NCP1399AA, NCP1399BA, NCP1399AC
Figure 46. NCP1399 CS Fault Principle
Dedicated Startup Sequence and Soft−Start
Hard switching conditions can occur in a resonant SMPS
application when the resonant tank operation is started with
50% duty cycle symmetry – refer to Figure 47. This hard
switching appears because the resonant tank initial
conditions are not optimal for the clean startup.
Figure 47. Hard switching cycle appears in the LLC application when resonant tank is
excited by 50% duty cycle during startup
The initial resonant capacitor voltage level can differ
resonant tank is low when the output is loaded and/or the
output voltage is low enough to made secondary rectifies
conducting during first switching cycles of startup phase.
The resonant frequency of the resonant tank is given by the
resonant capacitor capacitance and resonant inductance
−note that the magnetizing inductance does not participate
in resonance in this case. However, if the application
starts−up when the output capacitors is charged and there is
no load connected to the output, the secondary rectification
diodes is not conducting during each switching cycle of
startup sequence and thus the resonant frequency of resonant
tank is affected also by the magnetizing inductance. In this
case, the resonant frequency is much lower than in case of
startup into loaded/discharged output.
depending on how long delay was placed before application
operation restart. The resonant capacitor voltage is close to
zero level when application restarts after very long delay –
for example several seconds, when the resonant capacitor is
discharged by leakage to the power stage. However, the
resonant capacitor voltage value can be anywhere between
Vbulk and 0 V when the application restarts operation after
a short period of time – like during periodical SMPS
turn−on/off. Another factor that plays significant role during
resonant power supply startup is the actual load impedance
seen by the power stage during the first pulses of startup
sequence. This impedance is not only defined by resonant
tank components but also by the output loading conditions
and actual output voltage level. The load impedance of
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