Keeping the BMTL threshold low:
- Makes the converter to operate at
relatively higher switching frequencies. Due to the higher frequency at light
load, the amplitude of the magnetizing current will be smaller and care must be
taken to ensure ZVS is still achieved.
- Makes the converter to switch
more frequently during light load. Due to this, there will be enough energy
supplied by the auxiliary winding so that controller's VCC pin voltage won’t go
below VCCrestartJFET threshold voltage. Also, the output voltage ripple will be
small.
Keeping the BMTL threshold high:
- Longer periods of no switching
during light load. Due to this, higher output voltage ripple is expected
- Limits how high the switching frequency will go during light
load before ultimately dropping into burst mode. Because of this, during light
load, converter can easily achieve ZVS as the there is enough magnetizing
current during primary MOSFET's turn off instants.
- Need to have relatively larger
capacitance at the VCC and for boot strap so that controller doesn’t hit the
under-voltage limit during long burst off periods.