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VIPER53ESP-E Datasheet(PDF) 10 Page - STMicroelectronics |
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VIPER53ESP-E Datasheet(HTML) 10 Page - STMicroelectronics |
10 / 31 page Current Mode Topology VIPer53EDIP - E / VIPer53ESP - E 10/31 DocRev1 6 Current Mode Topology The VIPer53E implements the conventional current mode control method for regulating the output voltage. This kind of feedback includes two nested regulation loops: The inner loop controls the peak primary current cycle by cycle. When the Power MOSFET output transistor is on, the inductor current (primary side of the transformer) is monitored with a SenseFET technique and converted into a voltage. When VS reaches VCOMP, the power switch is turned off. This structure is completely integrated as shown on the Block Diagram of Figure on page 1, with the current amplifier, the PWM comparator, the blanking time function and the PWM latch. The following formula gives the peak current in the Power MOSFET according to the compensation voltage: The outer loop defines the level at which the inner loop regulates peak current in the power switch. For this purpose, VCOMP is driven by the feedback network (TL431 through an optocoupler in secondary feedback configuration, see Figure 3 on page 8) and is sets accordingly the peak drain current for each switching cycle. As the inner loop regulates the peak primary current in the primary side of the transformer, all input voltage changes are compensated for before impacting the output voltage. This results in an improved line regulation, instantaneous correction to line changes, and better stability for the voltage regulation loop. Current mode topology also provides a good converter start-up control. The compensation voltage can be controlled to increase slowly during the start-up phase, so the peak primary current will follow this soft voltage slope to provide a smooth output voltage rise, without any overshoot. The simpler voltage mode structure which only controls the duty cycle, leads generally to high current at start-up with the risk of transformer saturation. An integrated blanking filter inhibits the PWM comparator output for a short time after the integrated Power MOSFET is switched on. This function prevents anomalous or premature termination of the switching pulse in the case of current spikes caused by primary side transformer capacitance or secondary side rectifier reverse recovery time when working in continuous mode. I Dpe ak V COMP V COMPos – H COM P -------------------------------------------------- = |
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