Ripple current seen in flyback secondary capacitor

Hi,

I am doing an offline quasi-resonant flyback converter:

I wish to know how to calculate the ripple current in the secondary rail capacitor?

(-or even to know how to calculate the RMS value of the secondary diode current would be fine.)

-the problem is that the load of this flyback is two 5W , hysteretic mode LED drivers (with LM3404 controller)...these switch at about 100KHz.

I am certain that the ripple current seen in the secondary capacitor will vary depending on the relative phasing of the switching of the LED drivers -and also the relative phasing of the switching of the LED drivers with the switching of the flyback.

Do you agree with this?

The reason that i think this, is that i know for a fact that there are such things as IC's which do a boost power factor converter function, aswell as a controller for the downstream flyback, and the switching of these two stages is synchronized to reduce ripple in the capacitor that follows the boost stage.

-so surely the same principle would apply to my flyback and two "downstream" LED drivers?

LM3404 DATASHEET:
**broken link removed**

spec of quasi-resonat flyback:
V(in) = 230VAC
V(out) = 30V
C(in) = 150nF
C(out) = 330uF
Freq switch = ~55KHz, but varies with V(in)
P(out) = 10W


so in summary:
Do you agree that the ripple current in the secondary capacitor will vary depending on the phasing of the LED Drivers' switching?

Your assumption seems basically justified for synchronized switchers. In case of an unsychronized LED driver operating at a different frequency than the primary supply, the RMS contributions simply add quadratically.

thanks but supposing that the secondary load was of equivalent power, but simply a resistor......

....are you saying that the ripple current in the secondary cap. would be less for the case of a resistor?

Yes, compared to a secondary switcher with a frequency unrelated to the primary one.

....but surely a switcher consumes a certain amount of AC input current....
....and it must take this AC content from the flyback secondary....

.....so surely that means that there is now less AC ripple current in the secondary capacitor..
-because the switcher is going to be drawing some of it off.

Example:

In fact, i ran two simulations of 55KHz flybacks:
1. With a 13.3W resistive load
2. With a 13.3W pulsed current load (current pulsing at 50% duty cycle at 100KHz)

The ripple current in the main secondary electrolytic capacitor was the same in each case

Here is the schematic of the pulsed load simulation:
https://i31.tinypic.com/2vxh9p0.jpg

Here is the schematic of the non pulsed load simulation:
https://i25.tinypic.com/23tii3t.jpg


Would you now say that the ripple current in Flyback secondary caps is not at all influenced by whether or not the load current is being pulsed or not?






LTSpice simulation files:-
.........................................
here is the LTSpice simulation of the non-pulsed load flyback:
https://www.2shared.com/file/r3s9xUWL/fbk__open_loop__45Hz__150VAC__.html

here is the LTSpice simulation of the pulsed load flyback:
https://www.2shared.com/file/SSyl1pu9/fbk__open_loop__45Hz__150VAC__.html