DN06037/D Design Note – DN06037/D Low Power, Off-Line, CVCC Power Supply Device Application Input Voltage Output Power Topology I/O Isolation NCP1014 LED Driver, Chargers 90 – 260 Vac Up to 10W Off-Line Buck Non-isolated Other Specifications Parameter Output Voltage Ripple Nominal Current Max Current Min Current Others Output Specification 5 to 28 Vdc depending on selected Z1 zener value Dependent on L2 inductance and C6 ESR, typically 1% 50 to 350 mA typical 350 mA, set by R3 zero PFC (Yes/No) No, Pout < 25 watts Minimum Efficiency 65% (dependent on Vout and Iout combination) Inrush Limiting / Fuse Inrush resistor (R1) Operating Temp. Range Cooling Method/Supply Orientation Signal Level Control 0 to +60°C (dependent on U1 heatsinking) Convection None For applications where isolation from AC line is not required. Circuit Description This Design Note describes a simple, low power (10 W or less), constant voltage, constant current (CVCC) power supply intended for LED strings, battery chargers or similar applications where the CVCC load characteristic is necessary, and isolation from the AC mains is not required. The output voltage can be set from 5 to approximately 28 volts by using the appropriate zener diode for Z1. Output voltage sensing is accomplished by utilizing a level shifting circuit comprised of D7, C8, Z1, Q2 and the associated resistors. In applications where the output negative must be common with the input bulk capacitor negative, voltage sensing of the output directly is not possible. This is because the ground node of the controller is switched between the peak input dc voltage and the common line. In order to sense the voltage without an optocoupler in the feedback path, the level shifting “bootstrap” circuit acts as a sample and hold network which stores a sample of the output voltage on C8 (through D7) when the Mosfet in U1 is in the off state. During the on-time, U1’s ground reference is shifted up to to the input voltage level of choke L2, and sense diode D7 is back biased. When the voltage on C8 exceeds Z1’s February 2008, Rev. 0 breakdown level and the B-E drop on Q2, then the feedback pin of U1 (pin 2) is pulled down to achieve regulation. The output current is regulated during the constant current mode by the peak sensing circuit of D5, R3, R2, C5 and Q1. The max current is actually limited by the NCP1014 controller’s internal circuitry to about 450 mA, but is controlled externally by this circuit and the value of R3 by detecting the peak inductor current which is a good representation of the dc output current. Due to the additive magnetizing component of L2, the true load current should be limited to about 350 mA maximum for reliable constant current operation. A lower inductance than approximately 2.5 mH for L2 will probably lower the effective usable output current, so a minimum inductance value of 2.5 mH is recommended to keep the choke’s magnetizing current low. For lower currents and/or output voltages some compromises can be made on the maximum inductance value, however, lower values could also impact the output ripple if output capacitor C6 has high ESR. The schematic, V/I plot, and switching waveform are for an 18 volt, 325 mA prototype. www.onsemi.com 1 DN06037/D The output power will also be thermally limited, so for power levels above 5 watts, the ground tab on U1 (SOT223 package) should be soldered to a sufficient spread of copper pc board clad to assure that the internal over-temperature circuit is not activated. Key Features y Input filter (pi-network) for conducted EMI attenuation. y Constant current, constant voltage output regulation with minimal components and no optocoupler for feedback. y NCP1014 monolithic current mode controller with internal 700V Mosfet for maximum simplicity (see data sheet at www.onsemi.com. y Circuit easily configured for different output voltage and current requirements. y Inherent short circuit and over-temperature protection. y No optocoupler required for voltage sensing. Schematic D5 Q1 MMBT2222 MMSD4148 C5 R1 5.1, 2W AC input D1, 2, 3, 4 MRA4007 x 4 L1, 1 mH 0.6 A F1 0.5A 2 C2 + 1 C1 4.7nF "x" 85 - 265Vac U1 4 C4 22uF 25V R2 100 C9 1nF R6 100 MMBT2222 R7 1K + 1.8 ohm 0.25W C7 C6 470uF 35V D6 MURA 160 NCP1014 100 kHz (SOT-223) Q2 L2, 2.5 mH, 0.5A R3 3 C3 10 uF, 400Vdc x2 1nF 0.1uF 50V Output 18 V, 325 mA _ R4 10K Z1 MMSZ18T1 D7 MURA160 R5 100K + C8 4.7uF 35V Notes: 1. Vout set by Z1 (Vout = Vz) 2. I out set by R3 (Iout = 0.65/R3); Imax = 350 mA 3. L1 is Coilcraft MSS1260-105KL or similar. 4. Thick lines indicate recommended ground plane area 5. U1 should be heatsunk via ground tab to copper clad area 6. Crossed schematic lines are not connected Output Choke Design: L2 can be constructed by winding 200 turns of #28 magnet wire on the bobbin of an EF-16 (E16/8/5) ferrite core with a cross sectional area (Ae) of 0.2 square centimeters (or similar ferrite core and bobbin), and gapping the core to achieve an inductance of 2.5 mH when measured across the winding. February 2008, Rev. 0 www.onsemi.com 2 DN06037/D 1 NCP1014 Buck CCCV V/I Profile @ 120 Vac input 20 18 16 V out (V) 14 12 10 8 6 4 2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 I out (A) Note the “current tail” below 5 volts output due to the very short pulse width and the subsequent propagation delay effects in the controller. This sets a lower useful output level to the supply when in the CVCC mode to about 5 volts. Efficiency: 18 volts, 275 mA output (CV mode): 77% 17 volts, 325 mA output (CVCC knee): 76% 14 volts 375 mA output (CC mode): 74% Input Voltage to Choke (Cathode of D6) During Operation at CVCC Knee (120 Vac input): 1 © 2008 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 Frank Cathell, e-mail: [email protected] February 2008, Rev. 0 www.onsemi.com 3