Difference between flyback and single forward? - Page 1
Forum section: electronics / projects

author: seyedsaeed
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:41:09 am
content: Hi.I want to design a 500W switching power supply, but I don't know which to choose between flyback and single forward.Input = 220 VacOutput = 30V, 15AI mostly agree with single Forward.

author: moffy
author_rank: Super Contributor
timestamp: December 23, 2023, 07:45:48 am
content: For 500W I don't think flyback or forward topologies are appropriate:https://au.mouser.com/applications/power-supply-topology-forward/

author: Andy Chee
author_rank: Super Contributor
timestamp: December 23, 2023, 08:25:00 am
content: I'd go with a half-bridge design. ZVS would be nice, but hard-switched should be easier to get going.

author: seyedsaeed
author_rank: Regular Contributor
timestamp: December 23, 2023, 09:37:57 am
content: MEAN WELLNES-350-36single forward

author: moffy
author_rank: Super Contributor
timestamp: December 23, 2023, 10:59:11 am
content: Quote from: seyedsaeed on December 23, 2023, 09:37:57 am350W, I thought you wanted 500W?Look at the schematics for PC PSUs that is similar power and output voltage can be tailored by the TX.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2023, 01:25:21 pm
content: Two-switch forward and LLC resonant are most common in this space, I think.Flyback is discouraged by its high input and output ripple currents, large magnetics requirement, and the criticality of low leakage inductance.Half-wave forward has similar limitations to flyback, and adds an output inductor besides.Two-switch forward resolves the leakage issue, and greatly improves EMI.Half-bridge (full wave) forward could improve on magnetics utilization, but this doesn't amount to much, as flux density is driven by core losses rather than saturation, at least for operating frequency above 40kHz or so.LLC improves on forward by bringing soft switching, lower EMI, and potentially eliminating the output inductor (but a primary resonant inductor may be necessary; sometimes it can be integrated in the main transformer is all).Magnetics can be reduced by raising the operating frequency -- though in a cost and efficiency optimized design, it's quite hard to push above the EMI lower limit of 150kHz, even today. Better semiconductors (SiC, GaN) are more expensive, and can be quite a bit harder to use (few controllers designed for them as yet; faster edge rates = more critical layout; smaller SOA = challenging to design for robustness; etc.).As for which oneyouwant to make, or are able to make -- we have no idea, but you might want to keep it simple if you aren't very experienced at these. Or else, reserve plenty of time to familiarize yourself with new topologies, and budget to cover for burned components and multiple board revisions.Tim

author: seyedsaeed
author_rank: Regular Contributor
timestamp: December 24, 2023, 05:51:19 am
content: Quote from: T3sl4co1l on December 23, 2023, 01:25:21 pm5 years ago I made 100 watt single forward and 7 years ago 500 watt half bridge but in half bridge the problem is pulse transformer.I want to know why it is not possible to make 500 watts with single forward?

author: Andy Chee
author_rank: Super Contributor
timestamp: December 24, 2023, 06:13:46 am
content: Quote from: seyedsaeed on December 24, 2023, 05:51:19 amSingle switch forward converter for 500 watts is possible, but the problem is transformer size and output inductor size.You are the designer, so you have to choose whether this is acceptable.Also, newer half bridge designs do not use pulse transformers, they use dedicated half bridge drivers with bootstrapping for the high side gate supply. e.g. IRS2112https://www.infineon.com/dgdl/ir2112.pdf?fileId=5546d462533600a4015355c81cb71685

author: seyedsaeed
author_rank: Regular Contributor
timestamp: December 24, 2023, 08:35:04 am
content: Single switch forward converter for 500 watts is possible, but the problem is transformer size and output inductor size.You are the designer, so you have to choose whether this is acceptable.Also, newer half bridge designs do not use pulse transformers, they use dedicated half bridge drivers with bootstrapping for the high side gate supply. e.g. IRS2112https://www.infineon.com/dgdl/ir2112.pdf?fileId=5546d462533600a4015355c81cb71685Yes, I have used IR21xx before, but I doubt if IR21xx is safe in industrial design and I am worried.It is said that in an ideal design, the use of single switch forward does not cause problems at 500 watts, but in practice, problems such as saturation and... may occur?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 24, 2023, 09:04:29 am
content: With a single switch on only uses part of the transformer magnetization range ( half minus the remanece) and thus needs a larger transformer.The other problem that there is much more ripple current to deal with, both at the input and output.If one really don't wants the high side driver or gate drive transformer, one could also use a push pull version with a split primary winding instead, though this has additional voltage spikes and is less attractive when starting from a high voltage - it is fine when starting from some 12 V.Flyback and single forward are different in which half of the output wave they use. For good operation they want a different transformer. If needed one can combine the 2 parts and also use the forward part in a flyback design. The forward part would give an additional opposite sign (if the same winding is used) voltage that follows the input voltage, so no real regulation there, but additional power.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2023, 12:19:45 pm
content: Quote from: Kleinstein on December 24, 2023, 09:04:29 amIF the limitation is saturation. I mentioned this in my post above; most applications are above 100kHz and core loss limited, and so half/full wave is irrelevant for core size purposes.This leaves the advantage to full-wave of a longer conduction angle, i.e. better copper utilization, but this is a less proportionate savings.Actually I suppose they're equally proportional, from one point of view: both are direct factors into the area-product design procedure. Just that each one individually has a minor impact on overall design, all things considered (the overall design is determined by the geometric average of factors, you could say).Copper utilization itself is a sub-proportional factor though (RMS goes as sqrt(D)), so if you're coming from ampacity or temp rise/efficiency, it's still correct to say this.QuoteYeah, both flyback and one-switch forward suffer not only ripple current, but extra voltage as well. Evidently this is an industrial application, perhaps the supply voltage is quite high (600VDC+?) and flyback / demag would be undesirable (necessitating 1200V+ transistors). Two-switch forward/flyback, and half-bridge, require switch peak rating modestly above supply, making them attractive at high voltages.QuoteWell you wouldn't hard rectify it, that blows up transistors. But you could put it into another series inductor. I'd have to think about how to wire that while still taking flyback. Let me see here. Take the usual two diode common-cathode (into choke) arrangement; one anode comes from the winding, other from GND. Swap the winding and its series diode, now they're a common-anode pair coming from GND. Now the winding is clamped to GND on the low side when driving forward, and the high side goes up, pushing current into the choke; during flyback, the choke is clamped to GND on one side, and the high side of the winding as well; the low side of the winding flies back, going positive and now it can be clamped into the output capacitor. Aha, that would do it.The resulting operation is still PWM controlled, as you have normal forward operation in one direction, and flyback timed by transformer magnetization in the other. The relative balance of forward and flyback power levels is controllable by varying that inductance; the balance also varies with load (V or I), I think, and you can only make the DCM-CCM transition track at one particular voltage. What else... output voltage is at most half the forward peak voltage (in forward CCM; it can be more in DCM), which forces flyback to be half the voltage and twice the time (flux balance condition), so you'd get this odd 33/67% waveform forced by this condition, but also forward CCM would have an even higher ratio in that case (or maybe it simply wouldn't reach CCM at all).Alternately, we could center-tap the secondary and draw flyback from that point, getting the flux a bit better balanced between the two modes.Tim