48VDC to 160VDC (for inverter) - Page 1
Forum section: electronics / projects

author: Dmeads
author_rank: Regular Contributor
timestamp: February 06, 2021, 07:16:19 am
content: Hello and happy friday!!I am designing a variable frequency drive (VFD) for an induction motor.I would like it to run off a 48V battery pack.A lot of VFDs and inverters I am seeing either have access to higher DC voltage, or rectify 110VAC @ 60Hz to get the voltage for the motor.I only have access to 48VDC.Researching Boost converter controller ICs on Digikey and mouser showed that the highest voltage output was 100VDC, which is a little low. I can get the motor to spin, but I would like to have access to 160-170VDC so I can get the full 110-120VAC the motor was meant to run on.Since I have this issue, I need some suggestions.Some possible solutions:- Just use 100VDC and use a transformer to step up the AC voltage I create from an H-bridge. (I have a pretty nice SPWM algorithm I made and I guess I am just worried the transformer could degrade it).- Ditch the 48V battery pack, just rectify from mains voltage and get 170VDC that way.- Make a 48V -> 170V boost converter and design a custom controller with a microcontroller (this sounds the hardest but I am willing to do it)The goal is to have averynice looking sine wave that is 110-120VAC, and use the VFD to change the speed of the motor.Which one of my solutions is best for my goal? Additionally, what would someone in the power electronics industry do if they had this problem?Thanks,-Dom

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2021, 07:42:33 am
content: I'd definitely go for designing the boost.The controller IC doesn't need to see the output voltage. It senses it through voltage divider. The great side of boost topology is the MOSFET which sits on the low side, so the gate driver in the controller IC only sees GND + Vgate.Many controller ICs (for external FET + diode) should be able to do it.Remember the importance of the reverse recovery characteristics of the diode. A SiC is an easy choice.

author: Dmeads
author_rank: Regular Contributor
timestamp: February 06, 2021, 07:53:11 am
content: Okay great!So i could just make the output 170VDC, and use a voltage divider to bring that down to whatever the max output is for the controller?Then it could "psuedo-sense" the output voltage properly?I would then just calculate the values for my passives and stuff based on the "real" output voltage

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2021, 09:00:50 am
content: Wellanycontroller brings the output down with a divider for Vfb since the internal bandgap reference is typically between 0.75V and 1.25V. This doesn't change.I don't know why "output voltage" is a search parameter when you look at controllers.The Vcc for the controller typically comes from the input side, so 48V in your case. Only when input voltage is so low (say, 3V) you can't run the IC with it properly, you can bootstrap the supply from the output.I think it's easy to find a controller which can tolerate this 48V - they have an internal regulator. Even if not, you can use external linear (or switch mode) converter to bring 48V down to whatever the chip works with.If you look at more integrated ICs which contain the MOSFET switch,thenthe output voltage is a parameter.Maybe some IC sense switch current by Vds sensing, then the IC would be exposed to the output voltage? Avoid such chips. It's easy to see from the datasheet schematic - if there is no connection to the switch node or output, then the IC never sees the output voltage.For example, the first randomly picked part listed as "boost" controller on Digikey, NCV8873, shows a diagram with no connection to Vout or Vsw. It has low-side current sense resistor, this is what you want. Digikey does not list output voltage as a parameter, obviously. It can be 1000000V. This is not a recommendation for that part, though, didn't look any further.Remember the importance of the output capacitance and output loop area in the boost layout.

author: ogden
author_rank: Super Contributor
timestamp: February 06, 2021, 09:17:41 am
content: For design work I would rather use safe and dependable lab power supply, most likely used, powerful enough 200v system supply. Boost converter indeed could be project, but separate one - *after* you finish VFD drive.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2021, 09:21:22 am
content: Yes I agree, if you have never designed such high-power boost converter, it's a non-trivial project which will take some time (and learning).If you are sure that will be an important part of the project anyway, you can start with it, in fact you may save time by first finishing and verifying the boost so you don't need to spend time on coming up with a large lab supply.OTOH, if you just need to get the motor running ASAP and are not sure if the self-designed boost is the right way to go on, start with getting some kind of temporary lab supply for the VFD.

author: poorchava
author_rank: Super Contributor
timestamp: February 06, 2021, 09:24:20 am
content: It depends on power required. If it's in 10's of watts, then boost is reasonable. Boost ratio of ~4 is not challenging as it results in about 80% duty cycle. Above that I'd say use a transformer. At 48V supply you can push it to 200kHz easily. Depending on power level you can go for push pull, half bridge or full bridge. I'd leave flyback and forward alone, as flyback above like 50W starts to be challenging, and forward generally underutilizes the core as it works in on direction only. I guess you don't even need to regulate the output voltage, you can implement feedback and stabilization at the final stage, maybe with some feedforwadd from 48V supply in case it comes from a battery or something.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 06, 2021, 01:04:37 pm
content: If isolation is not required, use a push pull design with a variation: add diodes from each MOSFET to a smoothing cap (can be a fairly small film cap), that gives you about double the input voltage, getting you more than halfway there. The secondary output is connected in series with that and that only needs to provide the remaining 60-70V or so.Quote from: ogden on February 06, 2021, 09:17:41 amCheap way to get an isolated rectified mains supply is to get a cheap inverter and tap off the internal DC bus, then power the inverter from a server power supply. Or I suppose you could modify some microwave oven transformers to output less voltage at a higher current, keep in mind they have to be run at less than full voltage (half is easy by connecting two in series) for continuous operation.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2021, 02:48:07 pm
content: IMHO, 48V -> 170V isn't a problem yet. It's 1:3.5. For such high power, definitely design a multiphase. For example, I did a 22V -> 75V 750W which has the same ratio as the OP, to drive brushed DC (universal) motors, I did a 4-phase design, efficiency somewhere around 96%. This was with off-the-shelf 2-phase controller IC (syncable for 4-phase) with low side current sensing.At any higher conversion rates than this, having to rate the MOSFET for both the full output voltage and full input current becomes the practical problem quite quickly.I did the previous prototype as center-tapped (coupled inductor) boost and of course it worked, but the complexity and winding the transformer was not worth it, also snubbing the stray inductance added unexpected losses. Even if boost is not theoretically optimal, being able to work with a simple design with the most basic and low cost off the shelf components like inductors is a plus, especially if you have to build a few of them. With one-off, you can to anything, and also if you do a million, you can do anything, but a batch of ten to thousand greatly benefits from the simple COTS component design.

author: penfold
author_rank: Frequent Contributor
timestamp: February 06, 2021, 03:57:11 pm
content: How nice isverynice btw? Is it a 3-phase motor? Power rating is probably the most critical factor here since it will probably determine whether or not you'll be better off with surface mount or TH components on heatsinkHave you considered using third harmonic injection as a means to reduce your DC link voltage requirement? Could help keep the step ratio a bit lowerFor 48V input in the 1 kW sort of range, I would tend to generally advocate push-pull transformer based approach (push-pull primary, single secondary full wave rectified). For one, you're keeping switch voltage stress in a much nicer area, which given the high currents, it makes for a nicer mosfet. And EMI can be significantly easier to control, the PCB layout lends itself to very nice tight loops and the physical sizes of the components has less of an impact on the length and width constraints of the copper traces which makes the design more scaleable (which is part of the reason EMI is easier to control).Transformer isolation between battery and inverter stage is also beneficial because inverters are just a nightmare for common mode noise.Ditto to multi phase if using boost converter, at 48V however, multiple phases below 10A/phase (more than 1 phase for every 10 A of current) or so will just lower your power density and make efficiency optimisation more difficult. I would still suggest interleaving the push-pull option, but you can get better utilisation at a higher power per phase.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 06, 2021, 05:07:47 pm
content: Multiphase eases thermal design, instead of one huge hotspot you have several.Any single phase tends to have requirements regarding layout inductance, forcing you to put heat-generating components (mosfet, diode, sense resistor, inductor's one pad to minimize switch node area, also the output capacitor) in one clump. But different phases can have some healthy gaps between them. For example, if you estimate the lateral heat transfer in ground plane and copper fills to be 20mm, 40mm gap between phases is sensible (assuming you have space for that).Multiphase is also a very simple way to design, basically just copypaste work done once. When designing one "single" phase of a multiphase converter, remember you can and should use higher than normal (if you were to design a single-phase converter) current ripple percentage.But I admit 1:3.5 is quite near the edge of a sensible high-power, high-efficiency boost design. 1:3 usually makes sense and 1:4 does not but of course specifics matter here.

author: ogden
author_rank: Super Contributor
timestamp: February 06, 2021, 05:51:40 pm
content: Don't forget which is primary target - VFD or boost converter for it.

author: Dmeads
author_rank: Regular Contributor
timestamp: February 06, 2021, 06:17:56 pm
content: Target power is 200W. I will probably just go down to 1:3 boost ratio. Still gives me over 100VAC which will be fine. I have gotten the motor to run off 82VAC, it just wasn't as powerful.the largest boost design I have done was 100W, so I have some experience with them, but i have only 3 designs under my belt, so if 1:3 boost ratio is more sensible and potentially more efficient, then so be it.I will research multi-phase designs because it sounds like that it something I need to do.No wonder digikey filter parameters dont list the output voltage, this makes sense now.

author: Dmeads
author_rank: Regular Contributor
timestamp: February 06, 2021, 06:38:59 pm
content: I will probably go with the LTC3784 from Analog Deviceshttps://www.digikey.com/en/products/detail/analog-devices-inc/LTC3784EGN-PBF/4740987Im a big LTspice guy so I should get some good sims with this model.

author: Dmeads
author_rank: Regular Contributor
timestamp: February 06, 2021, 06:57:44 pm
content: Actually I guess since the induction motor will draw 1A, for startup this should be about 3 times this or 3A, so target power for the boost will be more like 450W to make sure it can handle the motor start up.Do I need a synchronous converter for 3A? or can I get away with an non-synch and a good heatsink on the diode?The reason I am wondering about non-synch is that the synchrounous converter controller ICs im looking at have a charge pump circuit for the MOSFETs that only goes up to 70 or 80V.

author: james_s
author_rank: Super Contributor
timestamp: February 06, 2021, 07:08:50 pm
content: I see no reason why a synchronous converter would be required, 200W is not particularly high power.If I were doing this I would probably just use something like the old TL494 with a pair of mosfets driving a transformer with a center tapped primary. It's an old part but still available and relatively easy to use, it was used in a lot of older PC power supplies. I'm not an expert with power electronics but I've used this part with some success in the past.

author: penfold
author_rank: Frequent Contributor
timestamp: February 06, 2021, 07:51:04 pm
content: Quote from: Dmeads on February 06, 2021, 06:57:44 pmCan you use the inverter to "soft-start" the motor?

author: Dmeads
author_rank: Regular Contributor
timestamp: February 06, 2021, 07:58:02 pm
content: Quote from: penfold on February 06, 2021, 07:51:04 pmYes actually I could just ramp it up nice a slow thanks for the idea. That would help lower the boost power needed by a lot.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 06, 2021, 09:54:54 pm
content: Quote from: Dmeads on February 06, 2021, 06:57:44 pmSynchronous conversion makes no sense at those voltages unless you need regenerative (bidirectional) operation.

author: mzzj
author_rank: Super Contributor
timestamp: February 07, 2021, 06:48:18 am
content: Quote from: poorchava on February 06, 2021, 09:24:20 amI'm always recommending 2-transistor forward for beginners: it is very hard to make it wrong compared most other topologies.Scales from 20W to 4kW, no issues with body diode conduction, flux walking, or trying to snub badly coupled primaries. Also no shoot-trough.Penalty from one-direction core utilization is minimal above ~20kHz when you are anyways not limited by Bsat

author: Siwastaja
author_rank: Super Contributor
timestamp: February 07, 2021, 07:22:00 am
content: Don't do synchronous, the output voltage does become part of the controller rating and LTC3784 is good to only 60V.Some 500W 48V->150V is easy aiming 95% efficiency with completely off-the-shelf parts.Diode needs to be carefully considered for reverse recovery losses. SiC just easily gets rid of this problem altogether.For my similar 750W job, I originally did the math for 2-phase but 4-phase finally helped get the off-the-shelf mass-produced inductor cost down and also spread the heat better.I'm wondering where all the problems are because I basically designed my converter in a few hours (well obviously drawing the schematics and layout takes longer), ordered parts, built three units, tested and measured, everything's running fine at a tad over 96% efficiency, no apparent EMI problems, yet reading at EEVBlog forum comments, seemingly such boost ratio should be impossible or at least very difficult. I don't get it.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2021, 02:50:59 pm
content: An inverter doesn't need a regulated power supply. Use a self-oscillating topology, without any regulation. Here's an example I found on the data sheet of the TC33167, a switched mode power supply IC. The switching transistors need to be rated to double the maximum input voltage.http://www.ic72.com/pdf_file/t/85248.pdfAnother example is a electronic halogen lamp transformer, or compact fluorescent lamp ballast style inverter, with a rectifier on the secondary.https://www.st.com/resource/en/application_note/cd00003902-electronic-transformer-for-a-12v-halogen-lamp-stmicroelectronics.pdfhttps://www.researchgate.net/figure/Typical-Compact-Flash-Lamp-Ballast-Circuit-10-15-Compact-Fluorescent-Lamp-ballast_fig1_292835245https://sound-au.com/lamps/fluoro-lamps.html

author: penfold
author_rank: Frequent Contributor
timestamp: February 07, 2021, 03:29:10 pm
content: Quote from: Siwastaja on February 07, 2021, 07:22:00 amPower electronics can be very easy to fluke! And with that, whilst its fantastic that it can be incredibly easy to work with off-the-shelf controllers which have really well documented data sheets and application notes and so many pre-designed power magnetics, if you're largely following the application guidelines you can get pretty descent performance just applying good general circuit design practices. The problems come from when designers start using the highly integrated ICs slightly outside their best optimised cases or mix-and-match advice from 1970's app notes not really understanding the limitations and how more recent advances have impacted the older recommendations.The fact is, that with good optimisation (whether the designer's aware they've done it or not!), higher frequencies, lower inductances, nicer ferrite materials, "better" control methods, readily available multi-layer boards and SMT components... boost converters perform so much better than they would have done.I do agree that an interleaved boost would be a very suitable option, I just find it a far less forgiving topology with much less margin of error for weaknesses in layout

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2021, 04:22:56 am
content: My first choice for simplicity would be an inverter using a commonly available 33 volt AC power transformer operating at a slightly higher frequency than 60 Hz to step 48 volts up to 170 volts. If better regulation is required, then a primary side low dropout linear regulator can be used with a slightly higher step-up ratio; efficiency is still high because the primary side regulation range is so small.

author: james_s
author_rank: Super Contributor
timestamp: February 11, 2021, 08:11:18 am
content: For 200W? That's going to be a fairly big heavy transformer. If you already have a suitable transformer or get one for next to nothing it might make sense, but otherwise it's so easy to make a basic switchmode converter that it doesn't really make a lot of sense to mess with iron transformers. The simplest approach is probably a Royer oscillator, I lashed one of those together once out of parts I found on the floor in my project room and a random toroid, amazingly it actually worked on the first try and delivered several amps at around 6V. There are better performing options but it's hard to beat the simplicity, just the transformer, a pair of transistors and a couple resistors.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2021, 10:18:01 am
content: This is a funny thread because everyone seems to have acompletelydifferent idea how to do this "easily", and the ideas of others are seemingly unsuitable.Of course you can MacGyver this up with almost anything you have lying around, but that's completely up to what OP happens to have.I stand by my idea that boost converter is the simplest with easiest-to-understand schematic and working principle, with guaranteed zero need for building custom transformer(s) or inductor(s). Yes, you need to minimize the switching loop area, but OTOH, you don't need to think about transformer leakage inductance, minimizing it with proper winding patterns, obtaining litz wire, and adding snubbers nevertheless.In other words: if all the hardships of boost described in this thread prevent OP from succeeding, I'm 100% sure any transformer-based topology will definitely fail, they have inherently more things to go wrong (also more potential to learn).Transformer based circuit which needs the MOSFET Vds(max) to be theoretically rated "just" 2x input (not accounting for any parasitics) does not sound, IMHO, very appealing at all compared to simple boost which needs to be rated 3x input.Same with flyback for example, the output voltage is reflected back to the input by the turns ratio making high conversion rates appealing, but if the conversion rate is just 1:3, the difference is small, and the new added parasitics may make it actually worse unless you really know how to build (and snub) it properly.By all means go for transformer-based topology if you want to learn about designing and building them, but if the aim is to make a simple, easy to design, easy to build, low-cost and efficient converter, 1:3 boost is hard to beat. That'sexactlywhy I changed a transformer-based 1:7 topology to a boost-based 1:3.5 topology when I had the chance to halve the output voltage requirement, and oh boy did it get so much simpler to design and build.And I'm sure many will disagree.And yes, 1:3.5 boost is getting close to the edge of the "makes sense" boost sweet spot but 1:3 is clearly within the sane area.

author: penfold
author_rank: Frequent Contributor
timestamp: February 11, 2021, 11:43:24 am
content: Quote from: Siwastaja on February 11, 2021, 10:18:01 amCan you clarify that? I only disagree because the push-pull and forward are usually my go-to since I've never had an issue with one, and that includes the occasional lash-up when I've not had the ideal core to hand. In comparison, something I noticed from being a teaching assistant the vast majority of students' projects' problems involved boost and buck converters whereas a typical student producing a transformer based design tended to have fewer problems. Most problems were due to bad PCB layout or control loop - both of which are coincidentally(?) something the push-pull is a little bit more forgiving over.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2021, 02:23:07 pm
content: Did the students wind the transformers, were skin/proximity effect losses estimated, how about pri/sec coupling, was the criteria just "works somehow", how were the switching parts selected?Or was it a design-done-by-the-teacher, parts supplies by the teacher, including pre made transformer?Boost converter control loop compensation surely sucks due to the right half plane zero, but there are a lot of appnotes available calculating the correct loop compensation components. I think no topology is immune to selecting loop compensation components "randomly".Minimizing one simple loop (switch - diode - output capacitor) is IMHO easy. Something to keep in mind, though.I don't know, maybe push-pull transformer-isolated topology isn't that difficult for a beginner, either. I'm remembering my own experiences and my first buck and boost ever obviously were a breeze. I also spent time winding transformers, enjoyed it and were successful but I remember taking a lot more time - and measurements - doing it.

author: penfold
author_rank: Frequent Contributor
timestamp: February 11, 2021, 03:25:11 pm
content: Quote from: Siwastaja on February 11, 2021, 02:23:07 pmI'm mentally combining two cases of independent design and a more teaching based exercise,In cases where the student chose to design their own PSU instead of buying one, part of a larger project but not the sole design focus, "works when testing in application" would be fine, so metrics such as "doesn't catch fire", "doesn't melt insulation", "keep the output in regulation", "still works under load" would be enough. If the PSU was the sole design project then it would be a specific topology, probably something more "interesting" than push-pull, boost or buck and that's a whole new world of problems!For the more teacher lead exercise, the student was still expected to design their own transformer, and given a typical design procedure to follow which did include skin and proximity.Its that last point which steers me more toward transformer based designs (I do still agree that the boost is a technically suitable option when done right so I do apologise if I sound argumentative). In complete ignorance of the OP's PCB layout skills I would find it easier to recommend a good transformer design guide (Ned Mohan's for instance) and a guide for selecting snubber components than it would be to start describing ways of keeping current loops low and speculating the possible sources of certain oscillations through the medium of the inter-web, especially when some of these concepts may very well be brand new.

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2021, 07:16:24 pm
content: Quote from: james_s on February 11, 2021, 08:11:18 am200 watts is not very large for a 60 Hz transformer and operating at a higher frequency increases the power capability within the limits of the winding resistance. If the operating frequency is raised, then the transformer has to be sized for the winding current and not the output power. Hysteresis losses are higher but they started out small and remain small at lower frequencies.I like the inverter method for simple designs because it avoids the complexities of feedback control. The big disadvantage is lack of short circuit protection although that could be added.For myself, I would implement a high frequency design using a current mode switching regulator but it is not something I would recommend for the casual designer. And I might still use an inverter as outlined above with primary side regulation to avoid making a high voltage switching transformer.

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2021, 07:52:39 pm
content: I don't see the problem with either approach. A boost converter can be made with a transformer.https://www.eevblog.com/forum/projects/12v-to-400v-boost-converter/https://onlinelibrary.wiley.com/doi/10.1002/cta.1994Here's the principle applied to a self-oscillating topology. Q1 and Q2 need to be rated to double the supply voltage. The output voltage is equal to double the supply voltage, plus the normal expected voltage on T1's secondary winding. For example if T1 has a 1:1 ratio, the output voltage, will be triple the input.

author: ogden
author_rank: Super Contributor
timestamp: February 14, 2021, 08:52:38 am
content: I would discourage self-oscillating topologies for anything above few watts, well... unless it is homework for "PSU (in)efficiency" class. Better use push-pull SMPS controller IC or MCU with low side MOSFET drivers - if SMPS IC for some reason is not welcome.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2021, 10:26:36 am
content: Quote from: ogden on February 14, 2021, 08:52:38 amWhy? If it doesn't need to be regulated, what's the point in using an SMPS IC? Self-oscillating transformer drivers are used in many commercial products. They're simple, cheap, reliable and high efficiency.

author: ogden
author_rank: Super Contributor
timestamp: February 14, 2021, 10:43:09 am
content: Quote from: Zero999 on February 14, 2021, 10:26:36 amEfficiency.QuoteReally? I would love to see commercial product with [simple, cheap] self-oscillating supply providing 200W. You most likely were joking when said "high efficiency"?

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2021, 11:14:44 am
content: Quote from: ogden on February 14, 2021, 10:43:09 amNo, I'm serious. Self-oscillating transformer drivers can be very efficient: >90% is pretty common and easy to obtain. Classic examples are electronic halogen lamp transformers and CLF ballasts. The main reason why they aren't used for switched mode power supplies is because they aren't regulated. Here are some electronic transformers with >90% efficiency.https://www.powerled.uk.com/wp-content/uploads/sites/48/2020/05/AC-PLED-Series-Data-Sheet.pdfhttps://www.osram.com/ecat/HALOTRONIC-PROFESSIONAL%20HTL-Electronic%20transformers%20for%20halogen%20lamps-Electronic%20Control%20Gears%20for%20Lamps-Digital%20Systems/com/en/GPS01_1028160/They all use the same simple, self-oscillating topology:https://www.st.com/resource/en/application_note/cd00003902-electronic-transformer-for-a-12v-halogen-lamp-stmicroelectronics.pdf

author: ogden
author_rank: Super Contributor
timestamp: February 14, 2021, 12:24:52 pm
content: Quote from: Zero999 on February 14, 2021, 11:14:44 amI would not consider 90% as efficient, especially for hi power (200W) supply. Note that electronic transformer in ST appnote has AC output, thus no rectifier losses.QuoteWhere's proof thatOsram HALOTRONIC-PROFESSIONAL HTLyou mention uses such topology? Not exact model, but anyway - no IC. Your point proven. Thanx.http://ixbt.photo/?id=photo:610174http://ixbt.photo/?id=photo:610175

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2021, 01:23:04 pm
content: Quote from: ogden on February 14, 2021, 12:24:52 pm90% is the minimum and is pretty decent for such as simple circuit. The rectifier losses will be lower, at higher voltages. An couple of additional windings and MOSFETs on the secondary could be added to give synchronous rectification, but it isn't worth it for 160V.And, no, I don't have any proof that the Osram electronic transformers use such a topology, but given the data sheet says it has a 50kHz output and it's what's inside every other electronic halogen transformer I've seen, I'd be surprised if it's something different.It's possible it uses an oscillator & MOSFET driver IC, such as the IRS2153D, FAN7387, L6571 etc. but I don't see why they would go to the additional expense.If you don't want to go with self-oscillating, then use an oscillator & MOSFET driver IC, such as the IRS2153D, FAN7387, L6571 etc. The main advantage is no feedback winding on the transformer to worry about, but I doubt it will gain much efficiency-wise.https://www.infineon.com/dgdl/Infineon-IRS2153D-DataSheet-v01_00-EN.pdfhttps://www.mouser.com/datasheet/2/149/FAN7387-1006880.pdfhttps://www.st.com/en/power-management/l6571.htmlThe nice thing about the self-oscillating topology is it tends to set the frequency to the optimal point. Here's a data sheet for the LTC1697, a CCL lamp driver IC, which still uses external transistors, in the classic, self-oscillating, Royer converter topology. If it was that bad, the IC would control the driver transistors directly, rather than PWMing the Royer oscillator.https://www.analog.com/media/en/technical-documentation/data-sheets/1697f.pdfThe circuit I posted previously, should be more efficient, than an isolated converter. It's configured like an autotransformer, so the circuit is only doubling the voltage, which is connected in series with the primary side, giving triple the voltage, thus cutting the losses by around a third.Two of the diodes can be eliminated, if one winding is used for the primary and secondary, making it into a true autotransformer.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 14, 2021, 02:05:16 pm
content: Should and should. Such topology-based assumptions about efficiency and voltage peaks are of little value. For any topology, I suggest you always run a spice simulation with explicit series inductances all over the place and realistic coupling coeffs like 0.95 especially if you are not a professional well-taught SMPS transformer winding specialist.I actually compared the efficiency of two different converters made for the same task, one with autotransformer-like tapped boost, one with simple traditional boost, and the latter was more efficient (and much simpler to build), because the snubber losses needed to keep the MOSFETs working with the theoretical "you only need this and this Vds rating" were way above I thought they were in a transformer-based design. Also transformer losses were higher than inductor losses in the simple boost.Stray inductance is a bitch as are transformer coupling and winding AC losses.Classic non-isolated buck and boost are pretty appealing when the conversion rates are not too big so that duty cycles don't wander too far from 50%. (Multiphases are utterly trivial to design and build and ease this condition to some extent.) The reason is in their simplicity and lack of parts that cause cost and losses. PC motherboard manufacturers use multi-phase synchronous buck converters for surprisingly high (over 1:10) conversion rates.

author: ogden
author_rank: Super Contributor
timestamp: February 14, 2021, 02:45:47 pm
content: My argument is not against push-pull topology. I say that self-oscillating BJT's in push-pull converter cannot reach same switching performance, thus efficiency, as properly driven MOSFET's in converter of same (push-pull) topology.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2021, 07:43:31 pm
content: Not to mention startup and fault conditions, which are likely to destroy a basic oscillator.You're pretty much hand winding the oscillator transformer, unless you run at such a low frequency that a mains transformer is suitable. The boost can use off the shelf inductors (at least up to modest power levels; for >kW you're probably using many in series/parallel, or winding your own).Oscillator has no regulation, and is made ever worse by poor transformers (e.g. split-bobbin vs. shell vs. toroid mains-frequency type). Can be controlled, say with a buck reg in front of it, but, y'know..?The oscillator probably has lower EMI, owing to the low loop gain. (A typical switching reg has many stages of digital gates cascaded, effectively having extremely high loop gain, not that it's a useful metric at that point. It can even be difficult to tame that speed, with such heavily optimized power transistors as we have these days.)Tim

author: uer166
author_rank: Super Contributor
timestamp: February 14, 2021, 08:44:28 pm
content: I don't get the point of self-oscillating topologies. They might look simple, but the number of edge cases, states, startup and fault conditions, potential for lockup, makes it probably the most difficult ones to analyze and prove that they work. They might work "on average" but you certainly can't claim any reliability figures.It's much easier to be truly in control of what's happening and it'll work fine and be easy to validate. I'd go with a simple multiphase boost for this any day, the ratio isn't difficult at all (use SiC diodes for reverse recovery losses, straight-forward). Don't need to care about leakage inductances, winding transformers, coupling coefficients, relying on thermal noise to start-up a royer oscillator, bs, bs, and more bs.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2021, 09:43:52 pm
content: Don't want BJTs? Then it can be built with MOSFETs, which will be more efficient at low voltages. Don't do BJTs down too much though: they're more cost effective than MOSFETs, at higher voltages.I think one important point has been forgotten here:it's driving an inverter, so the lack of short circuit protection and regulation are less important. The inverter can implement short circuit protection and vary the duty cycle to compensate for changes in the voltage.I'm not aware of any reliability issues. Self-oscillating transformer circuits are used extensively in many products, without any issues. I've seen cold cathode lamp drives and halogen lamp transformers last for over ten years. More complex switched mode power supplies just as regularly go wrong.I'm not saying thou shall use self-oscillating transformer, just that it's not as bad as many people think. Like any topology, it has it's advantages and disadvantages. I'd certainly give it ago.

author: ogden
author_rank: Super Contributor
timestamp: February 14, 2021, 10:25:37 pm
content: Quote from: Zero999 on February 14, 2021, 09:43:52 pmChanging BJT to MOSFET won't improve efficiency much. Simple 2-transistor self-oscillating topology can't achieve fast turn-on/turn-off times by design. Slow switching will result in conduction losses disregarding transistor type. Fast MOSFET switching needs gate driver. As many SMPS controllers includes gate drivers, it is logical choice when one needs better than said 90% efficiency. Hope this explanation helps.Quote48VDC shall not be considered as high voltage.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2021, 10:59:07 pm
content: Quote from: ogden on February 14, 2021, 10:25:37 pmThat's not true at all. The feedback winding of the transformer has a very low impedance, which is ideal for driving either a BJT, or MOSFET. A BJT will take a little longer to turn off, due to the storage time, but once it starts to turn off, it doesn't spend very long in the linear region, so the switching losses aren't too high. The good things about this topology when it's used to drive BJTs are: the voltage is stepped down by the feedback winding, so there aren't the huge losses in the base resistors and the base voltage also goes negative, which vastly improves the off time. It's also great for MOSFETs, as it can rapidly charge/discharge the gate capacitance, and it can drive a high-side N-MOSFET in half bridge configuration, as the feedback to the positive side can be floating.QuoteI agree. MOSFETs would be a good choice for this application, probably better than BJTs, even if they have to be rated to double the supply, if the push-pull centre tapped primary is used, rather than half-bridge.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 15, 2021, 01:40:59 pm
content: Quote from: Zero999 on February 14, 2021, 09:43:52 pmIt's the opposite! Inverterisa short circuit! Depending on model, they have a shitload, or two shitloads of DC link capacitors.Don't count on it of them implementing precharge. They are supposed to take many tens of A of inrush.Obviously you can design the self-oscillating push-pull with a soft-start, or even better, current limit.Obviously you can wind a custom transformer with feedback windings.Or you can just use off-the-shelf boost controller IC (with softstart and current limit!), off-the-shelf MOSFET, off-the-shelf inductor and off-the-shelf diode and that's it!Custom magnetics and more-complex-than-non-isolated-buck-or-boost topologies aregreat, evenfun, but a simple buck or boost is a perfect fit when conversion rates are below 1:3 to 1:4 and power levels are below about 500W. This projects falls within this range.Now 48VDC to 320VDC at 5kW would be something else!

author: Gibson486
author_rank: Frequent Contributor
timestamp: February 15, 2021, 04:40:35 pm
content: That actually seems like a straight forward thing to design with a boost converter. 48 to 160 is actually not difficult, but the added current constraint is a little worrisome (your current on the input needs to be ATLEAST Vo/Vi times the output). Also, I am not sure how many IC chips will handle that 48V input. I have only done boosts with max of 24V. For a motor, you can actually try to make an unregulated boost converter. It is basically the boost converter without feedback (so no IC to do it for you...just use a 555 and go to town). Again, you will plenty of input current to support this. It is a good learning experience, but I would be inclined to do this more with AC wall output power as the input, than DC.

author: Zero999
author_rank: Super Contributor
timestamp: February 15, 2021, 07:00:49 pm
content: Quote from: Siwastaja on February 15, 2021, 01:40:59 pmGood point about the DC link. If the DC link capacitance is big, that pretty much rules out a straightforward boost converter, or an autotransformer, as they will blow up, when the output is short circuited.Anyway, I thought the original poster was designing the inverter, so is in control of the DC link capacitance? As it isn't being powered off the mains, the DC link capacitance can be smaller, assuming it's not going to be doing a crazy amount of braking.If you want to do it the easy way, just buy a cheap 48V to 120V inverter and connect the motor inverter to its output, but where's the fun in that?

author: uer166
author_rank: Super Contributor
timestamp: February 16, 2021, 01:15:41 am
content: Just don't forget to precharge the DC link to the battery voltage in the first place before connecting battery, or make sure that the boost diode can handle the inrush. The boost can't output less than the 48V input, but once they're connected, can handle any and all inrush on power-up if it's a current mode controller as it should be.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 17, 2021, 07:51:47 am
content: Quote from: Gibson486 on February 15, 2021, 04:40:35 pmMany controller ICs available rated way above 48V, but you don't need to limit yourself to such parts if you feel more familiar with something rated lower; just regulate the 48V down for the controller IC Vcc pin with a simple linear regulator.In the non-synchronous boost topology, the lower MOSFET (and low-side current sense resistor) sit right on the ground plane and that's all the controller IC needs to know.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 17, 2021, 08:01:38 am
content: Quote from: Zero999 on February 15, 2021, 07:00:49 pmI missed this completely.If they are designing a VFD for ACIM from scratch, it is at least an order of magnitude more difficult and complex than any of the possible voltage boosting stages, so I don't think it makes sense to concentrate so much on finding a boost circuit which can be built in absolute minimum time and skills. I think OP should be able to do this on their own, if not, I wonder how they are going to do that VFD?I think the whole thing could be integrated and designed in one big blob. As the design will have six-MOSFET bridge driven by the MCU anyway, I see no reason not to use the same MCU to drive the boost stage as well.The simple boost may need some precharge anyway because the boost diode conducts the input voltage to the output even if the boost is turned off. Often this isn't a huge problem because at lower voltage (here 48V) the inrush current limited by the total ESR is much lower than at full voltage, and boost current limit starts to work right when the boost is enabled, but for a really robust system in cases with very large very low-ESR capacitor banks or actual short circuits, even the simple boost requires some additional protection (like an extra FET switch in the input, driven by a controller) to protect thediode.