Ncp1014: Low Power, Off-line Buck, Cvcc Power Supply

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 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.
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Ncp1014: Low Power, Off-line Buck, Cvcc Power Supply application circuits
Ncp1014: Low Power, Off-line Buck, Cvcc Power Supply application circuits



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