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VIPER20ADIP-E Datasheet(PDF) 11 Page - STMicroelectronics |
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VIPER20ADIP-E Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 34 page VIPer20A-E Operation description 11/34 5 Operation description 5.1 Current mode topology: The current mode control method, like the one integrated in the devices, uses two control loops - an inner current control loop and an outer loop for voltage control. 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 VS proportional to this current. When VS reaches VCOMP (the amplified output voltage error) the power switch is switched off. Thus, the outer voltage control loop defines the level at which the inner loop regulates peak current through the power switch and the primary winding of the transformer. Excellent open loop D.C. and dynamic line regulation is ensured due to the inherent input voltage feedforward characteristic of the current mode control. This results in improved line regulation, instantaneous correction to line changes, and better stability for the voltage regulation loop. Current mode topology also ensures good limitation in case there is a short circuit. During the first phase the output current increases slowly following the dynamic of the regulation loop. Then it reaches the maximum limitation current internally set and finally stops because the power supply on VDD is no longer correct. For specific applications the maximum peak current internally set can be overridden by externally limiting the voltage excursion on the COMP pin. 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 case there are current spikes caused by primary side capacitance or secondary side rectifier reverse recovery time. 5.2 Stand-by mode Stand-by operation in nearly open load conditions automatically leads to a burst mode operation allowing voltage regulation on the secondary side. The transition from normal operation to burst mode operation happens for a power PSTBY given by : Where: LP is the primary inductance of the transformer. FSW is the normal switching frequency. ISTBY is the minimum controllable current, corresponding to the minimum on time that the device is able to provide in normal operation. This current can be computed as : tb + td is the sum of the blanking time and of the propagation time of the internal current sense and comparator, and represents roughly the minimum on time of the device. Note: that PSTBY may be affected by the efficiency of the converter at low load, and must include the power drawn on the primary auxiliary voltage. PSTBY 1 2 --- LPI 2STBYFSW = ISTBY tb td + ()VIN Lp ----------------------------- = |
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