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FS7M0680 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FS7M0680 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
10 / 18 page FS7M0680, FS7M0880 10 Functional Description 1. Startup : Figure 4 shows the typical startup circuit and transformer auxiliary winding for FS7M-series. Because all the protections are implemented as latch mode, AC startup is typically used to provide a fast reset as shown in Figure 4. Before FPS begins switching operation, only startup current (typically 40uA) is consumed and the current supplied from the AC line charges the external capacitor (Ca) that is connected to the Vcc pin. When Vcc reaches start voltage of 15V (VSTART), FPS begins switching, and the current consumed by FPS increases to 8mA. Then, FPS continues its normal switching operation and the power required for this device is supplied from the transformer auxiliary winding, unless Vcc drops below the stop voltage of 9V (VSTOP). To guarantee the stable operation of the control IC, Vcc has under voltage lockout (UVLO) with 6V hysteresis. Figure 5 shows the relation between the FPS operating supply current and the supply voltage (Vcc). Figure 4. Startup circuit Figure 5. Relation between operating supply current and Vcc voltage The minimum average of the current supplied from the AC is given by where Vacmin is the minimum input voltage, Vstart is the Vcc start voltage (15V) and Rstr is the startup resistor. The startup resistor should be chosen so that Isupavg is larger than the maximum startup current (80uA). Once the resistor value is determined, the maximum loss in the startup resistor is obtained as where Vacmax is the maximum input voltage. The startup resistor should have proper rated dissipation wattage. 2. Feedback Control : FS7M-series employs current mode control, as shown in Figure 6. An opto-coupler (such as the H11A817A) and shunt regulator (such as the KA431) are typically used to implement the feedback network. Comparing the feedback voltage with the voltage across the Rsense resistor plus an offset voltage makes it possible to control the switching duty cycle. When the reference pin voltage of the KA431 exceeds the internal reference voltage of 2.5V, the H11A817A LED current increases, thus pulling down the feedback voltage and reducing the duty cycle. This event typically happens when the input voltage is increased or the output load is decreased. 2.1 Pulse-by-pulse current limit: Because current mode control is employed, the peak current through the Sense FET is limited by the inverting input of PWM comparator (Vfb*) as shown in Figure 6. The feedback current (IFB) and internal resistors are designed so that the maximum cathode voltage of diode D2 is about 2.8V, which occurs when all IFB flows through the internal resistors. Since D1 is blocked when the feedback voltage (Vfb) exceeds 2.8V, the maximum voltage of the cathode of D2 is clamped at this voltage, thus clamping Vfb*. Therefore, the peak value of the current through the Sense FET is limited. 2.2 Leading edge blanking (LEB) : At the instant the internal Sense FET is turned on, there usually exists a high current spike through the Sense FET, caused by external resonant capacitor across the MOSFET and secondary-side rectifier reverse recovery. Excessive voltage across the Rsense resistor would lead to incorrect feedback operation in the current mode PWM control. To counter this effect, the FPS employs a leading edge blanking (LEB) circuit. This circuit inhibits the PWM comparator for a short time (TLEB) after the Sense FET is turned on. FS7M -series 1N4007 Rstr Vcc C a Da I sup AC line (V ac min - V ac max) C DC Icc Vcc Vstop=9V 40uA 8mA Vstart=15V Vz Power Up Power Down Isup avg 2Vac min ⋅ π ------------------------------ Vstart 2 -------------- – 1 Rstr ---------- ⋅ = Loss 1 Rstr ---------- Vac max () 2 Vstart 2 + 2 --------------------------------------------------- 22 Vstart Vac max ⋅⋅ π ------------------------------------------------------ – ⋅ = |
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Similar Description - FS7M0680 |
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