Hi
we are using UCC256402 LLC Controller for our 200W project(28V output)
we are having problems with soft start. At initial condition the slope is very high it is rising upto 20V within very less time. we tried different combinations of LL/SS pin capacitor and resistors and changing soft start precharge value. But the slope is same.Can we reduce it?
Channel 1: Output voltage
Channel 2: LL/SS pin
Regards,
AM
Hi AM:
I use simulation model to check this, I really find the first open soft-start is not good, I check the VLL_ini by C=220nF, Rupper=309k, Rlower=150k. it is nearly 0.2V. I don't know the Transformer turn ratio, I just assume it by 1/0.14, then I change the VCR caps to Cvcr_H=26pF, Cvcr_L=3.3nF, shown by the following simulation fig under 28V/0.5A:
if changing to 28V/8A, the first stage's open loop soft start is improved.
with such a change, the 200W operation shall be checked by VCR voltage, it shows VCR_PK=5V, internal Vcomp=4V, that is OK, the open startup tank current is <4A, if checking 28V/0.5A, the internal Vcomp is 2V, the burst mode is not that easy to enter now, if you need a burst mode, you need to optimize these VCR caps and BMT setting value.
Be noted, you can design VCR cap value to promise Max VCR and light mode's burst mode setting from internal Vcomp
just refer to the formula by following examples
To tune the first stage open soft start performance, we can consider adding initial voltage on SS/LL so that the following start up frequency can be achieved. at the same time, you can check the tank current during this process, to see if the pk tank current can be controlled,
to achieve the overall 30ms startup time, the second soft start stage on the feedback circuit is also important, please refer that in my simulation circuit.
So, just to see if this can help you.
