IRISMPS3 International Rectifier, IRISMPS3 Datasheet - Page 6

• Manufacturer Part Number: IRISMPS3
• Description: International Rectifier Kansas Street El Segundo CA 90245 USA
• Manufacturer: International Rectifier

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
2
transformer. Remember that the energy stored is 1/2LI
and V=LdI/dt so with a lower voltage across the
transformer the rate of change of current is lower, so it will take longer to reach the same primary current level.
6
www.irf.com