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2022-09-23 11:09:02
FS8S0765RCB Fairchild Power Switch (FPS™)
feature
Burst mode operation to reduce power, consumption in standby mode External pins for synchronization and soft-start, wide operating frequency range up to 150kHz, low startup current (max: 80µA) low operating current (max: 15mA) pulse-by-pulse Current limiting, over voltage protection (auto restart mode) Overload protection (auto restart mode) Abnormal over current protection (auto restart mode) Internal thermal shutdown (auto restart mode) Under voltage lockout Internal high voltage sensor
illustrate
The FS8S0765 RCB is a Fairchild Power Switch (FPS) that is designed for offline switching power supplies of CRT displays with minimal external components. This device is a current mode pulse width modulation (PWM) controller packaged in combination with a high voltage power sensor. Integrated oscillator signal synchronized with external sync, under voltage lockout, optimized gate driver, temperature compensated precision current source loop compensation. The device also includes various fault protection, circuit protection such as overvoltage, overload protection, abnormal overcurrent protection, and overtemperature protection. Compared with discrete MOSFET and PWM controller solutions, FPS can reduce total cost, parts count, size , weight while increasing efficiency, productivity and system reliability. This unit is ideal for cost-effective monitoring of power supplies.
Absolute Maximum Ratings "Absolute Maximum Ratings" are values for which the safety of the device is not guaranteed. This device should not operate within these limitations. Parameter values defined in the Electrical Characteristics table are not guaranteed to be at absolute maximum ratings. The "Recommended Operating Conditions" table defines the conditions under which the equipment will actually operate. (TA=25°C unless otherwise specified)
notes:
1.Tj=25°C to 150°C
2. Repeat rating: maximum junction temperature limited pulse width
3.L=14mH, start Tj=25°C
4.L=13μH, start Tj=25°C
Note:
1. Although these parameters are guaranteed at design time, they are not tested in mass production.
2. These parameters, although guaranteed, are tested during the EDS (wafer test) process.
3. These parameters represent inductor current.
4. These parameters are to control the current in the chip.
Function description
1. Startup: to ensure the stable operation of the control device IC, FS8S0765RCB has a UVLO circuit band with 6V hysteresis. The graph shows the relationship between supply current (Icc) and supply voltage (Vcc). Before Vcc reaches 15V, the FPS only consumes 80mA of startup current, usually by the DC link through the startup resistor. When Vcc reaches 15V, the FPS starts to work and the operating current increases to 15mA as shown in the picture. Once the control chip starts to work, it continues to operate normally until Vcc is lower than the stop voltage of 9V.
2. Feedback control: FS8S0765RCB adopts primary side regulation, which allows to eliminate circuit components on the secondary side of feedback, such as optocoupler and TL431. Figure 23 shows the primary side control circuit. The primary side regulation voltage (Vpsr) controls the breakdown voltage (Dz) to the Zener diode. Due to the use of current mode control the drain current of the power MOSFET is controlled by the inverter input pulse width modulated comparator (Vfb*). when? The MOSFET is on and there is usually a high current. Primary side induced MOSFET current spike capacitance and secondary side rectifier reverse recovery. To prevent premature termination of switching pulses caused by current spikes, the FPS uses leading edge blanking (LEB). The leading edge blanking circuit suppresses the time after the shorted MOSFET of the PWM comparator is turned on.
3. Protection function: FS8S0765RCB has four self-protection functions such as abnormal overcurrent protection (AOCP), overload protection (OLP), overvoltage protection (OVP) and thermal shutdown (TSD). Because these protection circuits are fully integrated in integrated circuits with no external components, their reliability can be improved without increasing cost. If this failure situation occurs, the FPS enters an automatic restart operation. Once a fault occurs, the switching operation is terminated and the MOSFET remains off, which forces a reduction in Vcc. The protection resets when Vcc reaches 9V and the supply current drops to 80µA. Then, Vcc starts to be supplied through the startup resistor as the current increases. When Vcc reaches 15V, if the fault condition is removed. In this way, autorestart alternately enables and disables the power MOSFET until the fault is removed, as shown.
3.1 Abnormal Overcurrent Protection (AOCP): When the secondary rectifier diode or transformer pin is short-circuited, a steep current with extremely high di/dt can flow during the LEB period. Therefore, an abnormal overcurrent protection (AOCP) module is added to ensure reliability as shown in the figure. It senses the situation within 300ns after it shuts down abnormal overcurrent.
3.2 Overload Protection (OLP): When the load current exceeds the preset level for a predetermined time, the protection circuit should be activated to protect the SMPS. Because of the one-by-one current limiting capability, the maximum peak current through the SMPS is limited, and the maximum input given the input voltage will limit the power supply. If the output consumes more than this maximum power, the output voltage, together with the primary side regulation voltage, is reduced below the set voltage. This reduces the current through the primary side regulation transistor, increasing the feedback voltage (Vfb). If Vfb exceeds 2.7V, D1 is blocked and the 2µA current source starts charging the CFB with a slow source of Cfb charging compared to 0.9mA current. In this case, Vfb continues to increase to 7.5V, and the switch operation is terminated at this point, as shown. The delay time for shutdown is to charge Cfb from 2.7V to 7.5V with 2µA.
3.3 Over-Voltage Protection (OVP): In the case of primary side feedback circuit failure or open feedback loop due to solder defects, the current through the primary side control transistor becomes almost zero. Then, Vfb is forced to supply current to the secondary side at a preset maximum value in accordance with the overload condition until the overload protection is activated. Because more energy is supplied to the output before the overvoltage voltage may exceed the rated voltage load protection starts, resulting in secondary side equipment. To prevent this situation, over-voltage protection (OVP) circuits are employed. The block is activated when the Vcc voltage reaches 37V. 3.4 Thermal Shutdown (TSD): The sensor and control chip are built into one package. This enables the control IC to detect the sensory net. If the temperature exceeds 160°C, initiate thermal shutdown.
4. Soft start: The figure shows the soft start circuit. During initial startup, a 0.9 mA current source leaks out through Css and Rss. The leakage current decreases when Css is charged. Choosing a larger Css than Cfb can increase the feedback voltage slowly, forcing the sensor current to increase slowly. After Css reaches the steady state value, D3 is the blocking, soft switching circuit and feedback circuit. If the value of Css is too large, is it possible that Vfb increases to 7.5V, activating overload protection during soft start. To avoid this situation, it is recommended that the value of Css should not exceed 100 times that of the circulating fluidized bed.
5. Synchronization: To reduce the noise on the screen of the switching effect, the monitor's switching power supply synchronizes the frequency at which it switches to an external signal, usually a horizontal synchronization flyback signal. The switching frequency of the FPS can be from 20 kHz to 150 kHz depending on the external sync signal. An internal sync comparator detects the sync signal and determines when the sensor is set to turn on. The sensor turns on the negative edge of the synchronous comparator output. The reference voltage for the synchronous comparator is an inverted sawtooth with a fundamental frequency of 20kHz and 5.8V and 7.2V, as shown in Figure 29. The inverted sawtooth reference eliminates excessive switching noise when the first sync is turned on. The external sync recommends that the signal amplitude is greater than 4.2 volts
6. Burst Mode Operation: Minimize power standby dissipation, FS8S0765RCB burst mode operation. In burst mode, the FPS reduces the effective switching frequency and output voltage. This is when the soft-start voltage for the pin is higher than 3V. No sync signal is applied, and the feedback voltage is below 1V during burst mode operation with Vcc at 11V and 12V. Once the FPS goes into burst mode, it stops switching until Vcc drops to 11V. When Vcc reaches 11V, the FPS starts switching at 40kHz with a peak MOSFET current of 0.6A until Vcc reaches 12V. The switching operation is terminated again when Vcc reaches 12V until Vcc drops to 11V. The figure shows the working waveform. Start-up during soft initial start-up is shown in Section 1. During this period, there is no external synchronization signal and the switching frequency is 20kHz. The second section represents normal mode operation. The switching frequency is synchronized with an external synchronization signal. In part 3, the external sync signal is removed; however, the load and feedback voltage are still present. (Vfb) is higher than 1V. During this period, the FPS switching frequency is 20 kHz. Sections 4 and 5 show burst mode operation. At the end of Section 3, the load is removed and the feedback voltage (Vfb) drops to 1V, forcing the FPS to stop switching operation. In section 4, Vcc drops to 11V. Section 5, the hysteresis of Vcc is controlled between 11V and 12V. When an external sync signal is applied on pin 5 FPS resumes normal operation. To minimize power consumption during standby, it is recommended to set the Vcc value for normal operation as high as possible (approximately 29 volts).