DN06048/D Design Note – DN06048/D Reference Design for Sharp ZENIGATA LED Module Device NCP3065 Application Lighting Input Voltage 12 VDC or 12VAC Output Power 3.6 W Topology Buck-Boost I/O Isolation NONE Specifications AC Input Voltage DC Input Output Voltage Output Current 12V AC Line transformer (Source Resistance <0.1Ohm), MR16 Solid State AC Ballast 6V to 16VDC 8 – 12 V 350 mA / 550mA regulated Circuit Description Key Features This circuit is proposed for driving the Sharp ZENIGATA LED module in a variety of lighting applications. Configurations like this are found in 12 VAC 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 5 and 12 VAC wall adapters. The circuit is based on the NCP3065 operation at ~150 kHz in a non-isolated configuration. Key consideration in this design was achieving flat current regulation across input line variation and output voltage variation with a 12VAC input. y Small size for MR-16 applications y Buck-Boost operation y Wide input and output operation voltage y Regulated output current y Open LED Protection y Output Short Circuit Protection Sharp ZENIGATA LED Module Reference Design 0.457” x 1.148” (11mm x 29mm) August 2009, Rev7 www.onsemi.com 1 DN06048/D Schematic Figure 1 – Buck-Boost converter schematic August 2009, Rev7 www.onsemi.com 2 DN06048/D Basic Power Topology 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 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. 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. August 2009, Rev7 www.onsemi.com 3 DN06048/D Vin IQ Vgate ID Vsw Vout C IL RLoad Ton Vgate From Inductor Volt Second di Balance and: V =L dt Toff Vin Vi (Ton) Vo(Toff ) = L L Vsw Vo-Vf VinD = Vo(1 − D) IQ Vo D = Vin (1 − D) ID IL Vout is can be higher or lower than Vin for D = 0 to 1 Vout is Negative Figure 2 – Buck-Boost Operation August 2009, Rev7 www.onsemi.com 4 DN06048/D TSD NC Switch Collector Set dominant ILimit Comp Ipk Sense R Q S S Q R 0.2V Switch Emitter Set dominant Oscillator Vcc Ct Vref Comp Inv Ct GND Figure 3 – NCP3065 Burst Mode Controller Burst Mode Control The basic control loop consists of a 0.235V internal Reference, a Feedback Comparator, and two SetDominant 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. 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 4). A cycle of switch ON time is only allowed to start once the OFF time Inductor current crosses the Vref threshold. August 2009, Rev7 www.onsemi.com 5 DN06048/D Ipeak Ivalley Ton Ton Toff Toff Average Load Current = Area During Toff Figure 4 - Buck-Boost Inductor Current 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: ⎛ Vo ⎞ ⎞ ⎛ ⎜ ⎟⎟ ⎜ Vo 1 − 1 Vo ⎞ ⎛ ⎛ ⎞ Iave = ⎜ Ivalley + ⎜ ⎜ ⎟ Vo + Vin ⎟ ⎟⎜1 − ⎟ ⎜ 2 ⎜⎝ L ⎠ F ⎟ ⎟⎝ Vo + Vin ⎠ ⎟⎟ ⎜ ⎜ ⎠⎠ ⎝ ⎝ 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 5). 1.4 1.319 1.2 1 Iave( Vin) 0.8 0.6 0.408 0.4 0 3 5 10 Vin 15 20 20 Figure 5 - Average LED Current vs Vin DC (Without Vin Compensation) August 2009, Rev7 www.onsemi.com 6 DN06048/D 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. 0.4 0.35 0.3 Iave( Vin) 0.2 0.1 0 0 6 5 8 10 12 Vin, N 14 16 18 19 Figure 6 - Average LED Current vs Vin DC (With Vin Compensation) 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 5 and reduces the overall current error (see Figure 6). 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. ⎛ Vin × R9 ⎞ Vgs = Vin − ⎜ ⎟ ⎝ R9 + R 2 ⎠ Pulsed Feedback Resistor R7 and D5 are used to reduce the possibility of pulse skipping (see Figure 7). 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. 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. August 2009, Rev7 www.onsemi.com 7 DN06048/D Figure 7 - Pulsed Feedback Resistor AC Operation vs DC 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. Therefore, the input to the converter is a half wave rectified sine wave. Since the regulator is non-functional below ~4V there are dead spots in the regulation. So we end up with regulation for some finite portion ~80% of the 60Hz line cycle, and then no output for ~20%. This has the effect of reducing the average current by ~20% when operating with AC input. An additional AC compensation network is added to the Vin Compensation to account for the different operating point (see Figure 8). August 2009, Rev7 www.onsemi.com 8 DN06048/D Figure 8 - Pulsed Feedback Resistor Protection Z1 and R1, along with the Current limit feature 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. The surge device needs to be selected to a voltage that will never exceed the gate to source voltage of 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 Increasing Output CurrentThe reference design is configured for 350mA 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 250mOhms to 125mOhms. 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. August 2009, Rev7 www.onsemi.com 9 DN06048/D PC Board Figure 9 - Component Placement (Top) Figure 10 – Traces (Top View) Figure 11 - Component Placement (Bottom) Figure 12 – Traces (Bottom View) August 2009, Rev7 www.onsemi.com 10 DN06048/D Qty 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Ref F1 C3 C6 C1 C2 C4 C5 D1 D2 D3 D4 D6 Q1 D5 D8 Z1 L1 U1 M1 R4 R1 R3 R6 R7 R5 R2 R9 R8 D7 Value 1A 10uF 1nF 1uF 5.6nF 10uF 10uF 1A, 30V 1A, 30V 1A, 30V 1A, 30V 2A, 60V PNP 0.2A, 100V 0.2A, 100V 36V 68uH 40 V 1.5A PFET 1.2k 100 162k 196 22k 22k 1k 200 0.25 Part Number MFU0603FF01000P100 GRM21BF51A106ZE15L GRM188R71H102KA01D GRM188R61E105KA12D GRM188R71H562KA01D GRM32NF51E106ZA01L GRM32NF51E106ZA01L MBR130T1G MBR130T1G MBR130T1G MBR130T1G MBRS260T3 MBT3946DW1T1 MMSD4148T1 MMSD4148T1 MM5Z36VT1 MSS1278-683MLD NCP3065DR2G NTGS4111PT1G CRCW04021K20FKED CRCW0402100RFKED CRCW0402162KFKED CRCW0402196RFKED CRCW040222K0FKED CRCW040222K0FKED CRCW04021K00FKED CRCW0402200RFKED CSR1/20.25FICT-ND P6SMB22CAT3 Description Fuse Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor DIODE, SCHOTTKY DIODE, SCHOTTKY DIODE, SCHOTTKY DIODE, SCHOTTKY DIODE, SCHOTTKY General Purpose NPN Transistor Diode, Small Signal Diode, Small Signal DIODE, ZENER INDUCTOR, SM Switching Regulator MOSFET, P Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor ZENER, BACK TO BACK Manufacturer Tyco Murata Murata Murata Murata Murata Murata ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor ON Semiconductor On Semiconductor On Semiconductor ON Semiconductor Coilcraft ON Semiconductor On Semiconductor Vishay / Dale Vishay / Dale Vishay / Dale Vishay / Dale Vishay / Dale Vishay / Dale Vishay / Dale Vishay / Dale Vishay / Dale ON Semiconductor Table 1 – Bill of materials Figure 12 - LED Module Spec (Sharp P/N GW5BWC15L02) August 2009, Rev7 www.onsemi.com 11 DN06048/D Measurements 12V (ac) Data Iout vs VAC 0.4 0.38 0.36 Iout 0.34 0.32 0.3 8 9 10 11 12 13 14 15 AC Efficiency vs VAC 0.8000 0.7000 0.6000 0.5000 eff 0.4000 0.3000 0.2000 8 August 2009, Rev7 9 10 11 12 www.onsemi.com 13 14 15 12 DN06048/D Efficiency vs Vdc 0.8 0.75 0.7 0.65 Eff 0.6 0.55 0.5 7 9 11 13 15 17 19 1 © 2007 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 Tim Kaske and Tom Duffy, e-mail: [email protected] ; [email protected] August 2009, Rev7 www.onsemi.com 13