feature

Advanced Burst Mode Operation Low EMI Frequency Modulation Precision Fixed Operating Frequency Internal Startup Circuit Pulse Current Limit Reverse Current Protection Over Voltage Protection Overload Protection -in Soft Start experience

application

Switching Power Camera Adapters for VCRs, SVRs, Set-Top Boxes, DVDs for Switching Power Printers, Faxes and Scanners

illustrate

The fsdx0265rn (x stands for m, h) is an integrated pulse width modulator (pwm) and sensing fet designed for high performance offline switching power supplies (SMP) with minimal external components. The two devices are integrated high voltage power switching regulators that combine avalanche rugged sensing FETs and current mode pulse width modulation control blocks. Features of the integrated pwm controller include: fixed frequency oscillator for reduced EMI modulation, undervoltage lockout (uvlo) protection, leading edge blanking (leb), optimized gate on/off driver, thermal shutdown (TSD) Protection, Abnormal Over Current Protection (AOCP) and Temperature Compensated Source of Precision Current Loop Compensation and Fault Protection Circuitry. When combined with discrete mosfet and controller or rcc switching converter solutions, the fsdx0265rn reduces total component count, design size, weight and time to improve efficiency, productivity and system reliability. Both devices are a basic platform well suited for cost-effective design of flyback converters.

Typical circuit

Internal block diagram

Function description

1. Startup: In previous generations of Fairchild Power Switch (FPSTM) the VSTR pin had an external resistor to the DC input voltage line. In this generation, the start-up resistor is powered by an internal high voltage current source and the switch voltage VCC is turned off after 15 minutes of power supply is higher than 12V . VCC drops below 8V.

2. Feedback control: fsdx0265rn adopts current mode control, as shown in Figure 5. Optocouplers (such as h11a817a) and shunt regulators (such as ka431) are commonly used to implement feedback networks. Comparing the feedback voltage to the voltage across the sensor resistor plus the offset voltage controls the switching duty cycle. When the KA431 exceeds the internal reference voltage of 2.5V H11A817 A LED current increases, thereby reducing the feedback voltage and reducing the duty cycle. This activity occurs when the input voltage increases or the output load decreases.

3. Leading Edge Blanking (LEB): At the moment when the internal sensing FET is energized, there is usually a high current spike in the sensing FET, which is caused by the capacitance on the primary side and the reverse recovery of the rectifier diode on the secondary side. Excessive voltage on the RSENSE resistor can result in incorrect feedback operation control in current mode. To counteract this effect, fpstm uses an edge blanking (LEB) circuit. This circuit suppresses the short-time comparator (TLEB) turning on after the PWM sensing FET is activated.

4. Overload Protection (OLP): Overload is defined as load current exceeding a preset level due to unexpected events. In this case, the protection circuit should be activated to protect the SMPS, however, even when the switching power supply is operating normally, overloading during the load transition can activate the protection circuit. To avoid this undesired operation, overload protection circuits are designed to determine whether this is a transient condition or an overload condition as specified. Combined with the IPK current limit pin (if used) the current mode feedback path will limit the maximum PWM duty cycle when the current in the sensing FET reaches cycling. If the output consumes more than this maximum power, the output voltage (vo) will drop below the set value. This reduces the current through the optocoupler LED, it also reduces the current through the optocoupler transistor, which increases the feedback voltage (vfb). If VFB exceeds 3V, the feedback input diode is blocked and the 5UA IDELAY current source begins to slowly charge the CFB until it reaches VCC. Under this condition, VFB continues to increase until it reaches 6V, at which point the switching operation is terminated. The delay time for shutdown is the time it takes to charge the CFB from 3V to 6V with 5uA. Frequency Modulation: EMI reduction can be achieved by modulating the switching frequency of the power supply. FM can be measured by spreading energy over a wider bandwidth than EMI test equipment. This EMI reduction is related to the reference frequency. As shown the frequency changes from 65kHz to 69kHz within 4ms of the FSDM0265RN number.

The leakage inductance clamp is provided by R1 and C8 to keep the drain voltage below 650V under all conditions. Resistor r1 and capacitor c8 are chosen so that r1 dissipates power to prevent the drain voltage from rising through the leakage inductance. The FM characteristics of the fsdl0165rn allow the circuit shown to satisfy CISPr2ab with simple EMI filtering (C1, LF1 and C2) and output grounding. The second part goes through D12 D13, D14 and D15. Diode D15 for the 3.4V output is a Schottky diode to maximize efficiency. The diode D14v output of 5 is pn type, which concentrates the 5v output at 5v. The 3.3V and 5 V output voltages require two parallel capacitors to meet the ripple current requirements. Switching noise filtering is provided by l3, l2 and l1. Resistor r15 prevents peak charging of the 23V output at light loads. Regulated by the secondary reference ( TL431 ) voltage. Both the 3.3 V and 5 V outputs are sensed through R13 and R14. Resistor r22 provides bias for the tl431 and r21 sets the overall dc gain. Resistors r21, c209, r14 and r13 provide loop compensation.