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5

IRISMPS1

Fig 3) shows the same waveforms as in fig 2) again with a full load output, but this time with a 72VDC input.

Note that the operating frequency has now increased, and the effect of the quasi-resonant switching is now

more pronounced. This is evident by the quarter sine wave nature of the falling edge of the drain waveform, at

the bottom of which the FET is turned on, ensuring that it is turned on at the minimum possible voltage to

reduce the switching losses. Note also the steeper ramp in the FB pin voltage when the FET is on which results

in a shorter on time.

Fig 4) Drain (D) voltage of IRIS4007K (CH1) and the source(S) pin voltage (CH2) at 48VDC in/Full load

output

Fig 4) shows the waveforms for a 48V input and full load output, but this time it show the Source (S) pin

voltage with the drain voltage showing the effect of the primary current ramp, which is seen as a voltage

generated across the current sense resistor R10. Note that this waveform shows some noise on the source

pin voltage which is smothed out by the RC filter comprising R9 and C12. The intial voltage spike at the

start of the ramp is a result of discharging the quasi-resonant capacitor C5 and the primary winding

capacitance of the transformer.