target： input 4~20v output 12V ;the circuit is chosen based on 
case 1 no load output is 12V （normal）
case 2 load 120 Ω output is 12V （normal）
case 3 load （30 Ω ，20Ω ） when input is higher than 12V, output is 12V. When input is lower than 12V, output becomes to decrease with the decrease of input. pin "19" is normal when level is high but has sine ripple when level is low
inductance is 10mH