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ML4819CP 데이터시트(PDF) 8 Page - Micro Linear Corporation |
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ML4819CP 데이터시트(HTML) 8 Page - Micro Linear Corporation |
8 / 15 page ML4819 8 APPLICATIONS POWER FACTOR SECTION The power factor section in the ML4819 is similar to the power factor section in the ML4812 with the exception of the operation of the slope compensation circuit. Please refer to the ML4812 data sheet for more information. The following calculations refer to Figure 12 in this data sheet. The component designators in the equations below refer to the following components in Figure 12: RT = R16, CT = C6. INPUT INDUCTOR (L1) SELECTION The central component in the regulator is the input boost inductor. The value of this inductor controls various critical operational aspects of the regulator. If the value is too low, the input current distortion will be high and will result in low power factor and increased noise at the input. This will require more input filtering. In addition, when the value of the inductor is low the inductor dries out (runs out of current) at low currents. Thus the power factor will decrease at lower power levels and/or higher line voltages. If the inductor value is too high, then for a given operating current the required size of the inductor core will be large and/or the required number of turns will be high. So a balance must be reached between distortion and core size. One more condition where the inductor can dry out is analyzed below where it is shown to be maximum duty cycle dependent. For the boost converter at steady state: V V D OUT IN ON = − 1 (1) Where DON is the duty cycle [TON/(TON + TOFF)]. The input boost inductor will dry out when the following condition is satisfied: (2) or VD V INDRY ON MAX OUT =− []× 1 () (3) VINDRY: Voltage where the inductor dries out. VOUT: Output dc voltage. Effectively, the above relationship shows that the resetting volt-seconds are more than setting volt-seconds. In energy transfer terms this means that less energy is stored in the inductor during the ON time than it is asked to deliver during the OFF time. The net result is that the inductor dries out. The recommended maximum duty cycle is 95% at 100KHz to allow time for the input inductor to dump its energy to the output capacitors. For example: if: VOUT = 380V and DON(MAX) = 0.95 then substituting in (3) yields VINDRY = 20V. The effect of drying out is an increase in distortion at low input voltages. For a given output power, the instantaneous value of the input current is a function of the input sinusoidal voltage waveform. As the input voltage sweeps from zero volts to its maximum value and back, so does the current. The load of the power factor regulator is usually a switching power supply which is essentially a constant power load. As a result, an increase in the input voltage will be offset by a decrease in the input current. By combining the ideas set forth above, some ground rules can be obtained for the selection and design of the input inductor: Step 1: Find minimum operating current. I P V IN MIN PEAK IN MIN IN MAX () () () . = × 1 414 (4) VIN(MAX) = 260V PIN(MIN) = 50W then: IIN(MIN)PEAK = 0.272A Step 2: Choose a minimum current at which point the inductor current will be on the verge of drying out. For this example 40% of the peak current found in step 1 was chosen. VCC = 15V TA = 25 C 0 –4.0 –8.0 –12 –16 –20 –24 0 20 40 60 80 100 120 IREF, REFERENCE SOURCE CURRENT (mA) Figure 11. Reference Load Regulation Vt V D IN OUT ON MAX () () <× − [] 1 |
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