GM6108A

Off-line LED Lighting Driver Rev. 1.0 – March 25, 2011

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FEATURES

• 90V to 130VAC, 180V to 270VAC dual input range

• High power factor: better than 0.97

• High efficiency: up to 92% for sinusoidal buck, 88% for sinusoidal forward

• No need for any electrolytic capacitor

• 450mV current-limit voltage

• Natural current-limited soft start (no inrush current)

• Improved line regulation

• Over-temperature protection

• No-load protection, open feedback loop protection

• Adjustable switching frequency with natural spread spectrum

• 80uA start up current

• Available in MSOP-10 package

• Dimmable option (GM6182)

APPLICATIONS

• General LED lighting

• A19, PAR30, PAR38 LED light bulbs

• LED light tubes

• Outdoor spotlights

DESCRIPTION

The GM6108A is a high power-factor, constant current driver IC for offline LED lamps. It can accept dual input voltage range of 90V to 130VAC, and 180V to 270VAC. It is based on a sinusoidal buck/forward topology and achieves very high power factor, typically 0.97 or better. It supports both non-isolated (buck) and isolated (forward) designs. The GM6108A has a start-up current of less than 80uA. A bias winding on the buck converter’s inductor or the forward converter’s transformer provides the VDD power to sustain circuit operation after start up. The GM6108A provides an adjustable switching frequency, up to 200kHz. It has cycle-by-cycle current limit. The GM6108A provides an accurate internal current limit reference of 450mV. The GM6108A provides on-chip over-temperature protection. It shuts down the switching when the junction temperature exceeds 150oC. The GM6108A is available in MSOP-10 package. Note: GreenMark patents:.US 7,295,452 and US 7,750,616.

TYPICAL APPLICATION

GM6108A

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ABSOLUTE MAXIMUM RATINGS Supply Voltage VDD, GATE to GND….………….….…-0.3V to 22V Power Outputs and Control

VCC, ISEN, VSINE to GND…..……….. -0.3V to 6.0V CT, FBK, CAL to GND ..........................-0.3V to 6.0V

Operating Junction Temperature …….–40oC to 125oC Storage Temperature Range.………….–65oC to 150oC Lead Temperature (Soldering, 10 sec.).…………300oC

Package Thermal Resistance T JA …….. 150oC/W

PACKAGE / ORDER INFORMATION

10 9 8 7 6

1 2 3 4 5

PIN DESCRIPTION

PIN NAME FUNCTION PIN NAME FUNCTION 1 VCC 5V LDO output 10 VDD Chip supply voltage 2 CT Sets Toff length and nominal switching

frequency 9 GATE Gate drive output for external

MOSFET 3 FBK Feedback input (from an opto-coupler) 8 ISEN Current sense voltage input 4 GND Ground 7 VSINE Scaled sinusoidal ac input waveform 5 CAL Calibration for high line input power 6 NC

GM6108A

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ELECTRICAL CHARACTERISTICS (VDD = 12.0V. Typical values are at TA = +25oC)

Parameter Conditions Min Typ Max Units Supply Power Section VDD supply voltage range 7.0 12.0 V Operating current Fsw =100kHz, Ciss = 1nF 2.3 3.0 mA Start-up current VDD = 9.0V 60 80 uA OVP trip point 17.5 18.5 19.0 V UVLO Section Turn-on threshold voltage

10.0 12.0 16.0 V

Turn-off threshold voltage 5.8 6.0 6.2 V LDO Section VCC output voltage Vin from 7V to 16V 4.9 5.5 6.0 V Protection Section Internal OTP shutdown threshold

Switching is turned off. †Note 1.

150 oC

Internal OTP shutdown reset

Switching is resumed †Note 1.

100 oC

Gate Drive Section Source current Fsw =100kHz, Ciss = 1nF 15 30 nsec Sink current Fsw =100kHz, Ciss = 1nF 15 30 nsec Current Sense Section Current limit reference voltage

400 450 500 mV

Current sense level-shift reference

† Note 2 1.45 1.50 1.55 V

Leading edge blanking time

350 nsec

Propagation delay From OCP to Vgate drops to 0.9*VDD †Note 1

100 nsec

PFC and Line Regulation Section Line regulation reference 2.50 V CAL pin on-state voltage Sink current = 0.1mA 0.3 V FBK pin output resistance 1.7 kΩ Oscillator Section Toff length Ct = 680pF. †Note 3 8.3 usec

†Note 1: The parameter is guaranteed by design. It is not tested in production. †Note 2: The current sense level-shift reference is correlated to a 1.20V internal reference voltage

through a voltage follower buffer. †Note 3: The CT capacitance required for a target Toff is, CT = Toff / 12.2k

GM6108A

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FUNCTIONAL BLOCK DIAGRAM

90V ~ 130VAC or

180V~ 260VAC

R3R1

R2

Cin

6.8u

AC V+

C5

0.2u

Qb

VDD

S

R

Toff

VSINE

GATE

ISEN

LDO

VCC

Gate

Drive

LEB

Line regulation

& Waveform

Modulator

Q

CT

GM6108A

FBK

GND

CAL

1.7k

OVP

Qvh

Qvh turns on

at high line

Level

Shift

0.45V

OTP

AC V-

1.50V

Fig. 1 Functional Block Diagram

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APPLICATION INFORMATION The GM6108A is a high power-factor, constant current driver IC for offline LED lighting. It can accept dual input range of 90V to 130VAC, and 180V to 270VAC. It is based on a proprietary sinusoidal buck topology to achieve very high power factor, typically 0.97 or better. The GM6108A supports both non-isolated (buck topology) and isolated (forward topology) designs. It has a start-up current of less than 80uA. A bias winding on the buck converter’s inductor or the forward converter’s transformer provides the VDD power to sustain circuit operation after start up.

Setting the Switching Frequency The GM6108A is basically a constant-Toff switching regulator. Its nominal switching frequency, Fsw, can be adjusted via connecting a capacitor on the CT pin, according to the following equation: RT*CT = Toff = (1-D) / Fsw, where RT is a 12.2kΩ equivalent timing resistance. For a 6W LED lighting with dual-input range such as shown in Fig. 2, Fsw is selected to be 100kHz. At Vpeak = 300V, D is about 15%, we find Toff is about 8.5u To set Toff = 8.5 usec, we need CT to be 698pF.

Natural Frequency Jittering The switching frequency of a GM6108A converter is naturally modulated as Vin varies and follows the AC line’s sinusoidal waveform. This natural frequency jittering helps the LED lighting fixture to meet the EMI compatibility test. For 220VAC input, Vin varies from 0V to 300V peak. Since the switching frequency is typically near 100kHz, duty cycle, D, follows the following equation: Vin*D = Vo = Vf where Vf is the combined forward voltage of the series-connected LEDs.

Toff = (1-D) *Tsw = (1-D)/Fsw That is, Fsw = (1-D) / Toff = (1 – Vf/Vin) / Toff

So if we keep Toff constant, the switching frequency Fsw will be higher when Vin is high. For 220VAC input, most of input power happens when Vin is above 100V. If Vf = 45V, and Toff = 8.5us, Fsw will be 65kHz at Vin = 100V. Fsw increases to 100kHz at Vin = 300V. Notice GM6108A improves Fsw range by making Toff shorter for lower Vin. Its Fsw range is typically from 80kHz to 100kHz.

Setting up Sinusoidal LED Current Waveform LED current waveform follows the rectified sinusoidal waveform of AC input voltage. Use a voltage divider R1, R2 to provide a proper amplitude at the VSINE pin. The required VSINE amplitude is 2.5V. For a dual-input range system, we recommend a voltage divider of R1 = 1M, R2 = 20K.

High Power Factor Because of the in-phase sinusoidal LED current waveform, a GM6108A application circuit achieves excellent power factor, typically better than 0.97. An important advantage of the GM6108A LED lighting driver is there is no need for a large electrolytic or tantalum filter capacitor after the input bridge rectifier. Most conventional LED driver designs require an input filter capacitor in the order of 1uF per watt. In contrast, GM6108A requires only a small input filter capacitor, C1, of about 0.22uF to filter out high-frequency switching noise from feeding back to the AC lines.

No Electrolytic Capacitors A large electrolytic capacitor takes up circuit board space and volume. Furthermore, its limited operating life, usually less than several thousand hours, will seriously downgrade the usable life of an LED lighting fixture. The operating life of an electrolytic capacitor can be expressed as

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Lop = Mv*Lb* 2 [(Tm-Ta)/10]

where Lop is the expected operating life in hours. Mv is a voltage multiplier for voltage de-rating. Lb is the expected operating life in hours at full rated voltage and temperature. Tm is the maximum permitted capacitor internal temperature in oC. Ta is the actual capacitor internal temperature in oC. In other words, an electrolytic capacitor’s life will be cut in half for every 10oC of temperature rise. In fact, a major challenge in designing a reliable LED lighting fixture confirming to the conventional incandescent light bulb form factor such as MR16, A19, PAR30, or PAR38 is the thermal management. The LED driver circuit is enclosed inside of a completely sealed chamber with elevated temperature. It is common to measure internal temperature of 100oC or more. This harsh operating condition often takes a toll on electrolytic capacitors. Therefore, to achieve long-life in LED lighting, it is desirable to eliminate the need for any electrolytic capacitors. .

Non-isolated Buck Topology Design Fig. 2 shows a reference design for a 6W LED A19 light bulb. It uses a 45V, 120mA LED arrays (15S x4P). The voltage on the current sense pin, ISEN, will follow a truncated sinusoidal waveform with peak value of 450mV. Also, there is a cycle-by-cycle over-current limit that terminates a switching cycle when the ISEN pin exceeds 450mV.

In Fig. 2 circuit, the target average LED current is 120mA. Since GM6108A modulates the LED current to fol low a s inusoidal waveform, therefore, Ipk is 120*3.14/2 = 188mA. With 30% current ripple, we have Rsen = 0.45V/(0.188A*1.15) = 2.08, a 2.0Ω current sense resistor is used. For the power MOSFET, we recommend a 400V NFET with Rds(on) of about 3.4Ω, and Coss of about 40pF. Its conduction loss is about 37mW. Switching loss at 100kHz is about 97mW at 220Vac input.

The inductor uses an EE10 ferrite core. The primary winding has 175 turns with an inductance of 5.6mH. The bias winding has 58 turns. The free-wheeling diode, D2, will see a full Vin when the MOSFET, Q1, turns on. We recommend a 1.0A, 400V ultra-fast recovery diode (ES1G). The bias winding voltage is in proportion to Vf at MOSFET off time. For Vf = 45V, and VDD = 14.5V, the bias winding number of turns is 175*14.5V/45V = 56T. Notice the reverse voltage on D3 is Vin*(58/175), when Q1 is on. We recommend a 200V ultra-fast recovery diode. Two 1N4148 in series can be used as well. No inrush current Fig. 3 shows the start-up waveform of the GM6108A driver. When VDD reaches UVLO cut-in level, the GM6108A starts switching. The start up can trigger at any phase angle of a 120Hz cycle. But Isen always follows the sinusoidal Vin waveform. Even at cold start-up, there is consistently no inrush current.

Switching Waveforms Fig. 4 shows the key waveforms of Fig. 2 circuit over a half cycle of 60Hz line frequency.

Thermal Design Notice the safety isolation in Fig. 2 circuit is provided by the insulation layer between the LED devices and their aluminum substrate. An adequate heat-sink design for an LED light bulb can keep its outside surface temperature below 60oC. This in general will keep the junction temperature of the LED devices below 85oC. However, the air sealed within the air-tight light bulb chamber will be heated up by the heat dissipation from the inductor, the free-wheeling diode, and the power MOSFET. In many un-suspecting designs, the air trapped inside may get as hot as 150oC whereas the heat sink surface temperature is only 60oC. This trapped hot air often leads to component and circuit failure, especially in those LED drivers using electrolytic capacitors.

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This circuit board hot spot situation is due to the lack of heat conduction path from the converter circuit board to the inside wall of heat-sink. Uncirculated air around the circuit board has very little heat transfer capability. The heat dissipated from the circuit board simply accumulates within the chamber, elevating the circuit board and components temperature to ever higher level. In certain light bulb design, convection holes are added to provide tiny amount of air flow in order to cool off internal temperature. Although effective, adding convection holes has the drawback of condensed moisture and trapped dust inside the lamp fixture. We recommend to apply heat conducting, but electrically insulating glue between the power dissipating devices (mainly the transformer, the power MOSFET, and the free-wheeling diode) and the inside wall of heat sink.

Using 30mA LED Arrays Due to their huge volume use in backlight applications for cellular phones, portable computers, and LCD TVs, the 30mA LEDs are very cost effective. Their efficiency is in general higher than that of 1W power LEDs. In addition, using a large amount of 30mA LEDs to form a distributed light emitting surface on top of a heat-sink substrate can help to resolve both the glare and the hot-spot issues. With all these advantages, 30mA LEDs are getting ever popular for general lighting applications.

92% High Efficiency With proper selection of power MOSFET and free-wheeling diode, and fine-tuning of the inductor design, a GM6108A buck driver can expect an efficiency of as high as 92%. High efficiency not only simplifies the heat sink design, it also improves the overall efficacy and the reliability of the lighting fixture.

One key to achieve over 90% efficiency is use to use LED modules with higher VF. This is due to the free-wheeling diode loss is about Vd/VF. For example, if if Vd = 1.2V, and VF= 20V, the diode loss is near 5.4%. If VF is increased to 48V, the diode loss will be reduced to about 2%. The 3.4% efficiency improvement is quite substantial.

Isolated Forward Topology Design In many applications, such as LED ceiling light tubes and LED street lights, isolated designs (forward topology) are preferred. Although a forward topology requires several additional parts, it does simplify the thermal design, and it is easier to pass safety regulations. However, there is generally a size and cost premium due to the addition of two diodes and a power transformer. The efficiency of a GM6108A forward converter design will be expected to be about 83% to 88%. A major benefit of using a forward topology is its transformer allows flexible turn-ratio design, and it can extend the duty cycle at high Vin. The extended duty cycle reduces the RMS value of the current flowing through the primary winding and the power MOSFET.

Fig. 5 shows a reference design for a universal input, 6W LED light bulb design. It uses an isolated forward topology. Notice the RCD snubber circuit on the primary-side also serves to reset the core flux during Toff. The output is six 1-W LEDs connected in series. Notice that Fig. 5 circuit includes an over-voltage shutdown circuit. This OVP circuit protects the driver from charging the output capacitor to high voltage if the LED load is inadvertently disconnected. The transformer uses an EEL-16 core. N1 is 160 turns of AWG32 wire. N2 is 80 turns of AWG30.wire. N1 has Irms of 82mA; N2 has Irms of 164mA.

For the power MOSFET, we recommend FQD2N60C. It is a 600V, 4.5Ω device. The important feature of this MOSFET is its low Coss of 25pF.

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RCD Snubber Design In a forward transformer, it is necessary to reset the core flux after each turn-on cycle. The minimum reset voltage required is according to the following equation, Vrst = Vin*D/(1-D) However, the voltage across C4 is typically higher than the minimum reset voltage. The reset voltage level and the RCD snubber design are related to several factors. Heuristically, we found a transformer with a good coupling (lower leakage inductance) between the windings will reduce the RCD snubber loss and the reset voltage level. In Fig. 5 circuit, C4 is 1n, R4 is 200K. Higher R3 will reduce the snubber loss, but the reset voltage will increase too. It is important to keep the reset voltage to less than 150V if a 600V MOSFET is used. Line Regulation and Calibration For low-line input range of 90Vac to 130Vac, GM6108A provides good line regulation of +/-3%. For high-line input range of 180Vac to 270Vac, GM6108A

also provides a good line regulation of +/- 3%. However, for universal input range of 90Vac to 270Vac, we need to place a resistor, Rcal, between CAL (Pin 5) and FBK (Pin 3) to achieve tight line regulation. The proper value for the calibration resistance is related to the lamp power rating and VF value. We recommend to start with a 20K value for Rcal. After actual measurement of high-line output power, increase Rcal if high-line power is lower than low-line power; reduce Rcal if high-line power is higher than low-line power. An 18W LED Tube Design Fig. 6 shows a reference design for an 18W LED tube driver. It also uses an EEL-16 transformer. N1 and N2 are 120 turns each. Switching Waveforms Fig. 7A shows Vds and Isen waveforms of Fig. 5 circuit at Vin = 110Vac over a half-cycle of 60Hz line frequency. Fig. 7B shows Vds and Isen waveforms of Fig. 5 circuit at Vin = 220Vac.

GM6108A

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90V ~

270VAC

VDD

GM6108A

VSINE

GATE

ISEN

R3

820k

VCC

R1

1M

R2

20k

C1

0.1u

CAL

0.2u

680p

GND

Q1

Rsen

2.0

175T

45V, 120mA

LED module

GM6108A app buck

C3

6.8u

EE10

175T: 56T

L1 = 5.6mH56T

D2D3 C4

3.3u

Q1: FQD3N40

400V, 3.4Ω

Coss = 40pF

D2: ES1G

47

C2

0.1u

2.2mH

Cx

0.1u

AC V+

AC V-

Conduction loss = 0.19A*0.19A*3.4*0.3 = 37mW

Switching loss = 40P*270V*270V*100k =146mW.

FBK

CT

330

1n

20k

Fig. 2 Universal Input 6W Non-Isolated LED Light Bulb

Top trace: Vds (100V/div); Mid trace: VDD (10V/div) Bottom trace: Isen (200mV/div); Time base: 10msec/div

Fig. 3 Start-up waveform

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Top Trace: Vds (100V/Div) at 120Hz cycle Second trace: Isen (0.2V/Div) at 120Hz cycle Third trace: Vds (100V/Div) zoom-in at θ Fourth trace: Isen (0.2V/Div) zoom-in at θ

Fig. 4 6W Buck Waveforms at 110Vac input

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Fig. 5 Universal Input 6W Forward LED Driver Circuit

Fig. 6 Universal Input 18W Forward LED Driver Circuit

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Top Trace: Vds (100V/Div) in 60Hz half-cycle Second trace: Isen (0.2V/Div) in 60Hz half-cycle Third trace: Vds (100V/Div) zoom-in at θ Fourth trace: Isen (0.2V/Div) zoom-in at θ

Fig. 7A 6W Forward Waveforms at 110Vac Input

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Top Trace: Vds (100V/Div) in 60Hz half-cycle Second trace: Isen (0.2V/Div) in 60Hz half-cycle Third trace: Vds (100V/Div) zoom-in at θ Fourth trace: Isen (0.2V/Div) zoom-in at θ

Fig. 7B 6W Forward Waveforms at 220Vac Input

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PACKAGE DIMENSIONS MSOP-10