Design guide IDP2303(A)
PFC design
Application Note
14
Revision 2.0
2017-05-03
In Figure 9, R1&R2 are the resistor voltage divider to detect the bus voltage. The voltage divider is effective
only when the BSS127 MOSFET is turned on.
The BSS127 gate drive consists of:
two small signal diodes D1 & D2 to introduce LLC and PFC gate signals as control inputs
R3 as a current limiter at turn on
R4 & C3 (the input capacitance of BSS127 or an additional capacitor) network, as an RC integrator
that converts “pulse” signal into a “step” signal.
In standby mode, during burst-on, when there is either PFC or LLC switching, the BSS127 MOSFET will be
turned on and the voltage divider is connected to the MCOM (Multi-function and Communication) pin to
detect bus voltage. During burst-off, when there is neither PFC nor LLC gate switching, the BSS127 MOSFET
is open and the voltage divider is out-of-circuit. Thus, the conduction loss in the voltage divider can be
avoided during burst-off, thereby saving power during standby.
During normal operation when the load is usually much larger, the IC is always active and the PFC and/or
LLC switch continuously so the voltage divider is always in-circuit. However, the conduction loss is only a
very small portion of the total loss.
With the IDP2303, the MFIO pin is used for PFC output ROVP.
As the MCOM pin is a multifunction pin, that is not dedicated for high impedance bus voltage sensing, it
must not be connect to the bus voltage divider at start up. Current leakage into the pin during system start-
up could affect the IC start-up behaviour. The proposed solution above with the BSS127 ensures a proper
start-up and an almost lossless 2nd OVP function, which is almost lossless during standby mode.
Below in Figure 10 are the typical waveforms of PFC (green) and LLC (blue) gate signals, the gate signal of
BSS127 (red) and voltage at the MCOM pin (yellow).
Figure 10
Typical waveforms during standby(left:20 ms/div; right: 20 ms/div)
After the IC wakes from sleep mode, the firmware starts to check the voltage at the MCOM pin after it
reaches a peak and is stable. If the ROVP threshold is reached, after a defined blanking time, the IC ceases
PFC switching.
If the ROVP function is not needed, customers can connect a resistor between the MCOM and GND pins to
pull the ROVP pin voltage below the threshold to disable ROVP.
