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AN4146 Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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AN4146 Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 24 page AN4146 APPLICATION NOTE 6 ©2005 Fairchild Semiconductor Corporation Figure 9. Output Voltage Drop in Standby Mode In standby mode, Vo2 is regulated by the feedback control and the voltage drop ratio of the Vo2 winding is defined in where VF2 is the diode forward voltage drop of the DR2, and Vo2 normaland V o2 stby are the output voltages of V o2 in normal mode and standby mode, respectively, as shown in Figure 9. Assuming that the Vcc auxiliary voltage (Va) is reduced with the ratio of Kdrop, Va in normal mode is obtained as where Va stby is the minimum voltage of V a in standby mode, which should be larger than Vcc stop voltage of FPS (typically 9V). Notice that the operating current is reduced in standby mode and therefore the voltage drop across Rcc is negligible. It is typical to have a voltage margin of 2-3V when determining Va stby. After determining Va normal, the number of turns for the Vcc auxiliary winding (Na) is obtained as where VFa is the forward voltage drop of Da as defined in Figure 8. - Vcc drop resistor (Rcc) : The current consumed by FPS in normal operation is given by where Iop and Idrv are the currents required for IC operation and MOSFET gate drive, respectively. Iop is given in the data sheet and Idrv is obtained as where Ciss is the input capacitance of the MOSFET and fs is the switching frequency. When considering Idrv, it is typical to assume that Vcc is Vz (18V) and fs is 90kHz. The condition for the Vcc drop resistor (Rcc) is given by The heat dissipation of Rcc in normal operation is given by where Vz is the zener breakdown voltage (typically 18V). When a large voltage drop of more than 20V is required, application circuit shown in Figure 11 is preferred to minimize the power dissipation in the voltage drop circuit. Figure. 10 Vcc Auxiliary Circuit for a Large Voltage Drop [STEP-8] Determine the startup resistor Figure 10 shows the typical circuit of Vcc winding for FSCQ-series. Initially, FPS consumes only startup current (max 50uA) before it begins switching. Therefore, the current supplied through the startup resistor (Rstr) can charge the capacitors Ca1 and Ca2 while supplying startup current to FPS. When Vcc reaches start voltage of 15V (V START), FPS begins switching and the current consumed by FPS increases. Then, the current required by FPS is supplied from the transformer’s auxiliary winding. V o2 V o2 stby V a 9V Standby mode Normal mode V a stby V o2 normal V a normal 18V V cc 2-3 V K drop V 02 stby V F2 + V 02 normal V F2 + ------------------------------------------ = (15) V a normal V a stby V Fa + K drop ---------------------------------V F2 – =16 () N a V a normal V Fa + V o1 V F1 + --------------------------------------- =N s1 ⋅ 17 () I cc I op I drv + =18 () I drv V cc C iss f s ⋅⋅ = 19 () R cc V co normal V z – I cc -------------------------------------- <20 () P a v co normal V z – () 2 R cc --------------------------------------------- = 21 () FSCQ-series V cc D a C a1 Vz (18V) 10kΩ 3-5 Ω C a2 |
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