It sounds as though you're trying to impose one resonant frequency where your various converters have different resonant frequencies.
I wonder if you ruled out the idea of interleaving several buck converters? That ought to make your project easier to design. You can apply an equal duty cycle to all the converters. It's not terribly serious if one inductor is an oddball value. With the right combination of logic gates you can make a control circuit which staggers 9 On-Off times, and alter duty cycle with one adjustment. Output has little ripple so that you don't need an output capacitor. The initial power supply has a steady input current draw.
And considering the amount of Watts going through your system, it might be wise to keep the outputs of your converters separate, so that if one malfunctions it doesn't affect the rest of the system as much.
To parallel effectively, you must have a master voltage feedback controller which dictates the average current from each LLC. Each LLC should have its own inner current regulation loop.
Frequency locking LLCs is tricky. TI app notes suggest using some duty cycle modulation to implement current sharing. Never tried it though.
The assumption is, with rectifying, that you have very large o/p caps to give a stable Vout - without this the paralleling you are seeking cannot succeed.
- - - Updated - - -
You are far better off going to a different topology with current mode control on the primary - and current doubler on the sec ... paralleling then becomes easy ...
We have paralleled 5kW units at 200A each to 1600A DC this way ...
Why I insist on LLC:
1- Severe oscillations on the secondary rectifiers.(especially at light load). Need heavy snubbers to damp. Yes but its only low volts so the snubber losses are OK.
Yes, it is manageable, should not be a critic problem.
2- Need 100V Mosfet or diodes for 10V output and 150V-200V class Mosfet or Diodes for 15V output, which raises cost significantly.(especially for 2700A output), LLC variant can be implemented with 40V Mosfet or diodes for 10V output.LCC have very calm, clean and natural secondary. Current doubler needs only 50V fets
I want to avoid active snubbers to reduce complexity and coponent count, so I am not sure if a simple RC-snubber will be enough for no load or light load condition
3- LLC half bridge can be implemented with capacitor-diode clamp current limitation, which adds hardware based short circuit protection. This arrangement is a very effective protection for switching semiconductors. Similar structure is hard to implement for PSFB - current limiting is easy for PSFB a CT in the input lead and/or in series with Tx
I agree ,but I was talking abaout a naturally and analog current limiting scheme that does not need any digital control. I am not sure if it is possible to implement a fully analog clamp for current limitation for PSFB, the only solution I may think is to limit the power capability of the transformer ,and using overdimensioned semiconductors which can deliver this current saturating current (in case of a secondary short)
4- Half bridge is effective, no need for full bridge, less cost improved efficiency. You can have half bridge parallel loaded resonant converter with current doubler o/p 100kHz to 200kHz
Thank you for the idea , I will investigate parallel loaded resonant converter with current doubler o/p 100kHz to 200kHz
5- Need for an expensive (10uF-22uF range) decoupling flux balancing capacitor in series. Not with current mode PSFB
Unfortunatelly I hate fast current mode control, I think it relies on precision measurement of current through ADC.This may make the things more complex and unreliable. It tend to use voltage mode,
- - - Updated - - -
Regrettably you will never get LLC to work in parallel the way you want - unless all the stages are identical with the same gate drive and you parallel the AC as per the other posting on this subject ....
Post #5 shows a parallel connected LLC with output capacitor, using this topology, there's no need to synchronize the converters. Load balancing may be necessary for variable frequency (voltage varying) LLC, in case of fixed frequency LLC in series resonance you can expect fair balancing without controller.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?