feature

Internal avalanche bump sensor Advanced burst mode operation consumes less than 1W 240 VAC and 0.5 watt load Precision fixed operating frequency (66kHz) Internal start-up circuit Improved pulse-by-pulse current limiting 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 Applications: Switching Power Adapters for LCD Displays and Set-Top Boxes

illustrate

The fsdm0465rs 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 integrated fixed frequency pwm controller oscillator, under voltage lockout, leading edge blanking (LEB), optimized gate driver, internal soft start, temperature compensated precision current source loop compensation and self protection circuitry. Compared with discrete MOSFET and PWM controllers, it can reduce overall cost, component count, and size while increasing efficiency, productivity and system reliability. This device is a basic platform converter well suited for cost-effective flyback designs.

Function description

1. Startup: In previous generations of Fairchild Power Switch (FPS 8482 ;) VCC pin has a resistor to externally start the DC input voltage line. In this generation the start-up resistor is replaced by an internal high voltage current source. At startup, an internal high voltage current source provides internal bias and charges an external capacitor (CVCC) connected to the VCC pin as shown. When VCC reaches 12 volts, the FSDM0465RS starts switching and the internal high voltage current source is disabled. The fsdm0465rs then 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: FSDM0465RS adopts current mode control, as shown in Figure 18. Optocouplers (such as H11A817 A) and shunt regulators (such as ka431) are often used to implement feedback networks. Combining the feedback voltage with the voltage on the RSENSE resistor plus the offset voltage makes it possible to control the switch duty cycle. When the reference pin voltage of the KA431 exceeds the internal reference voltage by 2.5V, the H11A817ALED current increases, thereby pulling down the feedback voltage and reducing the duty cycle. This event usually occurs when the input voltage increases or the output load decreases.

2.1 Pulse-by-pulse current limit: Because the current is controlled by mode, the limit comparator (VFB*) that is input by the pwm inverter through the sensefet is shown in the figure. Assuming that the 0.9mA current source only passes through the internal resistor (2.5R+R=2.8KΩ), the cathode voltage d2 of the diode is about 2.5v. Because d1 is blocked by a voltage (VFB) in excess of 2.5V at feedback, the cathode with a maximum voltage of D2 is clamped at this voltage, thus clamping VFB*. Therefore, the peak value of current passing through the senses is limited. 2.2 Leading Edge Blanking (LEB): When the internal SESEFET is turned on, there is usually a high current spike through the sensor network, reverse-recovered by the primary side capacitor and the secondary side rectifier. High voltage resistors on RSENSE can cause current mode PWM control. To counteract this effect, the FSDM0465RS employs leading edge blanking (LEB) circuitry. This circuit suppresses the sensor setting for a short time (TLEB) after it is turned on. 3. Protection circuit: fsdm0465rs 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, and the reliability of the system is improved without increasing the cost. In the event of a fault, the switch is held off by the termination sensor. This causes VCC to drop. When VCC reaches the UVLO stop voltage of 8V, the protection is reset, and the internal high-voltage current power supply charges the vcc capacitor through the vstr pin. when? VCC reaches the UVLO startup voltage, 12V, and the FSDM0465RS resumes normal operation. In this way, automatic restart can alternately enable and disable the switch of the power sensor until the fault condition is removed. 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, during overload loads, the protection circuit can initiate the transition. To avoid this undesired operation, the overload protection circuit is designed to activate after a specified time to determine if it is a transient condition or an overload condition. 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, thereby increasing the feedback voltage (VFB). If VFB exceeds 2.5V, D1 is blocked and the 3.5µA current source begins to slowly charge CB until VCC. In this case, vfb continues to increase until it reaches 6V when the switching operation is terminated as shown. The shutdown delay is 3.5µA the time it takes to charge the CB from 2.5V to 6.0V. Typically, 10~50ms latency is for most applications.

3.2 Over Voltage Protection (ovp): If the side feedback circuit is faulty or solder defect causes the feedback path to be open, the current through the optocoupler transistor is zero. The vfb then forces the preset maximum current to supply the SMPS with overload conditions 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. In general, VCC is related to the output voltage and the FSDM0465RS uses VCC instead of directly monitoring the output voltage. If VCC exceeds 19V, the ovp circuit activates causing the switch to terminate operation. Avoid designing VCC below 19 volts during normal operation. 3.3 Thermal Shutdown (TSD): The sensor and control chip are built into one package. This makes it easy to control the IC to detect the sensory net. Thermal shutdown is activated when the temperature exceeds approximately 150°C. 4.2 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 as shown. 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: fsdm0465rs has an internal soft start circuit, which increases the inversion input voltage of the pwm comparator and the sensor current after slow start. The typical soft-start time is 10 ms, and the pulse width of the power switching device is gradually increased to establish the correct working mechanism of the transformer, inductor and capacitor conditions. The voltage on the output capacitor is incremented in order to successfully establish the desired output voltage. It also helps prevent transformer saturation and lowers secondary diodes during startup. 5. Burst operation: Put standby mode, FSDM0465RS into burst mode operation. When the load decreases, the feedback voltage decreases. As shown, the device operates when the feedback voltage is lower than VbUrl (500mV). 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 (700mV) continue. Then the feedback voltage drops and the process repeats. Burst Mode alternately enables and disables switching of the power sensor to reduce switching losses in standby mode.

feature

High efficiency (>81% at 85Vac input) Low zero-load power consumption (<300MW at 240Vac input) Low power consumption in standby mode (<800mW at 240Vac input and 0.3W load) Small number of components Improve system reliability through various protection functions Internal Soft Start (10ms) Key Design Notes Resistors R102 and R105 are used to prevent start-up at low input voltages. There is no power-down in these resistors after start-up because the start pin is internally disconnected after start-up. The overload protection delay is designed to be about 50ms, and the C106 is 47nF. C106 can be lowered to 10 nF if faster triggering of olp is required. Zener diode ZD102 is used for safety testing such as UL. When the DRAIN pin and the FEEDBACK pin are shorted, the Zener diode fails and remains shorted, causing the fuse (F1) to blow, preventing the optocoupler (IC301) from exploding. This Zener diode also improves immunity to line surges.