1kW LLC converter

[chas1]

After completing a fullbridge and halfbridge converter and using them to power my amp projects, I continued to investigate other topologies to overcome the disadvantages of the above:
1.EMI
2.Large inductors on outputs
3.Difficulties caused light loading
4.Hard switching and others

I found this forum's help essential to the success of my projects and I am again seeking your inputs and comments.

A big thanks for all help in the past ,I have been bit by the promise of the LLC converter and I want to scale one to a kW.I have read many appnotes from Fairchild,Onsemi and others and one thing stands out,PFC frontend required.. therfore a 1kW PFC will be needed for reliable operation.Fairchild,onsemi and STmicro have controllers dedicated for this along with excellent appnotes and tools.(AND8257,AND8255,AN2450 and AND8311/D.I will start with the LLC design first and to open the project I would like to present a simulation of a dual output llc using LTspice(free spice simualtor).The attached file set includes all models and libraries for the simulation.Included is a single output sim(see appnote AND8255).

[luka]

I will see what you did over the weekend

Why is PFC front end needed?

[Cristi]

Some of the reasons why the PFC stage is required, besides the need for a high power factor, and universal mains input is that the converter efficiency depends on the supply voltage, being maximum at 400-450V level, since the switching losses are very low, and the conduction losses are predominant. another reason is that the PFC front end provides a pre-regulated DC voltage, with the associated 100/120Hz ripple, much lower amplitude that in case of direct mains rectification, which allows to design a converter with lower gain, tipically in range of 1.2-1.3, lower than the other case, where need 1.4-1.5 in this way, m can be increased, Lm becomes larger, this resulting in less circulating current, and less low-load losses.
The maximum power level which i could get using LLC converters was up to 2.8KW with the standard Half-bridge LLC converter and up to 6.4KW with full-bridge topology.

[luka]

ok that is what I was thinking, asked anyway.
So this supply works at locked duty, but changes freq. and that's the way you get regulation?
What IC will be used for design?

[chas1]

Luka

The operation from line and pfc can be simulated,but first what is needed IMHO is a gold standard spice simulation based on an appnote with waveforms this is why I picked AND8255 but Cristi is correct the pfc does allow for a more robust design and also for most of us 2kW should be more than sufficent,another draw back with this topology is most designs I have encountered are designed for a fixed output voltage,load can vary. (xfmr turns ratio).The other is transformer construction that is recommended is intergal magnetics(sectional bobbin)which I plan on modifying a ETD core with a 3mm spacer one for Lp and the other for Lm. My thoughts are a L6599 control IC feeding an IR2181 that drives a simular mosfet used in the simulation.I will post schematic along with design equations used later.

[chas1]

Luka

Yes as the load changes the the freq will lower or raise depending if the load moves toward zero or open(no load) this is the major advantage for audio amps at no load regulation is tight,however high currents can be present but to protect against this the controller mentioned as most other can be programed to go into skip cycle or shutdown PFC.

[luka]

what about core design? I'm getting used to, that freq is constant.
What about here, how do you design it, I think saturation is still possible here, right?
And no output inductors? how is this better then with normal HB? coz of the rectified sine, that amplitude is changing instead of duty

[chas1]

A very good online seminar,I only hope this link works you might have to register.

"+playertitle+"

[chas1]

Luka I checked link, ok.Watch this when you can.Also the design will be posted this weekend.Ripple is vey low under all load conditions.. (link is playtitle)

[Cristi]

Originally Posted by chas1

another draw back with this topology is most designs I have encountered are designed for a fixed output voltage,load can vary.

the topology doesn't allow to get a large output voltage range, is limited at max. +-10-15% of the designated value. but it does allow to get constant output voltage from zero load to maximum load, and still keeping ZVS.

Originally Posted by chas1

(xfmr turns ratio).The other is transformer construction that is recommended is intergal magnetics(sectional bobbin)which I plan on modifying a ETD core with a 3mm spacer one for Lp and the other for Lm.

there are 2 ways to get the leaking inductance, first is to use a separate series inductor (used just for high power levels) or to use sectional windings where the primary winding is on one side and secondary winding is on the other side of the transformer. there are few more methods, like winding the secondary over the primary, similar as a normal transformer, but with a thick insulation layer, to reduce the coupling, and even to shift the windings to a side and another. another method is to use a large EE core and place the windings on the outer legs, and the central leg must be grinded to get the desired Lm and Lp. the i used this approach for a high power converter, and one more method, with UU cores, were i could get 3.6KW
most of the LLC converters use special coil former, with sectional windings, which allow large leaking inductances, and very important, tight inductance tolerances compared with the other methods, of winding. the leakage inductance comes as a bonus, without need for an external series inductor, and what's more important, it's saturation current is equal with the current of an air-core inductor.

Originally Posted by chas1

My thoughts are a L6599 control IC feeding an IR2181 that drives a simular mosfet used in the simulation.I will post schematic along with design equations used later.

There are at least 15 dedicated suitable IC's for this converter, most of the power management IC manufacturers have at least one. L6599 has integrated drivers, no need external one for power up to several hundred watts, but need to be buffered for more power. an interesting IC is the UCC25600 from TI, this doesn't have drivers, and need an external transformer, which makes it suitable for both HB and FB topology. i used this one for the 3.6KW converter. ON semi has few, NXP, champion micro, and few more other companies.
since the LLC topology wasn't invented yesterday, it was quite used in Tv sets in 1990's such as SONY which used discrete parts for controller, later some dedicated ic (can't remember name now, just search for some Sony TV service manual) and i saw in some they even used bipolar transistors for switches. in telecom industry, there are few companies which use this topology, and allows them to get very high efficiency, (see Eltek-Valere Flat-pack 2 HE >96%) and some of them use either discrete solution or even DSP/MCU.

Do you know that an TL494 or a SG3525 can be used for a LLC controller ???