onsemi NCL2801LED2GEVB Deep Dimming LED Driver 150 W Evaluation Board User Manual
- June 9, 2024
- onsemi
Table of Contents
onsemi NCL2801LED2GEVB Deep Dimming LED Driver 150 W Evaluation Board User Manual
OVERVIEW
This manual covers the specification, theory of operation, testing and construction of the NCL2801LED2GEVB demo board. This demo board uses NCL2801 as the front end PFC and NCP13992 LLC for the output converter which demonstrates a 150W LED driver. This LED driver features deep dimming with a 0-10 V interface.
SPECIFICATIONS
The key features of this demo board include:
- High Efficiency
- CrM PFC
- LLC Half Bridge
- Dual Dimming Control
- Low Standby Power
- Integrated Thermal Shutdown and UVLO
THEORY OF OPERATION
Overview
The NCL2801LED2GEVB has 2 converters. The front end converter is based on the
NCL2801 PFC controller and the NCP13992 LLC controller regulates the output
current to the LEDs. The NCP13992 has hardware handshaking with the NCL2801 to
control no load power. At startup, the NCL13992 provides VCC to the NCL2801
while also monitoring the HVDC from the voltage divider that sets the
regulated boost voltage. If the boost voltage does not come into regulation
within 200 ms, the NCP13992 turns off VCC to the NCL2801. LLC converters work
best when the input and output voltage are within a narrow range. The PFC
provides the regulated voltage for the LLC input.
PFC
The front end converter is a CrM boost converter based on the NCL2801 PFC
controller. This controller is optimized for high power factor and low THD
over a broad range of line voltage and loads. The NCL2801 is designed to
control high power factor boost converters. This description will focus on
aspects which have been optimized to provide very low THDi and high
efficiency.
The circuit operates in Critical Conduction Mode (CrM) for high loads, and
transitions to Discontinuous Mode at lighter loads by forcing a dead time.
This innovative Valley Count Frequency Fold-back method reduces the switching
frequency while preserving the benefits of traditional CrM operation. The
start of the next switching cycle is timed to the power MOSFET drain voltage
ringing after the end of demagnetization which improves efficiency by
switching at the valley. Internal circuity allows near-unity power factor even
when the switching frequency is reduced. Introducing delay lowers the
switching frequency and can improve efficiency under certain load conditions.
Unlike typical CrM boost converters based on voltage mode control, the NCL2801
utilizes current mode control providing more precise operation. A multiplier
is required to condition the envelope of the input current waveform. This IC
features a novel multiplier design to deliver very low input current THDi over
a broad power range. An offset is introduced to the output of the multiplier
to compensate for non-ideal nature of the process. This function maintains
sinusoidal input current waveform especially near the zero crossings of the
applied input. Line Feedforward compensation adjusts the gain of the
controller to improve wide range control. Gain is reduced at high input
voltage and increased when the applied voltage drops to a lower level. This
gain change maintains the output of the error amplifier, or VCTR, in a more
desirable operating range away from low level noise and high level clipping.
The gain change occurs in the unused input voltage band between 150 and 180
Vac. The change is clearly visible by monitoring VCTR while applied voltage
passes through this range. Range change has no effect while operating in
typical global mains voltage ranges. High power factor converters use low loop
bandwidth to maintain high PF and low THDi performance. As such, response to
input voltage or output load changes is typically slow and suffers large
deviation from the regulated value. The NCL2801 features a Dynamic Response
Enhancer (DRE) which quickly restores the control loop to the required range
in response to changes in power. DRE maintains the output voltage even during
an extreme zero to 100% load change. The DRE function is also active during
initial startup to speed the process of charging the output capacitor. This
DRE function allows use of smaller and lower cost output capacitors in place
of larger values often used to mitigate the effects of load changes. Two Over
Voltage Protection (OVP) functions are included in this version of NCL2801.
The first OVP activates at 105% of nominal output voltage and gradually
reduces on-time to zero. This reduces the power processing gradually over a
period of time avoiding erratic control of the output voltage. This function
typically manages events like rapid changes in applied voltage or load. If the
output voltage continues to 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 The NCP13992 supplies its own VCC at startup.
Simultaneously it turns on the NCL2801 to produce boost voltage. The LLC power
stage begins operation when the boost voltage is high enough and supplies VCC
power to 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 than needed so that the VCC power does not reach UVLO
during extreme light load operation when the power stage is in skip mode.
Dimming In order to avoid flicker issues caused by skip mode operation, this deep dimming design uses 2 current control loops. The main loop controls current through the normal feedback control to the NCP13992. Below a predetermined threshold (20% in this case), the second control loop sets the current using a linear regulator made of Q3 & U9. While the main loop is in control, Q3 is saturated for maximum efficiency. Once Q3 enters the active mode and controls the LED current, Q7 regulates the voltage across Q3 to optimize power losses while allowing Q3 to be in the linear region and provide a very stable output current even though the LLC is in skip mode.
The blue line shows the internal dim voltage (note this is not the external 0-10 V dim signal) vs the normalized output current for the main output control. The orange line represents the internal dim voltage vs normalized output for the linear control. For any given dim setting, the lower setting of the 2 control loops will set the output current. As you can see, the blue line (main loop) sets the output current above 20% load and the orange line (linear loop) sets the current below 20%. This dual mode control provides deep stable dimming while maximizing efficiency for higher output loads.
PROTECTION
OVP/Thermal Protection The thermal protection is built into the NCP13992 and shuts down the NCP13992 when the die temperature exceeds 150°C. An NTC can be connected to pin 7 to program the thermal protection. Pulling up pin 7 through a Zener diode is a good way to set the OVP on the primary side. The primary VCC winding is the image of the output voltage through the turns ratio of the power transformer. This is a convenient way to set the OVP threshold.
SCHEMATIC
BILL OF MATERIAL
TEST DATA
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