DN05035/D Design Note – DN05035/D LED Driver with Synchronous Rectification Device NCL30000 NCP4303A NTD5865 Application Input Voltage Output Power LED Driver 90 – 135 V ac 14 Watts Topology Flyback w/ synchronous rectification I/O Isolation Yes Output Output Current Ripple Nominal Voltage Maximum Voltage 1.5 amps 640 mA pk-pk 9.1 volts 12.5 volts Typical Power Factor Typical THDi Typical Efficiency Inrush Limiting/Fuse Operating Temp. Range 0.976 19.7% 85.2% 1 amp -40 to 70 °C providing a significant reduction in dissipation and consequently an increase in efficiency. Circuit Description There are a number of system trends occurring in LED lighting that require improvements to the power architecture to optimize the system efficiency and overall cost. This design note will discuss how the trend to use new generations of LEDs than can be driven at a higher current thus requiring fewer LEDs alters the power architecture of the single stage power factor corrected flyback. As a result a PAR bulb that in the past might have used 9 LEDs (27 V nominal) at 500 mA can now achieve the same lumen output with 3 LEDs driven at 1.5A. As the output current increases and the output voltage decreases, the losses in the output rectifier become more significant. Drivers with low output voltage historically use Schottky output rectifiers to leverage the low forward drop offered by this type of semiconductor and minimize power loss. Devices with very low forward drop are available, but may not provide the level of performance the designer was anticipating. Some Schottky rectifiers display very high reverse leakage current especially at elevated temperature which can degrade efficiency. Fortunately there is another rectification option available which displays extremely low effective forward drop in low output voltage applications without reverse leakage issues. The conventional PN junction or even Schottky rectifier can be replaced by a MOSFET device Aug 2012, Rev. 0 Gate drive for a synchronous rectifier MOSFET is carefully controlled to achieve best performance. The MOSFET must be turned on quickly when secondary current begins to flow and then turned off before reverse current develops. ON Semiconductor’s NCP4303A provides the required synchronous rectifier control. The CREE XLAMP™ XM-L LED shown above is rated for up to 3 amps of drive current and an ANSI Warm White LED driven at 1.5A can provide 330-390 lumens nominal at a junction temperature of 100 ºC.A lighting solution based on these LEDs may require only a few devices to achieve the target lumen output. This example driver is intended to power three XM-L LEDs with a drive current of 1.5 amps. www.onsemi.com 1 DN05035/D Shown below are the design guidelines for this driver: • • • • • • • • winding begins to conduct. The gate voltage begins to rise about 26 ns after the MOSFET body diode begins to conduct. This rapid turn on minimizes loss by allowing the MOSFET channel to conduct bypassing the body diode. Input range: 90 – 135 V ac Output current: 1.5 amps Output voltage: 9.1 volts Efficiency: >85% Power factor: > 0.95 TRIAC Dimmer Compatible Isolated Output Open/Short Circuit protection Implementation The NCL30000LED1GEVB demonstration board covers most of the performance requirements listed above. With a few changes it can be modified for this application. The original transformer was designed to accommodate 4 to 15 LEDs at up to 700 mA. A new transformer was designed to optimize performance with three LEDs at 1.5 amps. Transformer design details are included in this Design Note. The MOSFET used for the synchronous rectifier needs to display a low forward voltage at the circuit operating current. Peak secondary current is 9.6 amps. The NTD5865 MOSFET is rated at 18 milliohms on resistance at 25 ºC. Expected voltage drop will be 9.6 amps times 0.018 ohms or 0.17 volts. Some increase is expected at high ambient but the voltage drop will remain lower than a typical Schottky rectifier and as such will improve efficiency. Figure 1: Synchronous MOSFET turn-on Figure 2 shows the synchronous MOSFET turn-off. The secondary current in a CrM flyback reduces to a very low level near the end of the switching cycle and the MOSFET is off some 800 ns before the next switching cycle. This delay ensures the transformer demagnetization signal is not affected. The NCP4303A detects current flow by monitoring the voltage drop across the synchronous MOSFET device. When the current reaches appropriate levels the gate is turned on or off as required. The threshold is programmable via an external resistor creating an offset voltage due to an internal 100 µA current source. For this application, a 10 ohm resistor is used to ensure the MOSFET is off before the end of the transformer discharge period to maintain proper CrM timing. Programmable timing thresholds are used to avoid false triggering due to ringing. Details on use of this synchronous rectifier controller can be found at the ON NCP4303 Semiconductor website in the documentation. The NCP4303 requires 10.5 volts Vcc worst case. This LED driver has a nominal 9.1 volt output. An additional 3 turn secondary bias winding was added to provide sufficient voltage to power the NCP4303. LED applications operating above 10.5 volts will not require this extra bias winding and accompanying small diode. The schematic is shown in Figure 3. Test Results The waveform in Figure 1 shows the synchronous MOSFET gate turning on after the transformer secondary Aug 2012, Rev. 0 Figure 2: Synchronous MOSFET turn-off Efficiency of this LED driver operating at 115 V ac input is 85.2%. For comparison purposes, the synchronous rectifier was removed and replaced with an MBRF30H60 Schottky rectifier. This device is rated 30 amps and 60 volts. Efficiency with this rectifier is 82.9%. The sychronous rectification circuit reduced the overall system losses by approximately 440 mW. The efficiency improved 2.3% using synchronous rectification. The higher efficiency reduces the temperature of the driver which can enhance operational lifetime since the output rectifier is normally placed near the output electrolytic capacitors. www.onsemi.com 2 DN05035/D Figure 3 : Schematic Aug 2012, Rev. 0 www.onsemi.com 3 DN05035/D 1 MAGNETICS DESIGN DATA SHEET Project / Customer: NCL30000 Part Description: 14 Watt LED Driver; 115V triac dimmable Schematic ID: T1 Inductance: 260 µH Bobbin Type: 10 pin horizontal CSH-EFD25-1S-10P Core Type: EFD25/13/9-3C90 Core Gap: Gap for 260 µH, ~0.009 inches Winding Number / Type Step Winding Start 1 ½Primary 6 2 3 Insulate Secondary 4 5 Insulate Sec Bias Fly9 6 7 Insulate ½Primary 3 8 9 Insulate Pri Bias 1 10 11 12 Insulate Assemble Shield 13 Insulate Fly1 Fly3 Fly5 Fly7 7 Aug2012 Turns / Material / Gauge / Insulation Data Finish Turns Material Notes 3 15 #26 Wind in one layer 1 Mylar Tape Fly2 6 #26 TEX-E Wind quadfilar Triple in one layer. Fly Fly4 insulated leads exit top of Fly6 bobbin over Fly8 pins 6-10 1 Mylar Tape Fly10 3 #26 TEX-E Spread evenly Triple in one layer insulated 1 Mylar Tape 5 15 #26 Wind in one layer 1 Mylar Tape 2 13 #26 Spread evenly in one layer 3 Mylar Tape Gap Final core wrap Copper Add shield over core Mylar Tape Insulate shield Hipot: 3KV from primary to secondary for 1 minute. Note: This transformer is suitable for 230 V ac applications. The switching MOSFET and synchronous rectifier MOSFET ratings should be adjusted for the increased voltage. On-time capacitor C9 may also be changed for optimal dimming performance. Aug 2012, Rev. 0 www.onsemi.com 4 DN05035/D 2 Bill of Materials The table below highlights the changes made to the NCL30000LED1GEVB demonstration board to implement this 1.5 amp 9.1 volt LED driver with synchronous rectification. Designator Qty Description C9 1 Ceramic capacitor R20 1 C11, C12 Value Tolerance Footprint 120pF 5% 603 AVX 06033A121JAT4A Resistor 0.18Ω 1% 1206 Rohm MCR18EZHFLR180 2 Electrolytic capacitor 3300 uF 16V 20% Radial UCC EKZE160ELL332MK35S T1 1 Transformer - Custom Q6 1 MOSFET 60V 18mΩ - DPAK ON Semiconductor NTD5865N-1G D12 1 Rectifier 12V 5% SOT-23 ON Semiconductor BZX84C12LT1G U5 1 Sync Rec Controller - - SOIC8 ON Semiconductor NCP4303ADR2G R29 1 Resistor 0.047Ω 1% 1206 Rohm MCR18EZHFLSR047 D14 1 Rectifier 250V 200mA - SOT-23 ON Semiconductor BAS21LT1G R32 1 Resistor 10k 1% 603 Panasonic ERJ-3EKF1002V R33 1 Resistor 4.7Ω 1% 603 Panasonic ERJ-3RQF4R7V R34 1 Resistor 10Ω 1% 603 Panasonic ERJ-3EKF10R0V R35 1 Resistor 9.1k 1% 603 Panasonic ERJ-3EKF9101V R36 1 Resistor 15k 1% 603 Panasonic ERJ-3EKF1502V C17 1 Ceramic capacitor 100nF 25V 10% 603 Panasonic ECJ-1VB1E104K C18 1 Electrolytic capacitor 10uF 50V 20% Radial Panasonic EEU-EB1H100S - Manufacturer Part Number 2 © 2012 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] Aug 2012, Rev. 0 www.onsemi.com 5