Datasheet LT3798 (Analog Devices) - 9

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