feature

Avalanche Internal Sensor Precision Fixed Operating Frequency: 67khz-=YTET Advanced Burst Mode Operation Internal Startup Circuit Pulse Dual Pulse Current Limit Overvoltage Protection Overload Protection (OLP) Internal Thermal Shutdown Function Self-Recovery Mode Voltage Lock (UV) Soft Start Secondary side adjustment

application

Chargers and Adapters for Mobile Phones White Goods Auxiliary Power Monitors

describe

The fsdm311a includes an integrated pulse width modulator (PWM) and sensor, specifically designed for high performance, off-line, switch-mode minimum external power (SMP) components. This device is an integrated high voltage sensor combined with a vdmos power switching regulator with a voltage mode PWM control block. Features of this integrated pwm controller include oscillator, undervoltage lockout (uvlo) protection, leading edge blanking (LEB), optimized gate switch driver, thermal shutdown (TSD) protection, temperature compensation, precision current loop compensation and fault protection source circuit. When combined with discrete mosfet and controller or RCC switching converter solutions the fsdm311a device reduces overall component count, design size and weight, while increasing efficiency, productivity and system reliability. This device provides a basic platform ideal for cost-effective flyback converters. Related Resources AN-4134: Design Guidelines for Offline Forward Converters Using Fairchild Power Switches (FPS 8482 ;) AN-4137: Offline Flyback Design Guidelines for Converters Using Fairchild Power Switches (FPS™) AN-4138: Battery Chargers Design Considerations Using Green Mode Fairchild Power Switches (FPS™) AN-4140: Transformer Design Considerations for Offline Flyback Converters Using Fairchild Power Switches (FPSµ) AN-4141: Troubleshooting and Design Tips for Fairchild Power Switches (FPS™) Flyback Applications AN-4147: Design Guidelines for RCD Shock Absorbers AN-4148 Flyback: Sound Noise Reduction Technology FPS™

Function description

1. Start-up: During start-up, the internal high voltage current source provides the internal bias voltage to the external VCC capacitor as shown in the figure. when? When VCC reaches 9V, the device starts switching and the internal high voltage current source stops charging the capacitor. The device operates normally with VCC not lower than 7V. After startup, the bias is powered by the auxiliary transformer winding.

Calculating the VCC capacitance is designed using the FSDM311A . At initial start-up, the maximum starting operating current Istart is about 100 microamps, providing current blocking for the UV low voltage and VREF. The charging current of the VCC capacitor is equal to ISTR – 100 µA. After VCC reaches the UVLO voltage, only the bias winding is directed to the device. When the bias winding voltage is not sufficient, the VCC level drops to the UVLO stop voltage and the internal current source is activated again to charge the VCC capacitor. To prevent this VCC fluctuation (charging/discharging), the VCC capacitor should be chosen between 10 and 47 µF.

2. Feedback Control: The FSDM311A is a voltage mode control device as shown. Typically, optocouplers and shunt regulators, such as the KA431 , are used to implement feedback networks. This feedback voltage and an internally generated sawtooth wave directly control the duty cycle. When the reference pin voltage of the parallel regulator exceeds the internal reference voltage of 2.5V, the optocoupler LED current increases and the feedback voltage vfb is pulled down, reducing the duty cycle. when the input voltage increases or the output load decreases.

3. Leading Edge Blanking (LEB): Instantaneous internal sensor turn-on, primary side capacitance and secondary side rectifier diode reverse recovery usually cause high current spikes through the sense net. High voltage resistors on RSENSE cause pulse current limit to be improperly protected. To avoid this, a leading edge blank (LEB) circuit disables the pulse-by-pulse current limit protection blocking for a fixed time (tleb) after the sensefet is turned on. 4. Protection circuit: fsdm311a has several protection functions such as overload protection (OLP); over voltage protection (ovp), under voltage lockout (uvlo) and thermal shutdown (tsd). Because these protection circuits are fully integrated in the integrated circuit with external components, increased reliability increases cost. Once a fault occurs, the switch is terminated and the sensor remains off, causing VCC to drop. When vcc reaches uvlo stop voltage, VSTOP (7V), protection resets the internal high voltage current source to charge VCC capacitor through VSTR pin. When vcc reaches the uvlo start voltage, V starts (9V) and the device resumes normal operation. This way, auto-restart can alternately enable and disable the power sensor settings until troubleshooting.

4.1 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 SMPS is operating normally, the overload protection (OLP) circuit can activate during load transitions. To avoid this unexpected operation, the OLP circuit is designed to activate after a specified time to determine if it is a transient condition or an overload condition. If the output consumes more than the maximum power determined by the ILIM, the output voltage (VO) is reduced to the rated voltage. This reduces the current through the optocoupler LED and also reduces the optocoupler transistor current, thereby increasing the feedback voltage (VFB). If VFB exceeds 3V, the feedback input diode blocks and the 5µA current source (IDELAY) begins to slowly charge the CFB to VCC. In this case, vfb increases to 4.5V when the switching operation is terminated as shown. This shutdown delay time is the time it takes to charge the CFB from 3V to 4.5V using a 5µA current source.

4.2 Thermal crack (TSD): The sensor is integrated with the thermal crack control IC, making it easy to control the temperature of the detected sensor. When the temperature excess is about 145°C, the temperature excess shutdown is activated. Five Soft-Start: The FPS has an internal soft-start circuit that slowly increases the feedback voltage when it starts to feel it now. A typical soft start time is 15 mm, as shown, where progressive increments of sensory current are approved during the starting phase. Soft-start circuits gradually increase the current limit to establish Proper Working Conditions for Transformers, Inductors, Capacitors, Switchgear. It also helps prevent transformer saturation and relieve stress on the secondary diode 6 . Burst Operation: Minimize Power Consumption In standby mode, the fsdm311a enters burst mode operation. When the load decreases, the feedback voltage decreases. The device automatically enters burst mode when the feedback voltage is lower than vburl (0.55v). At this point, switching stops and the output voltage begins to drop. This causes the feedback voltage to rise. Once it passes Vburh (0.70V), the switch starts again. The feedback voltage drops and the process repeats. Burst Mode operation alternately enables and disables the switching of the power mosfet to reduce switch losses during standby mode.