Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Gapping a Two Transistor Forward Converter Transformer

Status
Not open for further replies.
Z

zenerbjt

Guest
Hi
This concerns gapping a Transformer for a Two Transistor Forward SMPS. -And why a gap is needed!

Previous discussions have always generally concluded that a gap was not needed…
https://www.edaboard.com/threads/gapping-a-ferrite-core-for-full-bridge-smps-transformer.341980/
https://www.edaboard.com/threads/tw...verter-with-bootstrap-high-side-drive.340530/
https://www.edaboard.com/threads/push-pull-start-up-issue.348624/page-2
**broken link removed**

However, page 21 of DER-472 shows that a gap is needed in a 300W Two Transistor Forward. The second picture down (pg21) shows a gap being made with 0.02mm tape. The reason for the gap is given at the bottom of page 31. It says that the gap must be present so that the “no_load resonant frequency Between L_PRI and C_PRI does not get too low”. -Presumably , there is a problem if this resonant frequency becomes the same as the switching frequency (??)

DER-472
https://ac-dc.power.com/sites/default/files/PDFFiles/der472.pdf

Does anybody know why the primary resonant frequency, as discussed above, is so important, such that a gap is needed?
 

i don't know the specific answer to your question, however

if the driving frequency (i.e. the switching frequency) is the same as the resonant frequency,
the amplitude of the oscillation increases without bound, until destruction

see "galloping gertie" (i.e. the Tacoma Narrows Bridge, Tacoma Washington USA)
 
  • Like
Reactions: zenerbjt

    Z

    Points: 2
    Helpful Answer Positive Rating
Thanks, but the attached is an LTspice simulation of a two transistor forward with a 395pF capacitor across the primary, meaning that its resonance frequency with the primary is at 80khz...exactly the same as the switching frequency...however, this has no bad effect whatsoever.
(LTspice is a free download)
 

Attachments

  • 2TFC _Pri Resonance.zip
    1.9 KB · Views: 116

No gap is required if there is a peak current limit on the pri side, and a PWM limit, e.g. 48 %
 
The said "primary resonance" circuit has high impedance and respectively low stored energy, I don't expect consderable effect on the converter operation. The main reason for setting a certain magnetizing current in the forward converter transfoemer is to support the flux reset during flyback phase and reduce the residual flux.
 
No gap is required if there is a peak current limit on the pri side, and a PWM limit, e.g. 48 %
Thanks, yes i would have thought the same....but HiperTFS controllers have " Transformer reset control, prevents saturation under all conditions "
(as per page 1 of datasheet)
...and also have a primary current limit (despite being voltage mode controllers), so i am surprised that they gap the core.

They have also a PWM limit, but its slightly above 50%, since they have a higher_then_vin voltage to reset the core.

HiperTFS controller
--- Updated ---

Hi,
Page 27 of DER368 shows why a gap is needed...to avoid pulse skipping of the high side driver
I believe kind contributors here actually pointed to this
 
Last edited by a moderator:

presumably the pri current limit + the pwm limit = PI's " Transformer reset control, prevents saturation under all conditions " ....
 
  • Like
Reactions: zenerbjt

    Z

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top