NCP43080
www.onsemi.com
30
VCS1
VCS2
VC2
VCC−VLLD
DRV clamp
Skip operation
Normal operation
IC enters
disable mode
VDRVMAX
VLLDMAX
VLLD_REC
VLLD_DIS
t
Figure 61. NCP43080 Light Load Detection Behavior in LLC Application – Other Option
Power Dissipation Calculation
It is important to consider the power dissipation in the
MOSFET driver of a SR system. If no external gate resistor
is used and the internal gate resistance of the MOSFET is
very low, nearly all energy losses related to gate charge are
dissipated in the driver. Thus it is necessary to check the SR
driver power losses in the target application to avoid over
temperature and to optimize efficiency.
In SR systems the body diode of the SR MOSFET starts
conducting before SR MOSFET is turned−on, because there
is some delay from V
TH_CS_ON
detect to turn−on the driver.
On the other hand, the SR MOSFET turn off process always
starts before the drain to source voltage rises up
significantly. Therefore, the MOSFET switch always
operates under Zero Voltage Switching (ZVS) conditions
when in a synchronous rectification system.
The following steps show how to approximately calculate
the power dissipation and DIE temperature of the
NCP43080 controller. Note that real results can vary due to
the effects of the PCB layout on the thermal resistance.
Step 1 − MOSFET Gate−to Source Capacitance:
During ZVS operation the gate to drain capacitance does
not have a Miller effect like in hard switching systems
because the drain to source voltage does not change (or its
change is negligible).
