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ML4813CS Datasheet(PDF) 8 Page - Micro Linear Corporation |
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ML4813CS Datasheet(HTML) 8 Page - Micro Linear Corporation |
8 / 15 page ML4813 8 APPLICATIONS The ML4813 is used to implement a discontinuous mode flyback (buck-boost) power factor regulator as shown in Figure 7. This topology is particularly well suited for low power applications such as fluorescent ballasts and low power switching supplies. It is also a useful topology when there is a requirement for the output voltage to be lower than the peak input voltage, or where an isolated output is required. This is not possible with a boost topology, where the output voltage must always be higher than the maximum peak of the input voltage range. The typical input range for the flyback power factor regulator is from 90VAC to 260VAC. The regulator operates in the discontinuous current conduction mode. The inductor energy stored during the ON time of the power switch Q is completely delivered to the output capacitance during the OFF time. Under steady state conditions, the inductor current at the beginning of the ON time starts to ramp-up from 0 Amps to a value that is determined by the instantaneous value of the input full wave rectified voltage; the ON time as it is set by the error amplifier and the PWM comparator; and finally by the inductor itself. The expression for the inductor peak current is given by: I Vt L L IN ON q q 05 05 = (4) Where: IL(q) = instantaneous peak inductor current tON = Power MOSFET "ON" time VIN(q) = VP sin q = Instantaneous input voltage VP = Input peak voltage Figure 8 shows the relationship between the low frequency envelope and the high frequency inductor current. Note that for clarity the scale between the two waveforms has not been preserved. Normally for 60Hz input line and 100kHz switching frequency, each half of the sine wave contains approximately 833 high frequency triangular waveforms. The envelope of the peaks of the switch current, which in this case represents the current drawn from the input source, has a sinewave shape. This relationship is shown as: II LP qq 0 5= sin (5) By combining (4) and (5), the following useful relationship is obtained: t LI V ON P RMS = 2 (6) Figure 8. Switch and Line Currents in the Flyback PFC Circuit RFI FILTER ML4813 CONTROLLER VOUT- VOUT+ RS RL2 RH2 C1 D1 Q1 L1 RH1 RL1 VS – + INDUCTOR CURRENT SWITCH CURRENT SINUSOIDAL PEAK ENVELOPE AVERAGE CURRENT Figure 7. Simplified Application Circuit |
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