Two Transistor Forward vs Two Transistor Flyback - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: February 11, 2024, 02:16:58 pm
content: Which do you prefer for 125W (as below), Two Transistor Forward, or Two Transistor Flyback?(comparison LTspice's and PNGs attached)The Two Transistor Forward ("2TFor") is preferable over the Two Transistor Flyback ("2TFly")for the following spec....Vin = 200-373VDC (155-265VAC with three diode, three phase rectifier, and smoothing capacitor)Vout = 24VW(out) = 125WThe attached shows PNGs and LTspice's of the two converters for comparison.The 2TFor is preferrable because ....1...It does not need a gapped core transformer. (sometimes difficult to find in stock)(It can use a gapped output inductor core, but thats an easy part to do a multi-footprintfor, so that you dont suffer nil-stocks. Also, the output inductor of the 2TFor can be donewith a torroid, eg, a sendust torroid, which has no gap.) This means the that magnetics for the2TFor will be less likely to ever being found in a state of nil-stock, since the 2TFor can be totally donewith non-gapped magnetics...The Other Flybacks (Single FET) which take the isolated 24V as an input, and then give either a 24V or 20V isolated output, do use gappedcore PQ26/20's, but they could, with some extra components, be converted to One Transistor Forward convertersif it was wanted, and then none of the magnetics would need to have a gap. At this point, i have left the 24V converters asflybacks , since they take less room like that.2...The 2TFor has significantly Less Secondary RMS current in the transformer, compared to the 2TFly.3...The 2TFor has Two output diodes, but thats good because they share the dissipation between them. Unlikethe flyback where a single output diode takes all the dissipation. You can parallel diodes in the 2TFly,but doing so puts you in all the doubts about matching and sharing of current between paralleled diodes.4....The 2TFor has significantly Less ripple requirement for its output caps (approx 4.5 times less).This means the 2TFor can have a smaller output capacitor bank than the 2TFly.5...The 2TFly, Needs its input voltage to be significantly greater than Vout x Np/Ns. The difference between Vinand "Vout x NP/NS" is the voltage needed to discharge the leakage inductance current back down.6...The gap in the 2TFly's Flyback transformer causes a certain amount of fringing field heating of the coils on the former.This doesnt happen with the 2TFor because the core, advantageously, has no gap.7...Both the 2TFor and the 2TFly could be used down to 90VAC. The one done here has been done with a transformer which isgood down to 155VAC. It could simply be re-wound with a greater Ns/Np to be able to handle down to 90VAC. Since the applicationis only going to be used for European mains input, it seems pointless to wind for wide range mains, since it would mean lessadvantageous sharing of the output current (and hence sharing of the dissipation) between the output diodes of the 2TFor.

author: jonpaul
author_rank: Super Contributor
timestamp: February 11, 2024, 02:22:21 pm
content: ARGGH FTTS, AGAIN!!!So answer isNEITHERYIKES give us a break,As mentined in your many sismal posts USE A SIMPLE FW OR FBRead a book , read a tech paper, attend a seninar on basic SMPS toplolgy, deisng and application.IEEE, PESC, etc.j

author: Faringdon
author_rank: Super Contributor
timestamp: February 11, 2024, 05:21:24 pm
content: Thanks, though i appreciate a one transistor forward or a one transistor flyback could be hacked up to this.....the dissipations in the individual semi's would be larger, and we dont have much room for heatsinks.One transistor forward gives us the issue with [2Xvin + snubber voltage] across the primary fet after its turned off.One tran flyback, ayk, gives issue with primary clamp loss, and all 5A output current in the single output diode.