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X-11181-002 Arkusz danych(PDF) 9 Page - Linear Technology |
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X-11181-002 Arkusz danych(HTML) 9 Page - Linear Technology |
9 / 20 page LT3798 9 3798f Primary Side Current Control Loop The CTRL1/CTRL2/CTRL3 pins control the output current of the flyback controller. To simplify the loop, let’s assume the VIN_SENSE pin is held at a constant voltage above 1V eliminating the multiplier from the control loop. The error amplifier, A5, is configured as integrator with the external capacitor C6. The COMP+ node voltage is converted to a current into the multiplier with the V/I converter, A6. Since A7’s output is constant, the output of the multiplier is proportional to A6 and can be ignored. The output of the multiplier controls the peak current with its connection to the current comparator, A1. The output of the multiplier is also connected to the transmission gate, SW1, and to a 1M resistor. The transmission gate, SW1, turns on when the secondary current flows to the output capacitor. This is called the flyback period when the output diode D1 is on. The current through the 1M resistor gets integrated by A5. The lowest CTRL input is equal to the negative input of A5 in steady state. A current output regulator normally uses a sense resistor in series with the output current and uses a feedback loop to control the peak current of the switching converter. In this isolated case, the output current information is not available so instead the LT3798 calculates it using the in- formation available on the primary side of the transformer. The output current may be calculated by taking the average of the output diode current. As shown in Figure 1, the diode current is a triangle waveform with a base of the flyback time and a height of the peak secondary winding current. In a flyback topology, the secondary winding current is N times the primary winding current, where NPSistheprimary to secondary winding ratio. Instead of taking the area of the triangle, let’s think of it as a pulse width modulation (PWM) waveform. During the flyback time, the average current is half the peak secondary winding current and zero during the rest of the cycle. The equation to express the output current is: IOUT = 0.5 • IPK • NPS • D where D is equal to the percentage of the cycle that the flyback time represents. The LT3798 has access to the primary winding current, the input to the current com- parator, and when the flyback time starts and ends. Now the output current can be calculated by averaging a PWM waveform with a height of the current limit and a duty cycle of the flyback time over the entire cycle. In the feedback loop described above, the input to the integrator is such a waveform. The integrator adjusts the peak current until calculated output current equals the control voltage. If the calculated output current is low compared to the control pin, the error amplifier increases the voltage on the COMP+ node thus increasing the current comparator input. Primary Side Voltage Control The output voltage is available through the third winding on the primary side. A resistor divider attenuates the output voltage for the voltage error amplifier. A sample-and-hold circuit samples the attenuated output voltage and feeds it to the error amplifier. The output of the error amplifier is the VC pin. This node needs a capacitor to compensate the output voltage control loop. Power Factor Correction WhentheVIN_SENSEvoltageisconnectedtoaresistordivider of the supply voltage, the current limit is proportional to the supply voltage. The minimum of the two error ampli- fier outputs is multiplied with the VIN_SENSE pin voltage. If the LT3798 is configured with a fast control loop, slower changes from the VIN_SENSE pin would not interfere with the current limit or the output current. The COMP+ pin would adjust to the changes of the VIN_SENSE. The only way for the multiplier to function is to set the control loop to be an order of magnitude slower than the fundamental frequency of the VIN_SENSE signal. In an offline case, the OPERATION Figure 1. Secondary Diode Current and Switch Waveforms 3798 F01 TFLYBACK TPERIOD SECONDARY DIODE CURRENT SWITCH WAVEFORM IPK(sec) |
Podobny numer części - X-11181-002 |
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Podobny opis - X-11181-002 |
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