Features

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

application

Power supply for LCD TVs and monitors, VCRs, SVRs, DVD and DVD recorder adapters

describe

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.

Function description

1. Startup: In the previous generation of semiconductor power switches (FPS_), the VCC pin has an external startup resistor to the DC input voltage line. In this generation, the startup resistor is powered by an internal high voltage current source. At startup, an internal high voltage current source provides internal biasing and charging of an external capacitor (CVCC) connected to the VCC pin as shown in Figure 16. When VCC reaches 12V , the FSFM260/261/300 starts switching and the internal high voltage current source is disabled. Then, the fsfm260/261/300 continue to work normally switching operation, the power is supplied by the auxiliary transformer winding, unless VCC is lower than the stop voltage 8V.

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. Comparing the feedback voltage across the RSENSE resistor makes it possible to control 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 the current mode control, the comparator (VFB*) is modulated by the pulse width through the sensefet, as shown in Figure 17. when? The current through the phototransistor is zero, the CURRENT LIMIT pin (4) is left floating, and the supply (IFB) of the feedback current 0.9mA is only through the internal resistor (R + 2.5R = 2.8K). In this case, the peak drain current of the cathode voltage diode d2 is 2.5V and 1.5A at maximum, respectively. The pulse-by-pulse current limit can be adjusted with a resistor to GND (4) on the current limit pin. The current limit level uses an external resistor (RLIM)

2.2 Leading Edge Blanking (LEB): When the internal sensor is turned on, a high current spike occurs through the sensefet, caused by the primary side capacitor and secondary side rectifier reverse recovery. Too high a voltage on the RSENSE resistor 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 suppresses the pulse width modulation comparator (TLEB) for a short time after the sensor is turned on. 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 unwanted excess power is delivered to the secondary power supply side. To compensate, auxiliary power compensation can use the network in the figure. 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 power (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 Difference Gain Constant Power) where Ilim_spec is the specification; NA and NP are the turns and primary side of VCC; ifb is the supply at the internal current feedback pin, typically 0.9mA; and Δilim_comp is the must-eliminate. If the capacitors in the circuit are 1µF, then 100V is fine for all applications.

3. Protection circuit: fsfm260/261/300 has some 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, auto-restart can alternately enable and disable the switching of the power sensor until the fault is resolved. 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, therefore, the maximum input power is limited by a given input voltage. If the output consumes more than this maximum power, the output voltage (vo) drops 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 begins to slowly charge CB up to VCC. In this case, Vfb continues to increase until it reaches 6V, when the switching operation is terminated, the shutdown delay time is 5µA required to charge CB from 2.5V to 6.0V. Typically, 10~50ms delay time is for most applications.

3.2 Over Voltage Protection (ovp): If a side feedback circuit failure or solder defect causes an open circuit in the feedback path, current flows through the optocoupler transistor 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 allows the control IC to detect that the SSESFET initiates thermal shutdown when the temperature exceeds about 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 conditions, because severe current stress is applied to 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: The fsfm260/261/300 has a soft-start circuit with an internal added pwm comparator to invert the input voltage, along with the sensor current, slow 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 reduces stress on the secondary diode during startup. 5. Burst Operation: Put the fsfm260/261/300 in standby mode into burst mode operation. When the load decreases, the feedback voltage decreases. , the 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. than traditional pwm controllers and mosfet discrete solutions. 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 handling components. Traditional power ground and signal ground. The power ground is the main input voltage and the power supply, while the signal ground is the PWM ground to the controller. In fps, the two grounds are the same pin, GND. Often, separate grounds are not identical to the same traces, meeting at only 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 polyester film or ceramic VCC with electrolytic capacitors operate better with smoothness. 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 Capacitors connected between primary and secondary DC power supply ground surge immunity. Because the voltage range of the feedback line is small, it is affected by the noise of the drain pin. Those marks should not go through 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.