Part Number:UC3843
I am trying to design a 1000V,10W output flyback-SMPS with 12VDC input and frequency 100KHz using IC3843. I had initially used the WeBench tool to get the values of the circuit components. However, after wiring the hardware, the circuit didn't work. After days of debugging and consulting TI engineers, I found out that most of the values output by the tool were incorrect. So, I have decided to calculate all values using the IC's datasheet. Most of the values have straight forward calculations. However, I found some calculations to be ambiguous.
To calculate Cct, tdeadtime = 2.83us, by graph in Fig.9 (page 10), Cct is coming out to be 10nF
and from equation fosc = 1.72/(Rrt*Cct) Rrt is coming out to be 1720 Ohm. But Rrt should be >= 5K. So how do I calculate the correct value of Rrt?
Apart from that, for the calculation of Rcsf, the value of Rramp is required. To calculate the value of Rramp the only info given is "Choose the value of RRAMP to be much larger than the RRT resistor so that it does not load down the internal oscillator and result in a frequency shift." Is that the only binding condition?
Lastly, how to calculate the value of Ccsf?
