the switching frequency, you’ll have to look at the knock-on effects on inductor current ripple and therefore turn on and turn off currents. It would also change the point at which you go to discontinuous conduction mode. If you want to go full nerd on it (and why not) with an optimiser I suggest you design for peak efficiency at 50% load but still check temperatures are OK at 100% load.For improved efficiency at light load you could consider using a burst mode.

author: Amper
author_rank: Frequent Contributor
timestamp: January 28, 2019, 08:39:51 am
content: QuoteIn case you meant me, i said bipolar transistor and thats referring to ordinary old school bjt, not fet, no igbt, not even darlington.The point i wanted to make is that for this task you really dont need insane frequencies, power densities and ultra modern parts at all. They used low tech even for the time and the iron core inductor was only a 50mm toroid including winding.

author: webgiorgio
author_rank: Regular Contributor
timestamp: January 28, 2019, 09:44:17 am
content: I haven't found an inductor, I have to look in the bin of old large salvaged inductors.Online I keep getting common mode Toroidal chokes (with two windings in opposite direction) like this one.https://www.elfa.se/en/toroidal-choke-mh-10-wuerth-elektronik-744834101/p/11064067https://uk.rs-online.com/web/p/leaded-inductors/8711385/I guess that they are suited to be used with near zero flux, so I can't simply connect the two windings in series.QuoteWhen I say "transistor" I mean BJT.@Amper Very nice video! The small one gives an idea on how my converter will look like.Quote from: jbb on January 28, 2019, 04:28:41 amWhat is a simple way to do this?A second 555 timer with duty cycle dependent on converter current, to enable/disable the PWM of the first 100 kHz 555? (too rough?)Is a IC available to do this?

author: richard.cs
author_rank: Super Contributor
timestamp: January 28, 2019, 10:44:15 am
content: Another topology to consider; a half bridge driving a 1:1 isolation transformer, the output from the transformer is full wave rectified and placed in series with the input supply.Key features:Converter only handles half the output power so 500 WSquare wave output so minimal output capacitance is neededTwo switches and two capacitors on the inputFour diodes on the output, you could reduce this to two by going centre tapped but would probably push the copper losses upThoughts?

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 28, 2019, 05:05:34 pm
content: I think there is a safety aspect here that does not seem to have been considered in this thread. The GTI you name IS NOT ISOLATED meaning the PV side is at GRID POTENTIAL moreover the GTI does not have arc sensing meaning there can be a fire hazard on the PV side. Both these issues make it unsuitable for DIY electronics additions to the PV side.As an alternative had you thought of extending the existing string, are the new panels on the same roof face, if so you can add them as long as N*VOC <=700V.Another alternative would be to use a separate GTI for the new panels as GTI's are quite happy to operate in tandem (on the GRID side) or replace your existing GTI with something more suited to your combination of panels.

author: coppercone2
author_rank: Super Contributor
timestamp: January 28, 2019, 05:15:16 pm
content: Are there any available tantalum caps on the market that would make that gigantic lump of foil capacitors go away?Why are hermatic ones only built to 100v?.

author: Benta
author_rank: Super Contributor
timestamp: January 28, 2019, 05:27:03 pm
content: @webgiorgio:The coils you are linking to are common-mode noise suppression inductors and will in no way store the energy you are looking for. The current rating is for the 50 Hz going through.The kind of coil you are looking for will be around the size of two cigarette packs. Also core and copper loss will be significant and way higher than your estimations.That's why I suggested full bridge or forward.

author: webgiorgio
author_rank: Regular Contributor
timestamp: January 29, 2019, 01:00:58 pm
content: Quote from: richard.cs on January 28, 2019, 10:44:15 amNeat!If it is a single secondary with full bridge, the current can be is positive and negative in the winding.If it is center taped (is actually twice longer), each half winding will conduct for half period.So, I think the losses in the copper are the same, so, better with 2 diodes. Same amount of copper in both cases, if it is center taped the section is half and the length twice.This would also reduce the losses, a it is dealing with half the power. In this case I need to make it Vo=Vin.The GTi (grid Tied inverter) complies with the grid requirements, without isolation. It has a isolation fault monitoring, that would disconnect the AC in case of leak current to ground.I can't add inverters neither change number and orientation of panels, ans I need to comply with technical regulations.I am allowed only to add "optimizers" (dc-dc) on the DC side.

author: Benta
author_rank: Super Contributor
timestamp: January 29, 2019, 04:52:51 pm
content: Quote from: webgiorgio on January 29, 2019, 01:00:58 pmNo. The center tapped configuration has significantly higher copper losses, as only half of the secondary is used at a time and it only has half the winding space = double resistance.At these voltages I see no reason to avoid a bridge rectifier, this is only relevant in low voltage applications where diode drop is an issue.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2019, 09:32:22 pm
content: Quote from: coppercone2 on January 28, 2019, 05:15:16 pmI do not think so but there might be such a thing for aerospace and military applications. They also make really big ceramic capacitors for aerospace and military applications but they are similarly expensive.QuoteThe solid hermetic parts are available up to like 75 volts and the wet tantalums are available up to like 125 volts. Considering their cost, they make even expensive film capacitors the cheap option and the way to go.