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BZX100A bảng dữ liệu(PDF) 10 Page - Linear Technology |
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BZX100A bảng dữ liệu(HTML) 10 Page - Linear Technology |
10 / 24 page LT8300 10 8300f applicaTions inForMaTion Selecting Actual RFB Resistor Value The LT8300 uses a unique sampling scheme to regulate the isolated output voltage. Due to the sampling nature, the scheme contains repeatable delays and error sources, whichwillaffecttheoutputvoltageandforceare-evaluation of the RFB resistor value. Therefore, a simple two-step process is required to choose feedback resistor RFB. Rearrangement of the expression for VOUT in the Output Voltage section yields the starting value for RFB: RFB = NPS • VOUT + VF ( ) 100µA VOUT = Output voltage VF = Output diode forward voltage = ~0.3V NPS = Transformer effective primary-to-secondary turns ratio Power up the application with the starting RFB value and other components connected, and measure the regulated output voltage, VOUT(MEAS). The final RFB value can be adjusted to: RFB(FINAL) = VOUT VOUT(MEAS) •RFB OncethefinalRFBvalueisselected,theregulationaccuracy from board to board for a given application will be very consistent, typically under ±5% when including device variation of all the components in the system (assuming resistor tolerances and transformer windings matching within ±1%). However, if the transformer or the output diode is changed, or the layout is dramatically altered, there may be some change in VOUT. Output Power Aflybackconverterhasacomplicatedrelationshipbetween the input and output currents compared to a buck or a boostconverter.Aboostconverterhasarelativelyconstant maximum input current regardless of input voltage and a buck converter has a relatively constant maximum output current regardless of input voltage. This is due to the continuous non-switching behavior of the two currents. A flybackconverterhasbothdiscontinuousinputandoutput currentswhichmakeitsimilartoanon-isolatedbuck-boost converter. The duty cycle will affect the input and output currents, making it hard to predict output power. In ad- dition, the winding ratio can be changed to multiply the output current at the expense of a higher switch voltage. The graphs in Figures 1 to 4 show the typical maximum output power possible for the output voltages 3.3V, 5V, 12V, and 24V. The maximum output power curve is the calculated output power if the switch voltage is 120V dur- ing the switch-off time. 30V of margin is left for leakage inductance voltage spike. To achieve this power level at a given input, a winding ratio value must be calculated to stress the switch to 120V, resulting in some odd ratio values. The curves below the maximum output power curve are examples of common winding ratio values and the amount of output power at given input voltages. One design example would be a 5V output converter with a minimum input voltage of 36V and a maximum input voltage of 72V. A six-to-one winding ratio fits this design example perfectly and outputs equal to 2.44W at 72V but lowers to 1.87W at 36V. The following equations calculate output power: POUT = η• VIN •D •ISW(MAX) • 0.5 η = Efficiency = 85% D = DutyCycle = VOUT + VF ( )•NPS VOUT + VF ( )•NPS + VIN ISW(MAX) = Maximum switch current limit = 260mA |
Số phần tương tự - BZX100A |
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Mô tả tương tự - BZX100A |
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