Hello
I have a problem with startup operation of ucc28740.
When VVDD reaches the 21-V (UVLO turnon threshold) the controller starts switching. However after
first three switching-cycle the controller turns off. I have tried also with precharge output and Vdd capacitors but still doesn't work.
ch1-Vcs
ch2- AUX winding
ch3- Vout
Can anyone explain me why the system works only for 3 cycles and restarts?
Any feedback on this will be greatly appreciated!
Marek Michalczuk
Hi Marek,
Is the VDD pin voltage dropping down to the UVLO trip point?
If the VDD pin voltage drops below 7.75 V during start-up the chip will turn-off until the VDD capacitor voltage charges back up to 21 Vs.
If this is the case, the problem is either one of two things: The capacitor is not sized large enough, or the auxiliary winding is not providing enough energy to re-supply the VDD capacitor.
How did you size your capacitor?
I use the formula below:
The delta_t in the graph shown is the transistion time it takes aux winding to charge up the VDD capacitor - usually 3 to 5 ms.
Because the Ipp min is so low at start-up, the duty cycle of the chip is kept low, so it could be that not enough energy is being supplied from the primary to the auxiliary winding to sustain the VDD capacitor. This is also a problem in standby mode.
In your aux winding do you use a damp resistor to stop Aux output from ringing? This can also cause start-up problems. The voltage dropped across the damp resistor is too high when the IC turns the MOSFET on or off and it prevents the VDD cap from charging properly.
In the past I have used cascaded diode resistor networks with two capacitors shown below.
Hope this helps
