NCL2801LED2GEVB

www.onsemi.com

2

SPECIFICATIONS

Input voltage

90 Vac – 305 Vac

Output Voltage

50 Vdc

Nom.

Output Current

1.5 A

Max

Output Ripple

± 5%

Max

Efficiency

94%

Max

Switching Frequency

85 kHz − 140 kHz

Dimming Interface

0 – 10 V

Dimming Range

0.3% − 100%

PCB Size

180 mm

× 65 mm

The key features of this demo board include:

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

monitoring the HVDC from the voltage divider that sets the

regulated boost voltage. If the boost voltage does not come

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