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July 2011, Rev. 1 1
Design Note – DN05010/D
Accelerated NCL30000 Line Dimming
Device Application Input Voltage Output Power Topology I/O Isolation
NCL30000 LED Lighting Various Up to 60W Hi PF Single
Stage
Applies to Isolated or Non-isolated
Overview
The NCL30000 LED Driver can be configured to dim LED current by using a standard phase cut (leading edge or trailing edge) wall dimmer. The most commonly used dimmers use a TRIAC circuit to “chop” a portion of the input AC waveform thus reducing the RMS input voltage into the driver. Details on implementing TRIAC and electronic dimmer control can be found in application not
at onsemi.com and in many cases the driver can be dimmed across a current range of greater than 200:1.Unfortunately, not all dimmers have wide conduction angle adjust ranges and this limits the dynamic range over which the LED current can be reduced. This design note describes a simple circuit enhancement that changes the shape of the dimming curve of the NCL30000 to allow better current control range over a narrow range of conduction angle.
Key Features
Provides accelerated ‘power curve’ dimming
Simple circuit consisting of four parts
Smooth dimming response
Improves dimming range of some dimmers
Circuit Description
As discussed in AND8448, the basic control principle for line dimming the NCL30000 is to limit the maximum on time such that as the RMS input voltage is reduced, the power delivered to the load is bounded. Once the limit point is reached, any further adjustment of the dimmer results in LED current reduction.
The dimming function described above provides an almost linear relationship between the conduction angle of the dimmer control and the LED current and provides smooth dimming that takes full advantage of the complete travel of the dimmer with no dead spots once dimming has started. Since some dimmers do not have a wide conduction range, this control scheme can be easily modified to achieve a non-linear dimming relationship that provides a more rapid reduction of current as the dimmer setting is reduced followed by a more gradual change as the dimmer approaches the minimum setting.
In February 2011, the National Electrical Manufacturing Association (NEMA) of the United States published SSL- 6 “Solid State Lighting for Incandescent Replacement Dimming” which establishes a clear set of guidelines for dimming LED products. It includes a specification for the dimming characteristic of an LED light source versus input line conduction angle as well as illustrates how an incandescent bulb behaves within the standard.
The accelerated dimming characteristic can be implemented on the NCL30000 controller by adding a simple circuit shown in Figure 1. This dimming characteristic more closely emulates an incandescent bulb.
Figure 1: Partial Schematic of Enhanced Dimming Circuit Modification for isolated control
The NCL30000 COMP pin is controlled by feedback and is compared to an internal ramp to establish FET switch on-time and the current delivered to the LEDs. The COMP pin voltage reaches a maximum of about 5.5 volts as the dimmer conduction angle and therefore applied input voltage is reduced. Since the peak power delivered is limited by the maximum on-time, any further reduction
RIn 150K
CF 10uF
Rectified AC
R9 6.2K
Vcc
R14 4.7K
C9 470pF C7
1nF 5V
R15 100K
Enhanced Dimming Circuit
U1
U2
12 34
QB BC857
1
4 3 2
Comp MFP
NCL30000 CS
Ct
DRV GND ZCD
6 5 Vcc
8 7
RLo 10K
July 2011, Rev. 1 2 the power delivered to the LEDs.
The circuit shown in Figure 1 modifies the dimming slope by reducing the COMP pin level as a function of the average input voltage. This in turn reduces the FET switch on-time.
The circuit of Figure 1 is a voltage divider with a filter capacitor providing a DC level proportional to the average AC line voltage applied to the LED driver. The filter smoothes the chopped waveform created by the phase- cut dimmer. The PNP transistor QB acts as an impedance buffer and isolation diode coupling the filtered DC level to the COMP pin.
Resistor Rin should be selected depending on the input line voltage. Consider the dissipation and voltage stress for the selected device. A value of 150 kΩ is suggested for 115 Vac applications and 300 kΩ is used for 230 Vac applications. Two 150 kΩ resistors may be required to remain within the voltage rating if 1206 surface mount devices are selected.
Resistor RLo is selected to establish the AC input voltage where the COMP pin will begin to affect on-time.
Typically, the threshold will be 5.5 volts, but must be corrected for the emitter-base voltage of QB. Typical values for the lower resistor range from 6.8 kΩ to 10 kΩ. The selection depends on the desired dimmer position where accelerated dimming begins.
To demonstrate the improvement, the circuitry described above was added to an existing 11 W PAR30 Driver design. Details on the design can be found in application
not
The graph in Figure 2 shows dimming characteristics for the 115 Vac NCL30000 PAR30 LED lamp controlled by a Leviton Sureslide® dimmer
.
Shown are the original and the accelerated characteristic achieved with the added circuit. Note the LED current diminishes rapidly as the dimmer setting or conduction angle is reduced.As the conduction angle drops below 80 degrees the slope of the LED current reduces. This flattening provides better adjustment resolution. Additionally, some dimmers have a narrow range of conduction angle; some are limited to 60º minimum. So at 60º conduction angle with the improved dimming curve, the LED current is more than 60% less than the standard dimming curve.
Moreover with the improve circuit the LED current reached 2 mA (230:1 dimming) at 46º versus the standard curve where that same point is reached at 29º.
This is important as many dimmers do not have the wide adjustment range of the example test dimmer from Leviton.
Similar results are obtained with a 230Vac PAR30 LED lamp. Figure 3 shows dimming response for the unmodified lamp and two values of RLo. The effect of when accelerated dimming begins is clearly shown.
June 2011, Rev. 1 3 Figure 2: 115 Vac Dimming Characteristics (Rin = 150 kΩ)
Figure 3: 230 Vac Dimming Characteristics (Rin = 300 kΩ) 0
50 100 150 200 250 300 350 400 450 500
0 20 40 60 80 100 120 140 160 180
LE D C ur ren t ( mA )
Dimmer Conduction Angle (degrees)
TRIAC Dimming of 115V PAR30 Lamp
Baseline Enhanced
0 50 100 150 200 250 300 350 400 450 500
0 20 40 60 80 100 120 140 160 180
LE D C ur ren t ( mA
)Dimmer Conduction Angle (degrees)
TRIAC Dimming of 230V PAR30 Lamp
Standard Curve 10K ohm Enhanced 7.7K ohm Enhanced
June 2011, Rev. 1 4
References :
ON Semiconductor Application Not
ON Semiconductor Application Not
NEM
1© 2011 ON Semiconductor.
Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice.
Design note created by Jim Young, e-mail: [email protected]
