TND373/D Rev. 0, Oct -- 2009 1 W to 5 W LED Driver for MR16 LED Reference Design Documentation Package Disclaimer: ON Semiconductor is providing this reference design documentation package “AS IS” and the recipient assumes all risk associated with the use and/or commercialization of this design package. No licenses to ON Semiconductor’s or any third party’s Intellectual Property is conveyed by the transfer of this documentation. This reference design documentation package is provided only to assist the customers in evaluation and feasibility assessment of the reference design. It is expected that users may make further refinements to meet specific performance goals. http://onsemi.com 2 TND373/D TND373/D 1 W to 5 W LED Driver for MR16 LED http://onsemi.com TECHNICAL NOTE Introduction boost topologies. Once understood, the buck--boost topology can offer many advantages for cost effective HB LED lighting when Vin overlaps Vf. The market for High--Brightness LED’s (HB--LED) continues to rise rapidly. Over the last few years LED efficacy (lm/W) has more than doubled while flux per package continues to rise rapidly making them more useful for many new applications. We have seen revolutionary new products in handheld flashlights, architectural lighting and street lighting. The challenge continues to be making a more cost effective system versus incandescent and compact fluorescent bulbs. In many applications this challenge involves powering the HB--LED’s from a wide input--voltage range source. This is especially true in general illumination applications like track lighting where the power source is a 12 Vac or +12 Vdc source that can be very loosely regulated. The LED’s need to be driven by a current source rather than a voltage source since the forward voltage (3.4 V nominal) can vary more than ±20% over process tolerance and temperature. Moreover given the flux of current 1 W warm white power LEDs, it is common to need 3--4 LEDs to replace the light output of a 20 W incandescent. To obtain predictable and matched luminosity and chromaticity it is also desirable to drive the LED’s with a constant current. The buck--boost topology meets this requirement from an architectural perspective but it not as common as the standard buck or Overview This reference document describes a built and tested, GreenPoint® solution for a 1 to 5 W LED driver for MR16 LED replacement. The circuit is proposed for driving HB--LED (high--brightness LED) in a variety of lighting applications but is configured in size and features for an MR16 LED replacement. Configurations like this are found in 12 Vac / 12 Vdc track lighting applications, automotive applications, and low voltage AC landscaping applications as well as task lighting such as under--cabinet lights and desk lamps that might be powered from standard off--the--shelf Vac wall adapters. The circuit is based around the ON Semiconductor NCP3065 operating at ~150 kHz in a non--isolated configuration. A key consideration in this design was achieving flat current regulation across input line variation and output voltage variation with a 12 Vac input. It also features an auto--detect circuit in combination with the NCP3065 which allows input from a 12 Vdc or 12 Vac supply and still maintain targeted output current regulation. 12 VACIN NCP3065 Compensation Network Figure 1. Simplified Block Diagram Actual Size http://onsemi.com 3 TND373/D Basic Power Topology energy in L. In this stage, the capacitor C supplies energy to the output load; While in the Off--state, the inductor is connected to the output load and capacitor through the Output Diode, so energy is transferred to the load. The principle of the Buck--Boost converter is fairly simple (see Figure 2): While in the On--state, the input voltage source is directly connected to the inductor (L). This results in accumulating Vin ID IQ Vsw Vgate Vout C IL RLoad Ton Vgate From Inductor Volt Second V = L di Balance and: dt Toff Vin Vi(Ton) Vsw Vo--Vf L = Vo(Toff) L VinD = Vo(1 − D) IQ ID Vo Vin IL Figure 2. Buck--Boost Operation http://onsemi.com 4 = D (1 − D) TND373/D Figure 3. Vswitch Node Waveform TSD NC ILimit Comp Set Switch dominant Collector R Q S Ipk Sense S Switch Q Emitter Set R dominant 0.2V Oscillator Ct Vcc Vref Ct GND Comp Inv Figure 4. NCP3065 Burst Mode Controller Burst Mode Control R8 is used to sense the inductor current and is fed to the FB pin of the NCP3065. This application produces OFF time instantaneous (Ivalley) inductor current control (see Figure 5). A cycle of switch ON time is only allowed to start once the OFF time Inductor current crosses the Vref threshold. The basic control loop consists of a 235 mV internal Reference, a Feedback Comparator, and two Set--Dominant RS Latches. Basically the NCP3065 allows the Power FET for the Buck--Boost stage to switch ON as the Feedback Voltage falls below the reference voltage. The Power FET will be then be forced OFF unconditionally during Ct Ramp down. http://onsemi.com 5 TND373/D Ipeak Ivalley Ton Ton Toff Toff Average Load Current = Area During Toff Figure 5. Buck--Boost Inductor Current 0.4 Since the controller does not provide integral PWM control and utilizes only a comparator trip point for feedback, the peak to average load current is not in direct proportion as in a Buck Converter, but rather follows the following formula: ⎧V 1 − ⎪L ⎩ 1 valley + 2 o Vo V o+V F ⎫⎫ ⎪⎪1 − V ⎭⎭ in Iave, (Vin) I ave ⎧ =⎪I ⎩ 0.3 Vo o + V in 0.2 0.1 Where, Ivalley is the lowest inductor current point. Plotting Iave vs Vin shows a dramatic curve which would cause a significant change in light output of the LED (see Figure 6). 0 10 12 14 16 18 Figure 7. Average LED Current vs Vin DC (With Vin Compensation) 1.2 Iave, (Vin) 8 Vin, N 1.4 A resistive divider network consisting of R3, R5 and summing resistor R4 are used to add Vin proportional voltage to the FB pin in order to reduce the load current as Vin is increased. This has the effect of flattening the curve of Figure 6 and reduces the overall current error (see Figure 7). This average line can be DC shifted with R8 and the ends can be aligned by adjusting R5, R3 and R4. R9 and C6 are used to limit the gate to source voltage on the external switch at high input voltage. The resistor divider network of R9 and R2 are used to program and gate to source maximum. 1 0.8 0.6 0.4 6 0 5 10 Vin, N 15 20 Figure 6. Average LED Current vs Vin DC (Without Vin Compensation) V gs = V in − Therefore an input voltage feed--forward compensation network is used to reduce the error due to the nonlinear response of the Iout vs Vin curve. http://onsemi.com 6 V in ⋅ R9 R9 + R2 (eq. 1) TND373/D Pulsed Feedback Resistor R7 and D5 add current to the Ct timing capacitor C2. This effectively limits the maximum achievable duty cycle of the NCP3065. When conditions warrant low duty cycle, R7 and D5 make higher than desired duty cycles unavailable. D7 is necessary to block voltage during the OFF time, since this is Buck--Boost Topology. More information on Pulsed Feedback compensation is available in the NCP3065 data sheet. R7 and D5 are used to reduce the possibility of pulse skipping (see Figure 8). Since burst mode control involves only one feedback voltage, cross--detection per cycle and does not involve the use of a window comparator, it is possible to have skipped pulses which do not effect the DC regulation but could be visible as flicker in an LED application if the pulsing had a low frequency component. Figure 8. Pulsed Feedback Resistor AC Operation vs DC for some finite portion ~80% of the 120 Hz line cycle, and then no output for ~20%. This has the effect of reducing the average current by ~20% when operating with AC input. Thermal consideration should be taken when running with > 12 Vac. In most applications the module is potted to increase thermal dissipation. An additional AC compensation network is added to the Vin Compensation to account for the different operating point (see Figure 9). Since there is a half sine wave input to the Buck--Boost stage, there is a different operating point as compared with pure DC input. Since small size is a goal for this design very little input capacitance is used past the full bridge rectifier. As a result the line voltage can drop to as little as 3 V depending on the input capacitance selection. Therefore, the input to the converter is a full wave rectified sine wave. Since the regulator is non--functional below ~4 V there are dead spots in the regulation. So we end up with regulation http://onsemi.com 7 TND373/D Figure 9. Pulsed Feedback Resistor Protection the power FET with reasonable voltage margin. This may require some trial and error to select since the clamp voltage will stretch depending on how much energy needs to be absorbed. Z1 and R1, along with the Current limit function of the NCP3065, are used for open circuit protection. In the event of an open circuit at the load, the loop will try to increase the output voltage in order to satisfy the current demand which feeds back zero current. When (Vin + Vout) exceeds the voltage of Z1, current will flow in R1 which triggers the current limit function of the NCP3065. Short circuit protection is handled with a fuse, F1, on the input. Surge protection from inductive loads is an important consideration specifically in transformer fed systems that carry significant source inductance such as found with magnetic transformers used in landscape lighting applications. The surge device needs to be selected to a voltage that will never exceed the gate to source voltage of Increasing Output Current The reference design is configured for 350 mA average LED current. Increasing the current regulation point on the reference board is as simple as cutting the current sense resistor R8 in half from 250 mΩ to 125 mΩ. Also, the input fuse must be increased to accommodate the increased input current draw. Heat sinking may be required depending on the implementation of the housing and the environmental characteristics when moving to the higher power design. http://onsemi.com 8 TND373/D PC BOARD Figure 10. Component Placement (Top) Figure 11. Traces (Top View) Figure 12. Component Placement (Bottom) Figure 13. Traces (Bottom View) http://onsemi.com 9 TND373/D Figure 14. Reference Design 0.457” x 1.148” (11 mm x 29 mm) Remember this is an inverting output. So the negative output will connect to the anode of the LED, and the positive output will connect to the cathode of the LED. Also note, when trying to make measurements with a scope probe, that ground is NOT ground. The scope will need to be floating (ground connection removed from the AC wall source) or there will be a ground loop / short circuit that will cause the device to turn off. http://onsemi.com 10 TND373/D Figure 15. SCHEMATIC http://onsemi.com 11 TND373/D Table 1. BILL OF MATERIALS Qty Ref Value Part Number Description Manufacturer 1 F1 4A 0457004.NR Fuse LittleFuse 1 C3 10 mF GRM31MF51A106ZA01L Ceramic Chip Capacitor Murata 1 C6 1 nF GRM188R71H102KA01D Ceramic Chip Capacitor Murata 1 C1 1 mF GRM188R61E105KA12D Ceramic Chip Capacitor Murata 1 C2 5.6 nF GRM188R71H562KA01D Ceramic Chip Capacitor Murata 1 C4 10 mF GRM32NF51E106ZA01L Ceramic Chip Capacitor Murata 1 C5 10 mF GRM32NF51E106ZA01L Ceramic Chip Capacitor Murata 1 D1 1 A, 30 V MBR130T1G DIODE, SCHOTTKY ON Semiconductor 1 D2 1 A, 30 V MBR130T1G DIODE, SCHOTTKY ON Semiconductor 1 D3 1 A, 30 V MBR130T1G DIODE, SCHOTTKY ON Semiconductor 1 D4 1 A, 30 V MBR130T1G DIODE, SCHOTTKY ON Semiconductor 1 D6 2 A, 60 V MBRS260T3 DIODE, SCHOTTKY ON Semiconductor 1 Q1 PNP MBT3946DW1T1 General Purpose NPN Transistor ON Semiconductor 1 D5 0.2 A, 100 V MMSD4148T1 Diode, Small Signal ON Semiconductor 1 D8 0.2 A, 100 V MMSD4148T1 Diode, Small Signal ON Semiconductor 1 Z1 36 V MM5Z36VT1 DIODE, ZENER ON Semiconductor 1 L1 68 mH MSS1278-683MLD INDUCTOR, SM Coilcraft 1 U1 40 V 1.5 A NCP3065DR2G Switching Regulator ON Semiconductor 1 M1 P--FET NTGS4111PT1G MOSFET, P ON Semiconductor 1 R4 1.2k CRCW04021K20FKED Resistor Vishay / Dale 1 R1 100 CRCW0402100RFKED Resistor Vishay / Dale 1 R3 162k CRCW0402162KFKED Resistor Vishay / Dale 1 R6 196 CRCW0402196RFKED Resistor Vishay / Dale 1 R7 22k CRCW040222K0FKED Resistor Vishay / Dale 1 R5 22k CRCW040222K0FKED Resistor Vishay / Dale 1 R2 1k CRCW04021K00FKED Resistor Vishay / Dale 1 R9 200 CRCW0402200RFKED Resistor Vishay / Dale 1 R8 0.25 CSR1/20.25FICT-ND Resistor Vishay / Dale 1 D7 P6SMB22CAT3 ZENER, BACK TO BACK ON Semiconductor http://onsemi.com 12 TND373/D MEASUREMENTS (12 Vac Data) 0.4 0.8 0.75 0.38 0.7 0.36 0.65 0.34 0.6 0.32 0.55 Iout 0.3 8 9 10 11 12 13 14 Efficiency 15 0.5 7 Figure 16. Iout vs Vac 9 11 13 15 17 19 Figure 17. Efficiency vs Vdc GreenPoint is a registered trademark of Semiconductor Components Industries, LLC (SCILLC). ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC 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. “Typical” parameters which may be provided in SCILLC 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. 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