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STR4A162S bảng dữ liệu(PDF) 11 Page - Sanken electric |
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STR4A162S bảng dữ liệu(HTML) 11 Page - Sanken electric |
11 / 23 page STR4A100 Series STR4A100 - DSJ Rev.3.3 SANKEN ELECTRIC CO.,LTD. 11 Aug.06, 2015 http://www.sanken-ele.co.jp © SANKEN ELECTRIC CO.,LTD. 2011 process based on actual operation in the application, so that poor starting conditions may be avoided. Figure 10-3 shows the VCC pin voltage behavior during the startup period. After the VCC pin voltage increases to VCC(ON) = 15.2 V at startup, the IC starts the operation. Then circuit current increases and the VCC pin voltage decreases. At the same time, the auxiliary winding voltage, VD, increases in proportion to output voltage. These are all balanced to produce the VCC pin voltage. When the VCC pin voltage is decrease to VCC(OFF) = 8.1 V in startup operation, the IC stops switching operation and a startup failure occurs. When the output load is light at startup, the output voltage may become more than the target voltage due to the delay of feedback circuit. In this case, the FB pin voltage is decreased by the feedback control. When the FB pin voltage decreases to VFB(OFF) or less, the IC stops switching operation and the VCC pin voltage decreases. When the VCC pin voltage decreases to VCC(BIAS), the Bias Assist function is activated and the startup failure is prevented. IC starts operation VCC pin voltage VCC(ON) VCC(BIAS) VCC(OFF) Startup failure Startup success Target operating voltage Time Bias assist period Increase with rising of output voltage Figure 10-3 VCC pin voltage during startup period 10.4 Soft Start Function Figure 10-4 shows the behavior of VCC pin voltage and drain current during the startup period. The IC activates the soft start circuitry during the startup period. Soft start time is fixed to around 6 ms. during the soft start period, overcurrent threshold is increased step-wisely (5 steps). This function reduces the voltage and the current stress of a power MOSFET and the secondary side rectifier diode. Since the Leading Edge Blanking Function (refer to Section 10.6) is deactivated during the soft start period, there is the case that ON time is less than the Leading Edge Blanking Time, tBW. After the soft start period, D/ST pin current, ID, is limited by the Drain Current Limit, IDLIM, until the output voltage increases to the target operating voltage. This period is given as tLIM. In case tLIM is longer than the OLP Delay Time, tOLP, the output power is limited by the Overload Protection (OLP) operation. Thus, it is necessary to adjust the value of output capacitor and the turn ratio of auxiliary winding D so that the tLIM is less than tOLP = 58 ms (min.). VCC(ON) VCC(OFF) Time VCC pin voltage Startup of SMPS Normal opertion D/ST pin current, ID tLIM < tOLP (min.) Soft start period approximately 6 ms (fixed) Time Startup of IC tSTART IDLIM Figure 10-4 VCC and ID behavior during startup 10.5 Constant Output Voltage Control The IC achieves the constant voltage control of the power supply output by using the current-mode control method, which enhances the response speed and provides the stable operation. The FB/OLP pin voltage is internally added the slope compensation at the feedback control (refer to Section 5.Block Diagram), and the target voltage, VSC, is generated. The IC compares the voltage, VROCP, of a current detection resistor with the target voltage, VSC, by the internal FB comparator, and controls the peak value of VROCP so that it gets close to VSC, as shown in Figure 10-5 and Figure 10-6. ● Light load conditions When load conditions become lighter, the output voltage, VOUT, increases. Thus, the feedback current from the error amplifier on the secondary-side also increases. The feedback current is sunk at the FB/OLP pin, transferred through a photo-coupler, PC1, and the FB/OLP pin voltage decreases. Thus, VSC decreases, and the peak value of VROCP is controlled to be low, and the peak drain current of ID decreases. This control prevents the output voltage from increasing. ● Heavy load conditions When load conditions become greater, the IC performs the inverse operation to that described above. Thus, VSC increases and the peak drain current of ID increases.This control prevents the output voltage from decreasing. In the current mode control method, when the drain current waveform becomes trapezoidal in continuous operating mode, even if the peak current level set by the |
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