Strange Voltage Signal at the output of a Full Bridge Converter - Page 1
Forum section: electronics / projects

author: Jad_Electronics
author_rank: Newbie
timestamp: August 02, 2021, 03:01:00 pm
content: Hello Everyone,So i built a phase shift full bridge converter( isolated Buck converter) that is supposed to convert a rectified DC signal down. The voltage at the primary is switching at 60KHz, and using a synchronous Rectifier at the output (secondary side), I wounded a toroidal transformer with at turns ratio Nsec/Nprim = 0.331.Testing at 30 VDC at the bridge for a start, i observed a strange waveform at the two secondaries, i expected a square wave signal, instead it has peaks, and the peak is more that i expected (should be 10V).What could be the cause of this resonance behavior?Toroid primary inductance is 5.6mHCH3 is using an active differential probe across the primary, all other channels are referenced to the ground of the secondary side.I drew in orange and brown on top of the Oscilloscope snapshot, the signal form which i would have expectedAny suggestions,Thank you!

author: sperb
author_rank: Contributor
timestamp: August 03, 2021, 01:01:39 pm
content: Hi.Your primary side measurement is differential. Your secondary side is not. So if you want to see the secondary winding voltages, you need to do Vch1 - Vch2 for the first secondary and Vch2 - Vch4 for the second secondary.Or if the circuit allows, put the probe grounds on the center tap and measure each secondary with one probe (be aware of your grounds when probing).

author: strawberry
author_rank: Super Contributor
timestamp: August 03, 2021, 06:52:08 pm
content: secondary drain peak voltage should be double or 20Vpush pull for reference

author: TurboTom
author_rank: Super Contributor
timestamp: August 03, 2021, 07:09:37 pm
content: Quote from: strawberry on August 03, 2021, 06:52:08 pmYes, that's correct. And the funny shape may be the result of a resonance across the snubber networks. Did you actually load the converter's DC output? If not, you may want to try that.

author: Jad_Electronics
author_rank: Newbie
timestamp: August 08, 2021, 05:02:30 am
content: I measured the secondaries referenced to the center tap, and i measured the primary winding current Attached is the result.CH1=-Vsec1CH2: Vsec2CH3: V primaryCH4: I_PrimaryMaybe the secondary us resonating, it should stay at Vprim x Nsec/Np till the Primary voltage drops to zero, thus have a rectangular waveform. The output is loaded with an 16 Ohm Resistor, no snubbers at the Synch. rectifier MOSFETs are assembled.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2021, 05:33:00 am
content: What is the transformer nominal V/I rating, and what is the winding schedule?Have you done anything in regards to leakage inductance?Tim

author: TurboTom
author_rank: Super Contributor
timestamp: August 08, 2021, 08:58:30 am
content: I agree with Tim. The linear slope of the current when the primary is changing polarity is a clear indication of too much stray inductance between primary and secondary. You may want this in an LLC converter, but a forward converter like this is (suposed to be?) needs a tightly coupled transformer with secondaries wound in parallel and the primary split in two, one section inside and the other outside of the secondary.But the strange waveforms may also be related to an improper control of the active rectifier. You may try to replace the MOSFETS just for a test with two Schottkys. If the body diode of the mosfets gets to conduct some current (since the gate switches off too early), they may actually need too long to block when the opposite polarity half-wave comes along, leading to a temporary "short" in the secondary.Just noticed: these transistors that you specified in the sync rectifier are some ultra-beefy 650V types with fast body diodes that shouldn't cause this behavior, at least not at high currents. Did you actually use the SiHG039N60EF transistors or are they only inserted as a replacement to provide the TO-247AC casing?

author: Jad_Electronics
author_rank: Newbie
timestamp: August 09, 2021, 08:12:27 am
content: Thank you for the feedback guys.I winded the turns by hand on a toroidal core (T107/65/25-3E27 from Ferroxcube) It is "supposed to be" 4KWatt transformer. I have some leakage inductance (which i can measure), and surely some stray capacitance between primary and seconday windings.Nsec/Nprim = 0.331.L_primary= 6mH.N_Primary= 30 turns.L_Leakage = 16uH.I wouded the primary first across the whole length of the core, and after adding an isolation tape on it, it wounded the secondaries on top.At 4KWatt, Zero voltage switching (ZVS) at the H-bridge (Phase Shift Full Bridge scheme) is desired, that is why i didnt feel bothered by some leakage inductance to achieve that.I will try it with two passive rectifying diodes at the output, thank you for the idea..I do use those big Vishay MOSFETs, on both sides of the transformer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2021, 02:47:33 pm
content: What about the leakage between secondary halves? And, what effect would that have on circuit operation, have you simulated different values?Tim