Two transistor forward with caps across output diodes - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: June 16, 2023, 07:08:15 pm
content: Hi,The attached LTspice sim and schem has 10nF caps placed across the output diodes. This massively reduces dv/dt and di/dt in the secondary (and primary) side power circuits. There is no disadvantage of this. As you can see, there is a sinusoidal ring in the transformer currents, (due to the caps ringing with the transformer leakage L) which is some 10Apkpk in the secondary….but this is matched by the same referred ring in the primary, and thus the B field from this cancels in the transformer core…so no hysteresis heating.Why don’t all 2 Transistor forwards feature these diodes?, which make EMC much easier to pass.The 2 Tran Fwd is 390Vin, 400Wout and 40VoutIt also shows that its best to not interleave wind, as the leakage is handy for the L in the "ringing LC"

author: Gyro
author_rank: Super Contributor
timestamp: June 16, 2023, 09:39:13 pm
content: Are you serious or is this just your usual weekend trolling? You've really never heard of snubbers across secondary side rectifiers?Instead of plain capacitors, thenormalmethod is to provide damping of the ringing using series RC snubbers.P.S. You haven't specifiedwhatrectifiers you are using, Schottky? Fast? Fast soft recovery? IN4001s? ? ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 17, 2023, 05:26:48 am
content: A more accurate model of the transformer would show the increase in power dissipation* due to AC resistance being much higher at the ringing frequency, and also the increase in EMI due to high currents in the inductive loops around the transformer, and mode conversion around the primary and secondary.*Or rather the displacement of power dissipation, from the resistor in the snubber case, to the transformer and other components.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: June 17, 2023, 08:55:49 am
content: QuoteThanks, good point, but ayk, they can be overly dissipative in the R.QuoteThanks, i assume you mean the skin effect losses in the transformer windings....so yes, we have to make those windings good for handling that...use Litz, etc....and ayk, in many ways, you'd be OK with transfering some of your heat out of your snubber resistor and into your copper....ayk, when you also need to heatsink the snubber resistor, then thats a nuisance for cost and board space.Ayk, the good thing, is that those high frequency ringing currents , are not going to cause transformer core hysteresis losses, since their pri and sec referred fields, cancel out.QuoteIf you remove the "big" capacitors across the diodes, and instead use RC snubbering, then you have much more jagged waveforms of current and voltage around the transformer, which surely will be worse for EMC?QuoteThanks, you mean diff mode currents being changed to common mode?.......again, i would point to the above concernign the less jagged waveforms when the "big" capacitors are used.QuoteThanks, ultra fast

author: jonpaul
author_rank: Super Contributor
timestamp: June 17, 2023, 03:56:33 pm
content: FTTSYou posted this same garbage again and again....HAVE A GREAT DAY give us a break, its the weekend!Jon

author: Faringdon
author_rank: Super Contributor
timestamp: June 18, 2023, 12:29:05 pm
content: Thanks, but what you see in the top post, is the "resonant-isation" of a hard switched topology by the fact of having a transformer with leakage inductance, which is able to ring with "stray" capacitors, and thus give an "EMC-quiet" solution.....this is precisely the reason that hard switched SMPS at high powers need to be transformer isolated....even if they need no isolation....because they need this resonant-isation............the big exception is PFC stages........they are hard switched and non transformer isolated...but...they now use SiC FETs and diodes to reduce reverse recovery........also they cant so easily be done with transformer isolation , as the Vin goes from zero to some 340V....and it all gets a bit more complex if they use a transformer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 18, 2023, 12:48:37 pm
content: The proof is in the pudding.Show efficiency and EMC measurements. This has been out to the test lab, right?Tim