UCC28700-Q1: capacitor used for CS pin and VDD pin

Hello Dongbao,

Thank you for your interest in the UCC28700-Q1 flyback controller.

The answer is yes and no... the UCC28700-Q1 can function with this high value of CS capacitance, however, it raise two main concerns:

1.  220pF will add a lot of time delay to the Vcs signal.  It forms an R-C delay with the Rlc resistor, which is used to compensate for the effect of turn-off delay at high input line.  It is ironic that it tends to defeat the purpose of the Rlc resistance. This extra delay will allow higher peak currents at high line and the constant-current function will not be as "constant" as expected over the input range.

2.  A filter cap value this large suggests that either there is a lot of noise coupling into the CS input or the current spike at turn-on is longer than the Leading-Edge Blanking interval and needs to be heavily filtered.  Please check to make sure that the main MOSFET is not oversized for the application with a high Crss value. 
Or, you may try to slow down its turn-on dv/dt with a higher gate resistor value and anti-parallel diode for fast turn-off.  If there is noise coupling, try to re-route the pcb layout of the noise source away from the CS path.

Please note that the delay from the 220pF cap will allow higher peak currents at all input voltages, but it will be highest at high line. This results in more energy per pulse, and switching frequency will reduce to compensate.  Check your minimum load condition to make sure you have enough min load at high line to avoid OVP.

Always try to use the lowest Ccs value possible, to minimize the delay effects it introduces. If a large Ccs solve a problem, see if you can identify the source of that problem and solve it in a different way. 

Regards,
Ulrich