Effect of excessive turns on SMPS primary - Page 1
Forum section: electronics / projects

author: BennVenn
author_rank: Regular Contributor
timestamp: December 04, 2014, 11:13:41 pm
content: Hi everyone!Just a quick question that I'm sure the experts will clarify for me. I'm designing and building a 1KW DC-DC converter (24v-48v) and I'm comfortable with the calculations for primary turns taking into consideration Bmax, AL, etc... I've read using excessive turns will 'under utilise the core'. What exactly are the consequences of using excessive turns? I'd like to reduce core losses as much as practicable and from what I’ve read, aiming for a lower Bmax will give better idle current and reduce core losses.Will adding more turns than the calculation predicts reduce the power transfer through the transformer? My thoughts are adding inductance will decrease the current/time ramp and limit the peak current per cycle. Or are the reasons for reducing the turns purely from a copper loss/cost/space perspective?Thanks!

author: BennVenn
author_rank: Regular Contributor
timestamp: December 04, 2014, 11:27:07 pm
content: Thanks for the reply!Yes, just a simple ferrite core toroid/E-core transformer (haven't decided yet) Room will not be an issue, the calculations predict 6 turns primary, 12 turns secondary on a rather large core. That is with Bmax approaching its limits. I'd like to add more turns to add a safety margin as the temperature increases. Just wondering the consequences of adding 8 turns the the primary, 16 turns secondary in regards to power handling

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2014, 03:18:26 am
content: "Underutilized" = "high efficiency". Take your pick.You probably want to run at a more modest flux density, say 0.1T, 0.2T tops, since that thing will get hot. Ferrite isn't so happy when it's hot: Bmax drops. One way to think of Curie temperature is, Bmax goes to zero so it's effectively saturated all the time.Although losses generally decrease slightly at elevated temperatures, Bmax drops as well, which can cause... unintended consequences.As for parasitics, obviously using more wire will increase leakage, and necessitate an increase in wire size to conserve or reduce copper losses. Leakage can be made up for by using alternating primary and secondary layers, interleaved. Don't overdo it, or you'll end up with consequences of excess stray capacitance. Moral of the story being, as with all things, you want a balanced impedance (Z = sqrt(L/C)), suited to the inverter. At this voltage, you probably need a very low impedance (under an ohm?), so you will want to interleave quite heavily to keep efficiency high.Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: December 05, 2014, 05:50:59 am
content: BennVennA well designed transformer will generally, among other things, have the core losses = copper losses at full load. If you use excessive primary turns then you will decrease the core losses and increase the copper losses, moving away from the lowest TOTAL loss point.

author: oldway
author_rank: Super Contributor
timestamp: December 05, 2014, 12:16:06 pm
content: For profissional use, there is a great software for designing transformers (also ferrite) and inductors.http://www.rale.ch/

author: BennVenn
author_rank: Regular Contributor
timestamp: December 07, 2014, 09:23:20 pm
content: Thanks for the help everyone!I'll design the primary around peak expected core temperature with a healthy safety margin. I've read most ferrite cores will never fully recover from excessive temps so thermal protection will be included in the control circuitry.And thanks for that link, looks like it will help me out a lot.

author: BennVenn
author_rank: Regular Contributor
timestamp: December 13, 2014, 04:45:22 am
content: Is core heating just part of the design?I've set up a crude TL494 circuit to supply a non regulated 38khz push-pull output with 5% (default) dead time. The core I am using is a PC40 Ferrite toroid T51x13x31 (http://www.ing.unp.edu.ar/electronica/asignaturas/ee016/anexo/l-tdk-ferrite.pdf) For a calculated Bmax of 1500 (Conservative?) the formula resulted in 4.8 turns primary. For the purpose of testing I wound 6 turns bifilar with no secondary. I shorted half the primary and measured the leakage inductance to be 0.150uH.Total circuit current is 140ma at 14.4volts. Current consumption with the transformer removed is 53ma. I expect a percentage of the total current to be consumed by switching the gate capacitance.Even with excessive turns on the primary to reduce Bmax, the core still consumes 1.25watts. Referencing the datasheet, core losses are a result of a few factors including Bmax, frequency, temp and most importantly core volume (which will be quite a few times greater when I build my 1KW inverter) Given the extra turns I have wound, I will reduce the frequency to see how that influences losses as the datasheet shows this has the largest effect.Is 2 watts (1.25w core) a reasonable loss for a 200watt system? My 1KW inverter will be used in an EV so efficiency is important. Should I be shopping for a particular core material or just accept that there will be unavoidable losses? In which case go for a smaller core and run it closer to its maximum SOA?Gate drive looks fine on the scope, as does the drain waveform. Mosfets run cold, core is about 30degrees, 10degrees above ambient after running for 10min.

author: Someone
author_rank: Super Contributor
timestamp: December 13, 2014, 05:15:16 am
content: Quote from: BennVenn on December 04, 2014, 11:13:41 pmHigher turn counts will reduce the copper area in the window due to the minimum clearance between adjacent tracks (assuming a PCB core here).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 13, 2014, 08:04:39 am
content: 2W in a core that size is pretty reasonable. Temp rise will be higher once you get wire and load current on there, so you may want to cut back a little yet.Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: December 13, 2014, 08:12:58 am
content: Thanks for your time to reply evb149.I'm working my way through those links, the 720W full bridge converter was an interesting read (97.1%!!!). I'll attempt synchronous rectification in my design, it seems quite a bit of efficiency can be gained there when working with higher currents.The formula I used to determine primary turns did not consider primary current, only voltage, target Bmax, frequency and a few constants. This had me stumped too, as its quite obvious that field strength is a function of turns and current, voltage is irrelevant. Though these calculations are proven and I figured the lack of current in the equation is due to the known Bmax and Voltage. I'm going to back a turn off and check the core loss just to confirm.I'm not chasing 99% efficiency, I'd be happy if I hit above 90. My test circuit doesn't even have a secondary yet so the losses will accumulate as it progresses. If I can keep each stage as efficient as possible I might just hit my 90% target.Toroid is what I had available as much of a pain it is to wind. I'll grab a few EI/EE cores to test too. I'm leaning towards toroid though as heat dissipation from core and copper would be greater.I had planned on Push-Pull but after reading that TI Appnote I might try full bridge. This is more of a learning experience for me, I could go and buy a COTS unit and probably save a few $$$'s but teach a man to fish...Thanks againedit - I've taken a turn off, now total current is 167ma. Effectively added 0.4watts more heat to the core. Though I'm sure I'll gain that back in copper efficiency when the secondary is loaded

author: BennVenn
author_rank: Regular Contributor
timestamp: December 13, 2014, 08:17:10 am
content: Quote from: T3sl4co1l on December 13, 2014, 08:04:39 amHi Tim,I read in that datasheet that core losses will continue to drop as the temps approach 60-70 degrees, and then jump right back up. It will be interesting to see how low it drops when I get some load on the secondary and put some heat into it. After running for an hr with no secondary, the temp still hasn't risen past 30 degrees so I suppose it is at an equilibrium.