Phase-shifted full-bridge output asymmetric oscillation problem - Page 1
Forum section: electronics / general

author: heroxue
author_rank: Newbie
timestamp: January 11, 2024, 02:47:02 pm
content: Hello Everyone，Recently, I referenced the official TI circuit diagram to design a phase-shift full-bridge circuit with a total power of 6000W,Cf=4700uF*10=47mF, Lr is the leakage inductance of the transformer, is 1.4uH,the transformer turns ratio is 10:1. It seems to work fine when there is no load on the output. However, when a light load is applied ((Vin = 60V,output = 3V,Phase shift approximately 50%，Load from a few watts to tens of watts), the transformer secondary starts to exhibit asymmetric oscillations, and the oscillation amplitude increases as the load power increases. I have tested the following:1.Tried removing the output freewheeling inductor Lf (1.6 uH)，and there was a significant improvement in output under light loads (from a few watts to tens of watts).2.When paralleling a RC circuit (R = 40 ohms, C = 1000 pF/0.1 uF) with the output Schottky rectifier diode, there was a slight reduction in output spikes. However, after increasing the load, the amplitude still increased.I would like to ask everyone, what could be the possible reasons for the above issues?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2024, 03:41:27 pm
content: Most likely, pulse widths aren't perfectly symmetrical. Lack of DC bias correction (feedback) or coupling capacitor, typical symptom. Current probe on the primary is diagnostic.I must say, I am not thrilled by the combination of experience/knowledge and power level here. Or, surely six megawatts is a typo (but that's also kind of proving the point--?). Even if a 6kW converter fails catastophically, from design issues you weren't aware of, such power can easily kill a person. If a 6MW converter fails catastrophically, that's enough to kill several, and take out additional infrastructure in the process. Is that a risk you're willing to take?I can't meaningfully tell you to go away, or give up what you are doing, but I can at least beg of you, study your ass off, understand all details of operation, current flows, power flows, loop dynamics / strays / parasitics, control systems, protection systems, and work hard to perfect your craft before something bad happens. Preferably these skills are all developed at low power (you can do current-mode control and desat protection at mere watts, and loop dynamics make themselves known at... 10s, 100s of W? single W even, if you're using GaN, these days), and scaled up by modest factors at a time until one fully understands a converter of this scope. But it can be done by force (and hopefully without costing buckets of components---or worse) at the kW scale, I suppose.Tim

author: heroxue
author_rank: Newbie
timestamp: January 11, 2024, 04:01:08 pm
content: Thanks for Tim's reply. Thank you very much for the kind advice.The above is a clerical error. I have corrected it to 6000W. I finally want to achieve 28V@200A output. Yes, Maybe I should try to achieve 100W power first，the above waveform is output in open loop (no feedback control is added). Currently, the transformer primary is not Add a coupling capacitor, I will try to modify the above conditions (add a coupling capacitor or add a feedback control) and test again.Kun

author: temperance
author_rank: Super Contributor
timestamp: January 12, 2024, 02:33:23 am
content: In the second picture is this measured at the transformer? If so, there is a problem with the gate drivers.To be blunt, how are you going to achieve the power level you state if you can't diagnose this experimental open loop test delivering only 10 W all by yourself? Maybe in five years or ten years you might be able to reach this kind of power levels. Do you have an idea with kind of problems you will have to solve? At some point posting your problems here will not bring you meaningful answers. It's much more complicated and solving those problems is hard, darn hard at times.

author: Faringdon
author_rank: Super Contributor
timestamp: January 12, 2024, 08:25:46 pm
content: You shoudl not remove the output inductor.Do you want LTspice sim of PSFB?...i can send if you want.You do at low power level....you probably in discontinuous mode...it normal to have oscillating pri and sec coils like you show when in light load and discontinuos current.Yes you need rc snubber on schottkys.....are they rated for the voltage and the spike?...schottkys cant take much overvoltage...they delicate.You got primary side current limiting in there?..i hope so...and you have output feedback?Send picture because to show you have good pcb and are not on breadboard etc...you need decent pcb layout.The attached shows LTspice for PSFB.The thing about PSFB, is that you can turn the fets on superfast and not get emc issues....and you can also turn them on super slow and not get switchiong loss issues...because the diode of the fet is already in conduction. With a Plain Full Bridge, you need to staunch the fet turn on, and when you are doing 1kW+, then it just means you get too much switching loss.However, the secret of PSFB is being able to use a big enough capacitor across the fets (and knowing how big to make it) so that you can turn them off superquick.If you dont use caps across the fets when at high power , you are not doing a PSFB properly.The other use for PSFB at low power, is when you just rig it up with no caps across the fets and no rail diodes, for say 250W-700W PSUs, because it has fixed 50% duty and so you dont get sudden duty cycle change problems in the gate drive transformer....but this isnt the "well known" use for a PSFB.....what you are doing is just getting a plain full bridge, and shovelling a Phase shift controller in there instead....you arent particulalry bothering even to have a high leakage inductance in the transformer.The caps across the fets in PSFB are the reason you need to turn the fets on slowly,,,,because at light load you are discharging them through the fets.Really what you want is turn-off snubbers with PSFB.Pretty much everyone is using Sic fets with PSFB though...because there are otherwise serious reverse recovery issues waiting and lurking in the wings. Or you can use GAN with Sic pllel diode.

author: analityk
author_rank: Regular Contributor
timestamp: January 14, 2024, 12:51:34 pm
content: When I worked under psfb I based on this app note:https://www.ti.com/lit/ug/tidu248/tidu248.pdf?ts=1705051768269&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTIDM-PSFB-DCDC

author: Faringdon
author_rank: Super Contributor
timestamp: January 15, 2024, 07:24:39 am
content: Thanks, thats an interesting app note....strange gate drive for a PSFB though...PSFB is slow on and lightning fast off....their gate drive doesnt show being adapted for that.Slow on, because its got 0v across fet when you turn on so slow on is fine and less noisy.PSFB does nothing to reduce turn off switching loss.......so you need lightning fast off and turn off snubber.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2024, 04:24:26 pm
content: They're using gate drive transformers... the gate resistors likely are irrelevant to switching speed.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: January 16, 2024, 12:31:33 am
content: Bonjour Simply by publishing an app note , ("official TI circuit diagram") TI is demonstrating just one approach to a design, but this is not necessarily the simplest nor optimum topology choice.What are advantages of : Phase-shifted full-bridge over a conventional full bridge or double forward?At 6 KW I would use resonant design or other soft switching topology.You will need a 240V or 3 phase mains bus at 6 kW.Jon

author: temperance
author_rank: Super Contributor
timestamp: January 16, 2024, 01:05:22 am
content: @ jonpaulThe PSFB the OP is building is a ZVS converter and he did build something because he attached pictures showing measurements taken with an oscilloscope.