NCP1129, 12 Watt, Off-line Buck Regulator
■ This design note describes a simple, low power (15 W or less), constant voltage buck power supply intended for powering electronics for white goods, electrical meters, and industrial equipment where isolation from the AC mains is not required. The efficiency limitations of the off- line buck converter are also discussed with emphasis on switching losses caused by the freewheel diode recovery characteristics. The output voltage can be set from 12 to approximately 28 volts (or higher) by using the appropriate zener diode for Z1 and selection of trim resistor R4. The new ON Semiconductor NCP1129 co- packaged controller/Mosfet in a DIP 8 package is utilized as the buck switching element. Output reguation is accomplished by utilizing a simple zener/optocoupler sensing scheme. The optocoupler is necessary because the control logic is at a switching node common to the freewheel diode (D3) cathode. This was done along with half-wave input line rectification to allow a common connection from the input line neutral to the negative output terminal.
■ The main problem associated with off-line buck converters is the efficiency associated with the combination of low duty cycle (Vout less than 50V) and the power Mosfet and freewheel diode turn-on switching loss. ON Semiconductor application note AND8318 discusses one solution for both of these issues by using a tapped buck inductor. This solution, however, typically requires a non-standard inductor which may not be an “off-the-shelf” component.
■ At initial Mosfet turn-on, the freewheel diode recovery characteristics causes a significant leading edge spike of current to flow through both devices that can easily exceed 3 to 5 times the normal peak load current of the devices. This is particularly acute at high input line (230 Vac) if a fast recovery type of diode is used where the reverse recovery characteristics are slower with increased diode PRV voltage rating. Conventional high voltage Schottky diodes (typically several in series) and silicon carbide (SiC) diodes will greatly reduce the turn-on switching loss but there will still be a leading edge current spike associated with the devices’ junction capacitance. The table and waveforms shown below compare the representative efficiencies and switching characteristics when using the three diode types. The SiC diode definitely exhibited the best performance, but the present day cost of this part may be prohibitive for the intended application, and a pair of (or 3) series standard Schottky rectifiers are probably the best compromise. Tests have shown that the use of a typical ultra-fast recovery rectifier for D3 may prohibit continuous output currents much greater than about 500 mA due to excessive heating of the Mosfet and/or the freewheel diode itself. Obviously good pc board layout with liberal copper clad for heatsinking will certainly help the thermal issues.
■ Protection features in the NCP1129 include peak overcurrent limiting which can be set by proper selection of current sense resistor R7, and the Vcc pin will latch the chip off if the Vcc exceeds 27 volts, allowing for overvoltage detection. Over-temperature protection at 150C (internal) is also included.
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[ Smart Meters ][ Electric Meters ][ White Goods ][ constant voltage buck power supply ] |
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Application note & Design Guide |
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Please see the document for details |
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DIP |
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English Chinese Chinese and English Japanese |
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October 2013 |
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Rev. 0 |
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DN05053/D |
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136 KB |
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