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IRISMPS1

on. This is also the lowest point of the resonant voltage at the drain pin of the IRIS4007K, so results in reduced

switching losses.

If the DC input voltage changes but the load stays constant, the primary current ramp will now be steeper

resulting in a shorter ON time, but still the same off time as it still takes the same amount of time to transfer the

same energy to the output. The reduced ON time leads to a higher operating frequency.

If the DC input voltage remains constant, but the load is reduced, the secondary side voltage monitoring

circuit (ISO1A/R5/D3) will see an increase in the voltage, as the circuit is still passing the same energy to the

secondary side, but less current is being drawn. This causes the zener diode D3 to conduct, which leads to a

current flow in the optocoupler ISO1A, which gets passed across the transformer boundary to the phototransistor

part of the optocoupler ISO1B. This in turn creates a voltage drop across R9, generating an offset voltage at the

FB pin which reduces the current required through the current sense resistor R10 needed to reach a voltage

of 0.73typ (Vth1 threshold) at the FB pin, and hence less energy is put into the transformer, reducing both the

ON time and the OFF time.

Circuit Waveforms

The following plots show waveforms taken from the circuit under various stated conditions

Fig 1) Drain (D) voltage of IRIS4007K (CH1) and Vcc voltage (CH2) at start-up

Fig 1) shows the drain voltage of the IRIS4007K and the Vcc voltage during start-up with full load output

with a 48VDC input. Note the small dip in the Vcc voltage as the circuit starts to operate, and also the

reflected voltage at the drain due from the output, raising the drain voltage above the DC bus.