Features: Internal Avalanche Rugged Sensor Advanced Burst Mode Operation Consume 1W at 240V AC and 0.5W Load, Precision Fixed Operating Frequency: 67kHz Internal Startup Circuit Over Voltage Protection (ovp) Overload Protection (OLP) Internal Thermal Shutdown Function (TSD) Abnormal Over Current Protection (AOCP) Auto Restart Mode Under Voltage Lockout with Hysteresis (uvlo) Low Operating Current: 2.5mA Built-in Soft Start: 15ms

Applications: Power supplies for LCD TVs and monitors, VCRs, SVRs, set-top boxes, DVD and DVD recorder adapters Design guidelines for offline forwarding of related resources Converters using Fairchild Power Switches (FPS)! AN-4137: Offline Flyback Design Guidelines Converters Using Fairchild Power Switches (FPS)! AN-4140: Transformer Design Considering Off-Line Flyback Converter Switches (FPS_) Using Fairchild Power Supplies! AN-4141: Troubleshooting and Design Tips Fairchild Power Switch (FPS_) Flyback Applications AN-4145: Power Electromagnetic Compatibility Converters AN-4147: RCD Damper Design Guidelines for Flyback Converters AN-4148: Sound Noise Reduction Fairchild Power Switch (FPS_) Application

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The fsfm260/261/ 300 is an integrated pulse width specially designed modulator (PWM) and sensor high performance offline switching power supply provided with minimal external components (SMP). The device is an integrated high voltage power switching regulator combined with an Avalanche rugged sensor with a current mode PWM control block. This PWM controller includes an integrated fixed frequency oscillator, undervoltage lockout, leading edge blanking (LEB), optimized gate drivers, internal soft-start, temperature compensated precision current source loop compensation and self-protection circuitry. Compared with discrete mosfet and pwm controller solutions, it can reduce total cost, component count, size, weight while increasing efficiency, productivity and system reliability. This device is a low-cost design platform for a basic flyback converter.

Functional Description: 1. Startup: In previous generations of Scout switches (fps), the VCC pin has a resistor for an external start-up DC input voltage line. In this generation, the start-up resistor is replaced by an internal high voltage current source. At startup, the internal high voltage current source provides internal bias and charges the external capacitor (CVCC) connected to the VCC pin, when VCC reaches 12V , the FSFM260/261/300 starts switching and the internal high voltage current source is disabled. Then, the fsfm260/261/300 continues normal switching operation and power is provided by the auxiliary power supply. Transformer windings unless VCC is 8V below the stopper voltage.

2. Feedback control: FSFM260/261/300 current mode control, optocoupler (such as FOD817A) and shunt regulator (such as KA431 ) are usually used to implement feedback network. This is made possible by comparing the feedback voltage across the RSENSE resistor. Controls the switching duty cycle. When the voltage at the reference shunt regulator pin exceeds the internal reference voltage of 2.5V, the optocoupler LED current increases, pulling down the feedback voltage to shorten the duty cycle. This usually occurs when the input voltage increases or the output load increases and decreases. 2.1 Pulse-by-pulse current limit: Due to current mode control, the current through the phototransistor is zero, the current limit pin (4) is left floating, and the feedback current is 0.9 mA from the pulse width modulated comparator (VFB*) through the sensefet (IFB) only through the internal resistor (R+2.5R=2.8K). In this case, the cathode voltage diode D2 and the peak drain current have maximum values of 2.5V and 1.5A, respectively. The pulse-by-pulse current limit can be adjusted with a resistor to GND (4) on the current limit pin. The formula for the current limit level using an external resistor (RLIM) is as follows :

2.2 Leading Edge Blanking (LEB): When the internal sensor has been turned on, a high current spike occurs through the SENSEFET, caused by the primary side capacitance and secondary side rectification reverse recovery. Over voltage resistors that are too large can cause incorrect feedback operation in the current mode's pwm control. To combat this effect, the fsfm260/261/300 employs leading edge blanking (LEB) circuitry. This circuit inhibits the PWM comparator for a short time. (TLEB) After turning on the sensor. 2.3 Constant power limit circuit: Due to the circuit fps delay, the pulse-by-pulse current limit increases when the input voltage increases. This means that unnecessary excess power is delivered to the secondary. side. Compensation, auxiliary power compensation can use the network in Figure 18. rlim can adjust the pulse current by sinking the internal current source (ifb: typ. 0.9ma) depending on the ratio between the resistors. There are proposed compensation circuits, the additional current draw from the IFB is more proportional to the input voltage (Vdc), the input range is wide and the power is constant. Apply the appropriate current for the selection rlim, then check the minimum and maximum input voltages. Eliminate the difference gain (constant power), Ry can be calculated by the following formula: Among them, Ilim_spec is the specification; NA and NP are the turns primary side of VCC, IFB is the power supply at the internal current feedback pin with a typical value of 0.9mA; and Δilim_comp must be eliminated. If the capacitors in the circuit are 1µF, then 100V is fine for all applications.

3. Protection circuit: fsfm260/261/300 has several self-protection functions, such as overload protection (OLP), overvoltage protection (ovp) and thermal shutdown (TSD). Because these protection circuits are fully integrated into the integrated circuit, no external components are required, increasing reliability and increasing cost. Once a fault occurs, the switch is terminated and the sensor remains off. This causes VCC to drop. Stop voltage when vcc reaches uvlo, 8V, protection reset, internal high voltage current source to charge VCC capacitor through VSTR pin. When VCC reaches the UVLO start voltage, 12V, the FSFM260/261/300 resumes normal operation. In this way, automatic restarts can be alternated. Enable and disable switching of the power sensor until troubleshooting. 3.1 Overload Protection (OLP): Overload is defined as 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, overload protection circuitry can initiate transitions during loads. To avoid this undesired operation, the overload protection circuit is designed to activate. Determines whether a transient condition or an overload condition occurs after a certain time. Because of the pulse current limiting capability, the maximum peak current through the sensor is limited and, therefore, the maximum input power is limited. given input voltage. If the output consumes more than this maximum power, the output voltage (VO) decreases below the set voltage. This reduces the current through the optocoupler LED, and also reduces the optocoupler transistor current, increasing the feedback voltage (VFB). If VFB exceeds 2.5V, D1 is blocked and the 5µA current source starts charging CB slowly until VCC. In this case, vfb continues to increase until it reaches 6V, and when the switching operation is terminated, the shutdown delay is 5µA, the time required to charge CB from 2.5V to 60V. In general, a delay time of 10 to 50 ms is typical. Most applications. 3.2 Over Voltage Protection (ovp): If the side feedback circuit fails or solder defect causes an open circuit in the feedback path, the current through the optocoupler transistor is zero. Then, the vfb forces the maximum current supplied to the SMPS with the overload condition until the overload protection is activated. Because more energy is supplied to the output than is required, the output voltage can exceed the rated voltage before the overload protection is activated, causing the device to be on the second side. To prevent this, an overvoltage protection (ovp) circuit is employed. Generally speaking, VCC is related to the output voltage and the fsfm260/261/300 uses VCC instead of monitoring the output voltage directly. If VCC exceeds 19V, the ovp circuit is activated, terminating the switching operation. Avoid in normal operation, VCC should be designed below 19V. 3.3 Thermal Shutdown (TSD): The sensor and control chip are built into one package. This enables the control IC to detect the sensory net.

Thermal shutdown is initiated when the temperature exceeds approximately 140°C. 3.4 Abnormal Over Current Protection (AOCP): When the secondary rectifier diode or transformer pins are short-circuited, a steep current with extremely high di/dt can flow through the sensor during the LEB time. Even though the FPS has overload protection, it is not enough to protect the FPS in those abnormal situations, because severe current stress is put on the sensor until the OLP triggers. This IC has an internal AOCP circuit. When the gate open signal is applied to the power sensor, the AOCP block is enabled and monitors the current through the sensed resistor. The voltage across the resistor is the same as the preset AOCP level. If the sense resistor voltage is greater than the AOCP level, the set signal is applied to the latch, causing the smps to turn off. 4. Soft-start: fsfm260/261/300 has a soft-start circuit with an internal increased pwm comparator to reverse the input voltage, along with the sensor current, slowly after startup. Typical soft-start time is 15ms. The pulse width of the power switching device is gradually increased to establish the correct transformers, inductors and capacitors. The voltage on the output capacitor is gradually increased to smoothly establish the desired output voltage. It also helps prevent transformer saturation and reduce stress on the secondary diode during startup. 5. Burst operation: Put standby mode, fsfm260/261/300 into burst mode operation. When the load decreases, the feedback voltage decreases. device when the feedback voltage is lower than VbURL (350mV). At this point, switching stops and the output voltage begins to drop at a rate that depends on the backup current load. This causes the feedback voltage to rise. Once toggled through Vburh (500mV) continue. Then the feedback voltage drops and the process repeats. Burst Mode operates alternately enabling and disabling switching of the power sensor, reducing switching losses in standby mode.

PCB Layout Guidelines fps has better noise performance due to the combined scheme. immune to MOSFET discrete solutions than conventional PWM controllers. Additionally, internal drain current sensing is eliminated by the sense resistor. There are some suggestions for improving noise immunity and suppressing the inevitable noise in power processing components. Traditional power ground and signal ground. Power ground is the ground for the primary input voltage. Power supply, controller when signal ground is PWM ground. In FPS, the two reasons are the same. pin, ground. Often, separate grounds are not identical to the same traces, meeting only at one point, the GND pin. The wider pattern is good for both areas by lowering the current through the resistor. Capacitors for the VCC and FB pins should be as close as possible to the corresponding pins to avoid switching devices. Sometimes the smoothness of the electrolytic capacitors with polyester film or ceramic VCC is better to operate. The grounds of these capacitors need to be connected to signal ground not power ground. The cathode of the snubber diode should be close to the drain pin to minimize stray inductance. y The primary and secondary capacitors should be connected directly to the DC bus power ground to maximize the voltage range of the feedback line, so it is affected by the noise of the drain pin. Those marks should not be drawn across or near the drain. In the fsfm260/261/300, the drain pin is a heat radiation pin, so it is recommended to reduce the packaging temperature for wider PCB patterns. Drain pins are also high voltage switch pins; however, a PCB pattern that is too wide may reduce EMI immunity.