NCL30060GEVB
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introduced by the transformer leakage inductance.
Monitoring the bias winding to detect output voltage
directly would indicate a false open load condition. The
NCL30060 measures the ZCD pin 2 ms after the MOSFET
turns off to allow the ringing to subside and avoid erroneous
readings caused by leakage inductance.
When the NCL30060 detects an open load condition, the
MOSFET is turned off and is held off for 1.25 ms, at which
time another DRV pulse is issued. If the open load condition
is still present, the MOSFET will be turned off again for
1.25 ms. Should four events occur in succession,
the controller shuts down for 1 second to protect the system,
and then attempts a restart. Qualifying four events avoids an
interruption in operation due to disturbance such as surge or
static discharge.
Figure 14 below is the bias winding voltage in the top
trace and the DRV in the lower trace during an open load
condition. Note the 1.25 ms periods of no switching and
after the fourth consecutive event the controller shuts off for
the extended 1 second period.
Figure 14. Open Load Protection Shutdown
The CS/ZCD pin monitors primary current during the
MOSFET on-time and bias winding voltage during off-time.
D7 is a blocking diode which allows this dual sensing. Note
that capacitance on the CS/ZCD pin will affect converter
operation. Typically, this pin cannot be directly monitored
as probe capacitance can alter circuit timing. Additionally,
board capacitance and recovery characteristics of D7 can
affect converter operation. Best performance is achieved by
selecting a low capacitance diode with recovery time of less
than 35 ns for D7 to avoid residual voltage on the CS/ZCD
pin as the converter naturally progresses from on-time to
off-time. PCB traces should be kept as short as possible to
avoid parasitic capacitance.
Shorted Output Protection
During the on-time, energy is stored in the flyback
transformer and during the off-time the energy is delivered
to the secondary. When the converter is operating with low
output voltage, the off-time is extended as it is the product
of voltage and time which demagnetizes the transformer
initiating the next switching cycle in CrM operation. Normal
converter startup produces the same extended off-times as
shorted output requiring differentiation between these two
events for proper protection.
High power factor operation further compounds detection
of shorted output due to the fact the energy transfer follows
the rectified sine envelope of the applied power.
The extended off-time characteristic of a shorted output
may only occur near the peaks of the sine envelope making
a standard timer based solution not possible. A novel
asymmetrical detection method accounts for the extended
off-time occurring only at the peaks of the applied voltage.
Further details on shorted output detection can be found in
the NCL30060
datasheet.
Shown below is the typical response of the evaluation
board to a shorted output. This trace shows output current
flowing for about 40 ms before the shorted output detection
circuit shuts off the converter. After a 1 second delay,
the converter attempts a restart. When the shorted output is
removed, recovery is automatic.
Figure 15. Current Pulses during Shorted Output
Dimming Functions
The NCL30060 evaluation board accepts dimming
control functions through screw terminal connector J31.
The board is factory configured for 1−10 V control, but can
be easily modified for PWM dimming control by installing
alternate components on the PCB. The dimming interface is
referenced to the secondary ground, but does not share the
negative lead of the LED load. Do not make a connection
between the negative of J31 and the negative of output
connector J2. This will interfere with LED current
sensing.
1−10 Volt Dimming
The typical 1−10 V dimming control for lighting provides
full output when the dimming control is at 10 V and
minimum output at 1 V or below. The interface on the
NCL30060 evaluation board will accept a direct connection
to a voltage source, such as a variable dc supply to achieve
dimming over the 1 to 10 volt range. Multiple LED driver
