Large value of C19 produces inappropriate sync waveform and prevents the time-out circuit from turning off the rectifier MOSFETs. Try with 100 pF or...
The input impedance of an ideal transmission line terminated with its characteristic impedance is just this impedance, in your example equivalent to...
As shown by wwfeldmann, the post #1 equation has real roots. Similarly the equation in post #5 has two real roots, x=3.987 isn't among it, but x≈-1.2...
There seems to be a "small" problem. LT1243 is a current mode controller. As far as I understand, there's no way to generate pwm output without using...