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IRISMPS3

sense resistors R15/R16. There is a small spike at the beginning of the ramp which is due to the discharging of

the resonant capacitor and the winding capacitance of the transformer primary winding. Note that if the input

voltage was 90VAC and there was a full load output, there would be very little feedback, and the source voltage

would ramp all the way to the Vth(1) threshold of the IRIS4013K, but in this case the input voltage is 230VAC,

so the feedback has generated an offset voltage to reduce the peak current in the transformer, and hence the

source voltage ramps to about 0.4V, which keeps the output power constant.

Fig 5) Drain (D) voltage of IRIS4013K (CH1) and the source(S) pin voltage (CH3) at 90VAC in/3A load

output

Fig 5) shows the same waveforms as in fig 4), but this time with a 90VAC input and a 3A load. Note that

under these conditions the current ramp is less steep due to the lower voltage across the transformer primary

winding, and The source pin voltage ramps to a higher level in order to get the correct amount of energy into the

transformer the rate of change of current is lower, so it will take longer to reach the same primary current level.