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ML4812CP Datasheet(PDF) 8 Page - Micro Linear Corporation |
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ML4812CP Datasheet(HTML) 8 Page - Micro Linear Corporation |
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8 / 16 page  ML4812 8 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 voltages. For a given output power, the instantaneous value of the input current is a function of the input sinusoidal voltage waveform, i.e. as the input voltage sweeps from zero volts to a maximum value equal to its peak 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 PEAK IN IN (min) .(min) (max) = ´ 1414 (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. then: ILDRY = 100mA Step 3: The value of the inductance can now be found using previously calculated data. L VD If V mA KHz mH INDRY ON LDRY OSC 1 20 0 95 100 100 2 = ´ ´ = ´ ´ = (max) . (5) The inductor can be allowed to decrease in value when the current sweeps from minimum to maximum value. This allows the use of smaller core sizes. The only requirement is that the ramp compensation must be adequate for the lower inductance value of the core so that there is adequate compensation at high current. Step 4: The presence of the ramp compensation will change the dry out point, but the value found above can be considered a good starting point. Based on the amount of power factor correction the above value of L1 can be optimized after a few iterations. Gapped Ferrites, Molypermalloy, and Powdered Iron cores are typical choices for core material. The core material selected should have a high saturation point and acceptable losses at the operating frequency. One ferrite core that is suitable at around 200W is the #4119PL00-3C8 made by Philips Components (Ferroxcube). This ungapped core will require a total gap of 0.180" for this application. OSCILLATOR COMPONENT SELECTION The oscillator timing components can be calculated by using the following expression: f RC OSC TT = ´ 136 . (6) For example: Step 1: At 100kHz with 95% duty cycle TOFF = 500ns calculate CT using the following formula: C TI V pF T OFF DIS OSC = ´ = 1000 (7) Step 2: Calculate the required value of the timing resistor. R fC KHz pF k choose R k T OSC T T = ´ = ´ =W = W 136 136 100 1000 136 14 .. . (8) TYPICAL APPLICATIONS (Continued) |
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