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TND6225/D
Rev. 0, MAY − 2017

Designing LED Drivers for the
Challenges of Phase Cut Dimmers© Semiconductor Components Industries, LLC, 2017

May, 2017 − Rev. 0

1

Publication Order Number:
TND6225/D

Designing LED Drivers for the Challenges of Phase Cut
Dimmers
By Frazier Pruett, Application Engineer, ON Semiconductor
LED replacement bulbs are now firmly entrenched in the mainstay of lighting worldwide. Replacement
bulbs are available in all major household fixtures. LED bulb performance has always been excellent fromtheir introduction solving almost all of the CFL bulb issues from slow starting to temperature sensitivity.
Except for some specialty bulbs, CFL bulbs are not dimmer compatible. A majority of LED bulbs aredimmer compatible to the extent claimed by the manufacturers. The CEC is moving to make dimmer
compatibility a standard feature of LED bulbs for the California market. Dimmer compatibility is the single
biggest challenge facing the LED driver designer. Operational requirements with and without a connecteddimmer are difficult to meet with a single design approach. Good dimmer compatibility does not lend
itself to high power factor and low THD. The optimum design needs multiple personalities depending on
the operating state and enough intelligence to recognize the different operating states. Semiconductor
manufacturers such as ON Semiconductor® are releasing new LED drivers that ease the challenges ofdimmer compatibility.

Such is the case with the ON Semiconductor® NCL30095 and NCL30167. NCL30095 is a fully integrated
switcher solution phase-cut dimmable LED driver in a boost configuration. Phase-cut may be either
leading edge (typical for most TRIAC based dimmers) or trailing edge (dimmers compatible withelectronics transformers). The internal HV MOSFET has a nominal Rdson of 4.5Ω. The NCL30167 has thesame functions as the NCL30095 except that the HV MOSFET is external allowing for higher power
applications. The cascoded FET arrangement is one of the unique features of these LED drivers. The
output current is regulated for SSL7a compliance. Included functions of the NCL30096/NCL30167 are:closed loop current regulation, thermal foldback, conduction angle sensing, mode changes, and ZCD
sensing. The power stage operates in CrM.

This article will cover the following topics:Dimmer Compatibility

Rev 00 4/6/2017



NCL30095 / NCL30167 Features & Setupo

Current Set Point and regulationoMode ChangesoHold CurrentoVcc GenerationoCascode setup and layout considerationsoProtection Features


OVP



OTP
L1

J1

D4

FUSE
1
2
3

+HVDC

4.7mH

F1

C4

AC1

+

AC2

-

270nF 250V

ABS10

Rdamp

CON3

C6
68nF 250V

270

Typical Input Schematic Figure 1.

Dimmer compatibility
The input filter plays an important role in dimmer compatibility. There are 2 main types of phase control
dimmers: leading edge modulated and trailing edge modulated. Here is an example of the line current ina leading edge modulated dimmer:

Rev 00 4/6/2017

Line Current for a Leading Edge Dimmer Figure 2.
Notice that the line current is delayed from the zero crossing. In this case, dimmer turn on modulates theleading edge of the current to control the bulb. Trailing edge dimmer current turns off partly though theline cycle. Both techniques are equally effective at dimming the bulb. However, the implementation foreach technique is quite different. Leading edge dimmers are much more common because their designs
are much older and lower cost because the main switch is a TRIAC or SCR. TRIACs and SCRs can only beturned off when the current through the device drops below the holding current. This occurs naturally at
the zero crossing after the device has been turned on. Trailing edge dimmers use MOSFETs to control thecurrent. MOSFETs can be turned on or off regardless of the current through the device which makes themable to turn off partly through the line cycle. The MOSFETs and their associated control are more
expensive and complicated than the TRIAC/SCR controlled dimmers. Not only have TRIAC/SCR dimmers
existed longer than MOSFET dimmers but they are lower cost and still dominated the dimmer market.
While this is great for the consumer, TRIAC dimmers present a host of complex issues for the driver
designer.

Incandescent bulbs have some non-linearities that make them well suited to TRIAC dimmers. In particular:
1. Positive temperature coefficient
2. Light output that drops off rather sharply with applied voltage.
Rev 00 4/6/2017

The positive temperature coefficient comes from the tungsten filament. As the tungsten cools, its
resistance goes down. This makes the current almost constant over the dimming range providing holdcurrent for the TRIAC.

Light output from a tungsten filament drops rapidly as the filament cools. Typical filament temperaturesare 2,500K which puts them in the warm spectrum of light (more red). As the filament is dimmed, thepower in the filament drops but also the color temperature further shifts into the red region.
Incandescent bulbs are good examples of nearly perfect black body radiators. The human eye is lesssensitive to red light so the luminous output from the filament drops because of lower power and color
temperature shift. Light output is measured in lumens which is photopically adjusted for the response of
the human eye. The human eye is most sensitive to green light at 560nm.

Human Eye Response Figure 3.
This explanation serves to help understand why TRIAC dimmers are so well suited to work withincandescent bulbs. No light driver (LED or otherwise) behaves like an incandescent bulb. The single
biggest challenge for any type of electronic light source driver is maintaining sufficient hold current sothat TRIAC does not stop conducting from current starvation. A typical 60W incandescent bulb willproduce 800 lumens. An equivalent LED light source consumes only 8W to produce the same light output.
Reduced input current is the chief cause of TRIAC dimmer problems. The driver must draw continuous
Rev 00 4/6/2017

current of a high enough magnitude over the line cycle to keep the TRIAC conducting. The EMI filter
design is a critical part of this.

In figure 1, L1, C4, C6, and Rdamp form the EMI filter of the driver. When the TRIAC begins conducting, itapplies a step function of voltage to the EMI filter. The reactive elements of the EMI filter will ring at theirnatural frequency if not well damped. C4 and Rdamp form the damping network which dampens out
ringback current and keeps the TRIAC current above the hold current threshold. The ratio of C4/C6 iscritical to maintaining optimum filter damping. This ratio is between 2:1 and 4:1. A ratio below 2:1 isnearly impossible to completely dampen which results in compatibility problems with some dimmers. A
ratio greater than 4:1 simply dissipates excessive power. In this architecture, the EMI filter design isagnostic to the driver design. Some designers have resorted to complex dampening circuits that containactive components to reduce dissipation or acoustic noise. If you’re willing to bear the cost of the addedcomplexity, there are advantages to be gained.

Overview of NCL30095/NCL30167 features
The NCL30095/NCL30167 are constant current output boost regulators. Boost converters make input
current control very easy especially in low power CrM designs. Peak current is easily measured andcontrolled. The instantaneous average current is simply ½ of the peak current. The output voltage is set
by the LED array on the output. The LED voltage must be greater than the peak of the line voltage (aswith any boost converter) in order to control the LED current. The NCL30095/NCL300167 have closed
loop control of the LED current, overvoltage protection, thermal foldback, and hold current management.

Rev 00 4/6/2017

+HVDC

L1

D1

4.7mH

UFM14PL

D6
1N4148WS(no stuff)

R19
1

R12

33

499k

D4
18V

R11

499k

499k

Led+

1

LED-

C3
10uF 400V

C7
10n

R1

2

U1

R10

1

499k
NCL30095

499k

3

Drain

Gate

Drain

Source

Drain

CS1
Gnd

5
6
7

R14
18.7k

Comp

Vcc

Acc_ThCS2

Tf

FB/OVP

D7

14

NSR0340HT

13
12
11

D2

10

1N4148WS

8

1k

C6
C5
330nF

R5

t

C10
10n

R17

9

C8
10n

R2
499k

R4
3.9

C9

10uF

10n

C11
560p

R3
11.5k

R15

R16

15

15

1

2

D5

2

1

R13

SD05T1G

100k Ohm NTC

Main Schematic Figure 5.

Mode Changes
NCL30095/NCL30167 operate in different modes depending on the detection of a phase-cut dimmer. The
operating mode is on-time control and peak current control. When no dimmer is detected, the control
operates in on time control exclusively.

No Dimmer Operation Figure 4.

Rev 00 4/6/2017

The power factor is greater than 0.95 and the TDH is less than 20%. No hold current management isinvoked because no dimmer is present (or detected). Some distortion is seen at the zero crossings which
is typical for CrM based on time controllers. Here the control is in on time control only. Once a dimmer
is detected the control is both on time control and peak current control.

Peak Current control

On time control

Typical Input Current during Dimming Figure 5.
The peak current mode establishes the minimum input current to provide holding current for the TRIAC.
In this mode, the controller never allows the peak current to fall below 35% of the maximum peak current
set by R4 on CS1. Even at minimum dim level, the minimum peak current is enforced. Decreasing the
1𝑉value of R4 increases the hold current. This also increases the maximum switch current which is 𝑅4 . Aslong as the boost inductor does not saturate, increasing the peak current does not affect regulation
because of closed current loop.

Current Regulation
R15 and R16 set the LED current. The parallel combination of R15 and R16 make up Rsense. The output
current is calculated as follows:
0.25𝑉

Led Current = 𝑅𝑠𝑒𝑛𝑠𝑒
Rev 00 4/6/2017

D5 limits the peak current (by clamping the peak voltage) on R15 and R16 which occurs from inrush
current at initial application of input voltage. This is a one-time event since the boost voltage quickly
exceeds the peak of the line so no further inrush current is seen. R17 is necessary to limit current in orout of CS2 pin preventing damage to the controller. D5 must have adequate surge capability to handle
the inrush current.

ZCD
The current through R15 and R16 is the output current since it’s in series with the LEDs. The current in
the LEDs is filtered by C3 so the LEDs see mostly a DC current. The current in R15 and R16 is exactly thesame as the discharge current from the boost inductor L1. Consequently zero current detection is alsodone on the CS2 pin. When the decreasing voltage on CS4 reaches 40mV, the controller starts a newswitch cycle. Even at 40mV, the current in L1 has not quite reached zero. C11 causes some time delay
with R17 to allow for enough time for the current to truly reach zero thus allowing the drain voltage toswing down for ZVS. Some tuning of R17 and C11 will be necessary for each application.

Acc_thThe Acc_th pin detects the presence of a dimmer if the sensed conduction angle is less than 160°. The
Acc_th pin has a comparator for sensing the input voltage. The threshold is 400mV rising and 350mV
falling. The resistor divider R12, R13, and R14 have an influence on the detection angle because of thecomparator threshold on Acc_th. A lower voltage on Acc_th makes the controller enter dimmer detection
mode earlier which improves dimmer compatibly and accelerates the dim curve wrt phase angle.

The phase angle on the Acc_th pin is mapped into the output current set point. It is common practice toallow the power stage to operate in open loop control when dimming. However that can lead to irregular
output. The reference for the current control loop is established by the phase angle which keeps the LED
current always in regulation.

OVP
The voltage divider of R1, R2, and R3 set the over voltage threshold at the OVP pin. The threshold is 3V
and it is non-latching.
Rev 00 4/6/2017

Thermal Protection
A single NTC (R5) sets the thermal foldback characteristic. Pin 7 sources 85 µA and limits the voltage to
3V. As the NTC heats up, the resistance drops as does the voltage on Pin 7. Output current reduction
begins when the voltage on pin 7 reaches 1V. Output current drops linearly to 10% when the voltage on
pin 7 reaches 0.5V. Below 0.5V on pin 7, the output goes off. A small capacitor on pin 7 is recommended
for noise immunity.

Loop Compensation
The error amp is an OTA so the compensation network connects to the comp pin and ground. A single
capacitor is enough for good PF and THD. More complex compensation networks will make small
improvements in PF and THD. Compensation works like any PFC converter.

Power Stage
The power stage uses a cascode arrangement of MOSFETs.

Cascoded MOSFETS Figure 6.
The chief advantage of the cascode is that the HV MOSFET provides Vcc power as a linear regulator. This
results in:
Rev 00 4/6/2017

1. Fast start up
2. Stable Vcc over the entire dimming range
3. Small Vcc capacitor
4. Single winding boost inductor – no aux winding needed
5. Only one HV MOSFET required

Basic Operation
The gate of the HV MOSFET is held to a fixed voltage. In figure 5, this is done by R10, R11, D4, and C7.
Selection of D4 is critical because it sets the startup voltage and limits Vcc in operation. Vcc_on is 13V
maximum and Vcc_max is 20V. Vth of the HV MOSFET is 2-4V. So an 18V Zener assures that the Vcc atturn on will be 14V. Vcc will not exceed the Zener voltage in operation. The range of Zener voltages thatwill meet the startup and operating requirements is limited so 18V is a good target value.

The selection value for C7 is rather wide. In general, C7 needs to be large enough to have a low ripple
voltage in operation since the gate charges and discharges through C7. Large values of C7 delay thestartup. Typical values for C7 are 10nF - 1µF.

Clamping the LV MOSFETThe lower MOSFET will not have a drain voltage greater than Vcc. So the LV MOSFET is much smaller thanthe HV MOSFET even though both MOSFETs have about the same Rdson. The drain of the LV MOSFET
needs to be clamped with a Schottky diode (D7) to prevent conduction of the body diode. Body diodeconduction is not ordinarily any concern for MOSFETs in a standalone application. In this case, the LV
MOSFET is part of the control circuit die. Body diode conduction will draw substrate current on the die
and disrupt control. This is usually noted by an irregularity in the dimming curve. A conventional PN diodewill not work because its VI characteristics are similar to the body diode.

Vcc Capacitor Size
The Vcc capacitor is recharged whenever the HV MOSFET switches off. However in deep phase cut
dimming, recharge pulses may not occur over a large portion of the half line cycle because there is no
Rev 00 4/6/2017

available power from the dimmer. So the Vcc capacitor must hold up Vcc over a full half line cycle. This
sets the minimum capacitor size of C6. 4.7µF is the smallest value that will work. 10µF is therecommended minimum value since is provides good design margin and is still available in a SMD package.

Layout Considerations and HF Noise Issues
While cascode drives have significant advantages, there are also challenges related to them. In particular,
they are prone to HF oscillations and sensitive to layout. This is caused by the extremely high small signal
gain. Oscillations of 200MHz – 400MHz have been seen from the cascode drive. In the best case, theoscillations disrupt operation or show as radiated EMI.

In the worst case, the circuit will failcatastrophically. Oscillation is particularly evident at startup. A good scope with high bandwidth isneeded to see the oscillation which is most easily seen on the gate pin 14.

Care should be taken to minimize the trace lengths on the gate (pin 14), Source (pin 13), and CS1 (pin 12).
A ground plane under the controller is recommended to minimize impedance back to pin 11 (ground).
The gate circuit is especially critical to have a low impedance ground path back to the controller.

Here are some ways to dampen out the oscillations:
1. Gate resistor (R19) – a speed up diode can be placed in parallel with the resistor to speed turnoff
2. Ferrite bead on the gate
3. RC snubber on the source

Examples of Good Layout PracticeLet’s examine some good layout practices for the cascodes drives in general and NCL30095 in particular.

Rev 00 4/6/2017

One of the first things to notice is the ground plane under the control circuit (colored in pink). The
controller ground pin is pin 11. It is immediately connected to the ground plane through a via.
NoiN

Notice the tight current loop formed by the current sense resistor R4. Pin 11 and Pin 12 areadjacent to make this a naturally tight current loop.

Rev 00 4/6/2017

H

Here the loop for the gate of the HV FET is show in yellow. The gate resistor (R19) is right next
to pin 14 which is the gate of the HV FET. The gate capacitor (C7) is directed connector to R19 and theground plane.

Here are some general layout rules to keep in mind.
1. Placement is critical – high dv/dt and di/dt loops need to be kept short. A well placed layout
will route easily.
2. Don’t cross power and signal grounds – power and signal returns should be connected at
one point usually at a sense point or power semiconductor.
3. Keep high voltage away from signal level circuits – high voltage dividers usually havemultiple SMD resistors which can be separated if needed.
4. Input signal traces should be kept short to avoid noise sensitivity.
Ground Planes
Ground planes can be your best friend or your worst enemy if done wrong. Ground planes make a nice
way to connect to a circuit common. Don’t use the ground plane for signal and power currents. Thisdefinitely cause circuit problems. So pouring a ground plane over the entire board is bad idea for powerboards. Its common practice for digital boards but digital boards have much lower currents. Under thecontrol circuit is a good use for the ground plane on a power board. Ground planes makes good layouts
better but won’t make bad layouts good.

Rev 00 4/6/2017

ON Semiconductor andare trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries.
ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent
coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
Buyer is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards,
regardless of any support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or
specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer
application by customer’s technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are notdesigned, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification
in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized
application, Buyer shall indemnify and hold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, andexpenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if suchclaim alleges that ON Semiconductor was negligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This
literature is subject to all applicable copyright laws and is not for resale in any manner.

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