NCL2801LED2GEVB

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rise to 107% of nominal, a second OVP function stops all

switching to avoid a run−away situation. Switching resumes

when the output voltage returns to normal. The OVP

functions can be observed during initial startup and for large

reductions in output load. The CS pin links the switch

current to the boost control function. This pin also provides

over current protection on a cycle by cycle basis. A

programming feature is also managed by the CS pin. At

initial power up, the NCL2801 outputs a current through the

CS pin to read a resistor placed on the circuit board. A

lookup table links the measured resistance to one of six

levels. Resistance below 50 nominal is interpreted as a

shorted pin, or assembly fault which stops the converter

from operating. Nominal impedances of 150, 330, 620, and

1 k are linked to one of four thresholds determining when the

control function changes from CrM to DCM operation. A

resistance measurement greater than about 1.3 k nominal is

considered an open circuit, or assembly fault which disables

the converter. This feature allows easy configuration of the

operating mode and detects faults on the circuit board. This

EVB is fitted with 150 which invokes the lowest CrM to

DCM threshold.

LLC Power Stage

The power stage is a resonant LLC half bridge. The

NCP13992 controls the output by controlling the switching

frequency so the output duty cycle is 50%. The NCP13992

operates in current mode so the frequency is slaved to the

primary current. Current control has similar effects in LLC

converters has as other current controlled converters:

1) Simplified loop stability 2) Inherent overload protection

3) Stable frequency operation. LLC converters operate at a

fixed duty cycle like most resonant converters. Increasing

frequency reduces the output current but there are limits to

the upper frequency operation. As the load approaches zero,

the NCP13992 enters skip modes to regulate the output

voltage.

The LLC delivers maximum output at its lowest operating

frequency. There is an on time limiter which is hard coded

into the NCP13992. The on time limit is selectable by

choosing the desired suffix in the part number. The power

component values are chosen to work at the frequency limits

of the controller.

HV Start

Simultaneously it turns on the NCL2801 to produce boost

voltage. The LLC power stage begins operation when the

NCP13992 and NCL2801 from the bootstrap winding on the

power transformer. The bootstrap voltage is regulated by Q5

to about 15 V because the bootstrap voltage is about 25 V in

normal operation. The bootstrap voltage needs to be higher

during extreme light load operation when the power stage is

in skip mode.