Choice of resonant capacitor value in LLC converter?

[Faringdon]

Hi,
Would you agree that in the attached 70kHz  LLC converters, the choice of  C(resonant) should be 258nF rather than 138nF?
..Both cases are shown.
(LTspice sim and jpeg schem att
ached)

The LTspice simulation shows that when both are short circuited, the one with C(res) = 258nF  suffers less overcurrent, and less overvoltage to the resonant capacitors. It will be much easier to source caps at the voltage  rating needed if the higher C(res) is chosen. Generally it appears that C(res) should be chosen to be as high as possible. However, there comes a point when the resonant inductor has to be so low that its not practical to realise it accurately (since inductors tend to have an integer number of turns)….As such, would you  agree, C(res) should be increased until the “paired” value of L(res) is no longer able to be accurately realised (within reasonable cost and effort constraints).
Another point, is that  raising the C(Res) value (and correspondingly reducing the L(res) value so that f(res) is kept the same) results in  the LLC Gain peak getting pushed lower in frequency with respect to the resonant frequency…which in turn means you are further away from the  dreaded “capacitive region”  when you are at your operating frequency….So you therefore have less chance of  straying  into the deadly capacitive region by using a high C(res).

As such, we can conclude that C(res) should generally be selected to be at as high a Faradic value as possible.
Would you agree?

[Faringdon]

Hi,

Why are high values of resonant capacitor frowned upon in LLC converters? I mean, why  does SQRT(L/C)  have to be above a certain value?   
(Where L = resonant inductor; C = resonant capacitor)
Why are LLC converters with a low value of SQRT(L/C) deemed to be less able to handle short circuit output? There is no logical reason for this?   

The  four  attached jpegs show three cases of  Half Bridge LLC design component values.
This LLC has 390vin, 2kW, 180Vout.
In all cases, Resonant frequency is set to 70kHz.
In all cases, the NP/NS is 1:1
In all cases, Transformer L(mag) = 1.45mH
There is also an external L(mag) in each case.
Case 1…L(res) = 10uH
Case 2….L(res) = 20uH
Case 3….L(res) = 37uH
Case 4…L(res) = 5uH

…..In the 4 above cases, the C(res) value was adjusted so that f(resonance) is 70kHz.
In Case 1, the External magnetising inductor could be advantageously increased to 200uH, and as shown, still keep the 70kHz operating point well above the “peak of doom” (lower resonance frequency).
In case 2, the External magnetising inductor, unfortunately had to be reduced to 150uH, in order to keep the “peak of doom” far enough away from the intended frequency of operation
In case 3, the External magnetising inductor, unfortunately had to be reduced down to 60uH , in order to keep the  “peak of doom” far enough below the intended operating frequency.

In cases 2 and 3, the ETD59 offtheshelf gapped core was seeing high delta B in the core, due to the lower inductance. A bigger gapped core could not be chosen, since no offthe shelf gapped cores exist above ETD59 in size.
As such, case 2 is chosen, and has L(res) = 10uH and C(res) = 517nF. Case 2 is very “flat”, but no problem with this is seen.

Case 4 looks great, and allows an L(mag) increase to 300uH, and still keep the peak of doom well away from the intended frequency of operation…however, it will be difficult to wind a whole number of turns onto an offtheshelf gapped ETD59 core and get the inductance equal to 5uH, or even near it…so this case 4 could not be chosen. If a custom gapped core could be used, then case 4 would be ok, but custom gapping is too expensive.

(actually, in all the above cases, the intended operating frequency is slightly above 70kHz, as you can see…..ie, the operation has been chosen to be slightly above resonance, as this is deemed to have good properties with regard to output short circuit protection?)