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: 80uA)

Low operating current (Max: 15mA)

Pulse-by-Pulse Current Limiting

Overvoltage protection (auto restart mode)

Overload protection (auto restart mode)

Abnormal overcurrent protection (auto restart mode)

Internal thermal shutdown (auto restart mode)

undervoltage lockout

Internal high pressure sensor

illustrate

The FS8S0965RCB is a Fairchild Power Switch (FPS), the smallest external component designed specifically for offline switching power supplies of CRT displays. This device is the feel of the current mode combined with the high voltage power supply's PWM controller in one package. The PWM controller features an integrated oscillator with an external synchronizing signal, under-voltage lockout, optimized gate driver and temperature-compensated precision current source loop compensation. The device also includes various fault overvoltage protection, overload protection circuit protection, abnormal overcurrent protection, and overtemperature protection. Compared to discrete MOSFETs and employing PWM controller solutions, FPS can reduce overall cost, parts count, size and weight while increasing efficiency, productivity and system reliability. This device is well suited for cost-effective monitoring power

Function description

1. Startup: ensure the control chip, FS8S0965RCB has a 6V UVLO circuit hysteresis band. The graph shows the relationship between supply current (Icc) and supply voltage (Vcc). Before Vcc reaches 15V, the FPS draws only 80 microamps of startup current, usually from the DC link through the startup resistor. When Vcc reaches 15V, the FPS starts working and the current increases to 15mA. Once the control IC starts operating, it will continue its normal operation until Vcc is 9 volts below the stop voltage

2. Feedback control: FS8S0965RCB adopts primary side regulation, which allows to eliminate secondary side components of feedback circuit, such as optocoupler and TL431. The diagram shows the primary side control circuit. This primary side regulation voltage (Vpsr) controls the breakdown voltage of the Zener diode (Dz). Since mode control is now used, the leakage current of the power MOSFET is limited by the inverting input of the PWM comparator (Vfb*). When the MOSFET is turned on, there is usually a MOSFET current spike recovery caused by primary side capacitance and secondary side rectification reverse. To prevent premature termination of switching pulses due to current spikes, FPS employs leading edge blanking (LEB). The leading edge blanking circuit is turned on when the MOSFET is turned on.

3. Protection function: FS8S0965RCB comes with 4 protection functions including abnormal over current protection (AOCP), overload protection (OLP), over voltage protection (OVP) and thermal shutdown (TSD). Because these protection circuits are fully integrated into the integrated circuit with external components, no cost increase is required. In these failure situations, the FPS enters an automatic restart operation. Once a fault occurs, the switching operation is terminated and the MOSFET remains off, which will force a reduction in Vcc. When Vcc reaches 9V, the protection is reset and the supply current is reduced to 80µA. Then, Vcc starts to increase with the current supplied through the startup resistor. When Vcc reaches 15V, if the fault condition is removed. In this way, auto-restart alternately enables and disables the power switching MOSFET until the fault condition is removed, as shown in Figure 3.

3.1 Abnormal Over Current 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 time. Therefore, abnormal overcurrent protection (AOCP) modules are added to ensure that as shown. Turn off the sensor within 300 seconds after detecting abnormal overcurrent.

3.2 Overload Protection (OLP): When the load current exceeds the preset level for more than the preset time, the protection circuit should be activated to protect the skim milk powder. Because of the pulse current limiting capability, the maximum peak current through the SMPS is limited and therefore the maximum input power is limited. at a given input voltage. If the output consumption exceeds this maximum power, the output voltage along with the primary side regulation voltage drops below the set voltage. This reduces the current increasing the feedback voltage (Vfb) of the transistor through primary side regulation. If Vfb exceeds 2.7V, D1 is blocked and Cfb starts to charge the CFB slowly compared to when the 2uA current source is 2uA current source compared to 0.9mA. In this case, Vfb continues to increase to 7.5V, at which point the switching operation is terminated, as shown. Delay shutdown time is Cfb from 2.7V with 2u to 7.5V

3.3 Over Voltage Protection (OVP): When the primary side feedback circuit or feedback loop fails due to the solder defect leading to an open circuit, the current through the primary side control transistor is almost zero. Then, Vfb ramps up to an overload condition in a similar fashion, forcing a preset maximum current to the secondary side until the overload protection is activated. Because the energy supplied to the output is greater than required, the output voltage may exceed the rated voltage before the load protection starts, causing the equipment on the secondary side. To prevent this, an overvoltage protection (OVP) circuit is employed. When the Vcc voltage reaches 37V, the OVP block is activated. 3.4 Thermal Shutdown (TSD): The sensor and control chip are built into one package. This enables the detection of sensory nets. Thermal shutdown is initiated when the temperature exceeds approximately 160°C.

4. Soft start: The figure shows the soft start circuit. At initial startup, a 0.9 mA current source leaks through Css and Rss. When Css is charged, the leakage current decreases. Therefore, by choosing the ratio Cfb, it is possible to slowly increase the feedback voltage to force the induced current to increase slowly. After Css reaches the steady-state value, D3 is blocked and the soft-switching circuit is separated from the feedback circuit. If the value of Css is too large, it is possible that Vfb increases to 7.5V, which activates the overload protection soft-start time. To avoid this, it is recommended that the value of Css should not exceed the circulating fluidized bed.

5. Synchronization: In order to reduce the influence of switching noise on the screen, the switching power supply of the monitor is synchronized to the frequency of the external signal, which is usually a horizontal synchronization flyback signal. Switching frequency FPS can be from 20khz to 150khz according to external sync signal. An internal sync comparator detects the sync signal and determines when the sensor is turned on. The sensor turns on the comparator output on the negative edge of the sync. The reference voltage for the synchronous comparator is a base frequency of 20kHz and varies between 5.8V and 7.2V as shown. The inverted sawtooth reference gets rid of excessive switching noise when the first sync is turned on. It is recommended that the external sync signal has an amplitude greater than 4.2V.

6. Burst Mode Operation: To minimize power standby dissipation, the FS8S0965RCB operates in burst mode. In burst mode, the FPS reduces the switching frequency and output voltage. The FPS enters burst mode when the soft-start pin voltage is higher than 3V, no synchronization signal is applied, and the feedback voltage is lower than 1V. During burst mode operation, Vcc is controlled with hysteresis between 11V and 12V. Once the FPS enters burst mode, the operating voltage drops to 11V until Vcc stops switching, when Vcc reaches 11V, the FPS starts switching with a switching frequency of 40kHz and a peak MOSFET current of 0.6 A until Vcc reaches 12 V. When Vcc reaches 12V, the switching operation is terminated again until Vcc drops to 11V. The figure shows the working waveform. The soft-start during this first start-up is shown in Section 1. During this period, the switching frequency is 20kHz with no external sync signal. Section 2 represents normal mode operation. The switching frequency is synchronized with an external synchronization signal. The external sync signal has been removed in Section 3. However, the load is still so that the feedback voltage (VFB) is higher than 1V. During this period, the FPS performs a normal switching operation with a switching frequency of 20kHz. Sections 4 and 5 show burst mode operation. At the end of the third section, the load is removed and the feedback voltage (Vfb) drops below 1V forcing the FPS to stop switching operations. In segment 4, Vcc drops to 11V. In paragraph 5, the hysteresis of Vcc is controlled between 11V and 12V. When an external sync signal is applied to pin 5, the FPS resumes its normal operation. To minimize power consumption in standby mode, it is recommended to set the Vcc value as high as possible (about 29 volts) during normal operation.