IRISMPS1 IRF [International Rectifier], IRISMPS1 Datasheet - Page 3

• Manufacturer Part Number: IRISMPS1
• Description: REFERENCE DESIGN
• Manufacturer: IRF [International Rectifier]

on. This is also the lowest point of the resonant voltage at the drain pin of the IRIS4007K, so results in reduced
switching losses.
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.
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.
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.
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If the DC input voltage changes but the load stays constant, the primary current ramp will now be steeper
If the DC input voltage remains constant, but the load is reduced, the secondary side voltage monitoring
Circuit Waveforms
The following plots show waveforms taken from the circuit under various stated conditions
Fig 1) shows the drain voltage of the IRIS4007K and the Vcc voltage during start-up with full load output
Fig 1) Drain (D) voltage of IRIS4007K (CH1) and Vcc voltage (CH2) at start-up
IRISMPS1
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