I used LM5180 to convert DC-24V to DC-24V.
output current under normal state is around 5mA.
Heavy load is limited to 50mA.
basic design requirement:
input range:１８～２８ＶＤＣ
transformer is 30uH, np:ns =1:2
under normal/ heavy load/ light load condition, output OK.
under no load condition, if i remove the output Zener, output is about 70V, .
two layer of PCB.
my problem is:
1. if I use my finger to touch the board especially close the RSET pin, or if I use probe to measure RSET pin, the output abnormal, transformer noise in the audio range which can be heard very clear. the FB point curve is above below. how does this phenomenon appeared?
CH1 is FB pin, CH2 is output transformer/ VF anode.
2. i can hear a very light audio sound of the transformer even under normal condition. the transformer is varnished. why?
3. i see the switching frequency is from 12K to 350K, when the IC would've be working under audio frequency range?
4. under normal condition the RSET pin curve is as below, is that correct?
