Features

Burst Mode Operation for Reduced Power Consumption External Pins for Synchronization Wide Operating Frequency Range Up to 130kHz Internal Startup Circuit Low Operating Current (Max: 6mA) Pulse-by-Pulse Current Limit Overload protection (auto-restart mode) Abnormal over-current protection (auto-restart mode) Internal thermal shutdown (auto-restart mode) Undervoltage lockout Internal high voltage sensor (650V)

Application CRT Monitor Description: The fses0765rg is a special fairchild power switch (fps) designed for offline switching power external components of CRT monitors. This device incorporates a current mode high voltage power sensor integrated pwm controller package. This pwm controller features an integrated oscillator synchronized to an external sync signal, undervoltage lockout, optimized gate driver and temperature compensated precision current source compensation. The device also includes various fault overvoltage protection, overload protection circuit protection, abnormal overcurrent protection and overcurrent protection temperature protection. With discrete mosfet and pwm controller solutions, fps can reduce total cost while calculating parts count, size and weight to improve efficiency, productivity and system reliability. This device is ideal for cost-effective CRT display power supply.

Notes: 1. The maximum practical continuous power design ambient temperature for the open frame is 50°C. 2. 230 VAC or 100/115 VAC with frequency multiplier. 3. The maximum output power can be limited by the junction. temperature

Function description

1. In previous generations of Fairchild Power Switch (FPSTM) VCC pins had external resistors for starting the DC input voltage line. In this generation the start-up resistor is replaced by an internal high voltage current source. At startup, the internal high voltage current source provides internal bias and charges the external capacitor (CVCC) connected to VCC pin 4 as shown. When VCC reaches 12V, the FPS starts switching and the internal high voltage current source is disabled. Then, the fps continues its normal switching operation and power is supplied from the auxiliary transformer winding unless VCC falls below the stop voltage of 9V.

2. Feedback control: FSES0765RG adopts current mode control, as shown in the figure. Optocouplers (like h11a817a) and shunt regulators (like ka431) are often used to implement feedback networks. Adding the feedback voltage to the rsense resistor plus the offset voltage controls the switch duty cycle. When the voltage of the reference pin KA431 exceeds the internal reference voltage within 2.5V, the current of the H11A817A LED 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 mode adopts control, the peak current through the sensing FET is limited by the reverse input of the pwm comparator (vfb*) as shown in the figure. Assuming that the 0.9mA current source only flows through the internal resistor (2.5R+r=2.8kΩ), the cathode voltage of diode d2 is about 2.5v. Since diode D1 is blocked when the feedback voltage (VFB) exceeds 2.5V, the maximum voltage at the cathode of D2 is clamped at this voltage, therefore, the peak value of the sensefet current is limited. 2.2 Leading edge blanking (LEB): When the internal sensing FET is turned on, there is usually a high level. By sensing the current spike of the FET, the capacitor and the secondary side rectifier reverse recovery is induced by the primary side. Excessive voltage on the RSENSE resistor can cause incorrect feedback operation control in current mode. To counteract this effect, fps uses a leading edge blanking (LEB) circuit. This circuit suppresses the short-time comparator (TLEB) turning on after the PWM sensing FET is activated.

3. Protection circuit: fses0765rg has several protection functions such as overload protection (OLP), abnormal over current protection (AOCP), over voltage protection (ovp) and thermal shutdown (tsd). Because these protection circuits are fully integrated in the integrated circuit without the need for external components, reliability can be improved without increasing cost. Once a fault occurs, the switch is terminated and the sensing FET remains off. This causes VCC to drop. Stop voltage when vcc reaches uvlo, 9V, protection reset, internal high voltage current source to charge VCC capacitor through VSTR pin. When the vcc reaches the uvlo startup voltage of 12v, the fps resumes normal operation. In this way, auto-restart alternately enables and disables the power switching detection FET until the fault is removed (see figure).

3.1 Overload Protection (OLP): Overload occurs when the load current exceeds the preset value due to unexpected reasons. In this case, the protection circuit should be activated to protect the switching power supply. However, even when the switching power supply is operating normally, overload protection can activate the circuit during load transitions. To avoid this unintended operation, the overload protection circuit is designed to activate after a specified time to determine if this is a transient or overload condition. Due to the pulse-by-pulse current limiting capability, the maximum peak current through the sensing FET is limited and, 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, thereby increasing the feedback voltage (vfb). If VFB exceeds 2.5V, D1 is blocked and the 2UA current source (idelay) starts charging cb slowly until vcc. Under this condition, VFB continues to increase until it reaches 7.5V, then terminates the switching operation as shown in Figure 7. The delay time for shutdown is the time required to charge the 2.5V to 7.5V breaker with 2uA. Generally speaking, 10~50ms delay time is typical in most applications.

3.2 Abnormal Over Current Protection (AOCP): Even though the FPS has OLP (overload protection) and current mode pwm feedback, these protections are not enough when the secondary diode is shorted or the transformer pin is shorted. The FPS has an internal AOCP (Anomalous Over Current Protection) circuit as shown in the Sense FET when the power is turned on, the AOCP block is enabled and monitors the current through the sense resistor. The voltage passed through the resistor is then compared to the preset AOCP level. If the sense resistor voltage is greater than the AOCP level, a reset signal is applied to the latch, causing a shutdown.

3.3 Over Voltage Protection (ovp): If the secondary side feedback circuit fails or solder defect causes the feedback channel to open, the current through the optocoupler transistor is almost zero. The ventricular fibrillation then rises in a manner similar to an overload situation, forcing a preset maximum current to be supplied to the SMPS until overload protection is activated. Because the energy ratio is required to be supplied to the output, the output voltage can exceed the rated voltage before the overload protection is activated, resulting in a second side. To prevent this from happening, a voltage protection (ovp) circuit is used. In general, vcc is proportional to the output voltage, and fps is used instead of directly monitoring the output voltage. If VCC exceeds 19V, an ovp circuit is activated, resulting in termination of switching operation. To avoid accidental activation of the ovp during normal operation, the vcc should be designed to be lower than 19V. 3.4 Thermal Shutdown (TSD): The sensing FET and control chip are built into one package. This enables the control chip field effect transistors from the sensing heat generation. Thermal shutdown is initiated when the temperature exceeds approximately 150°C. 4. Synchronization: Because fses0765rg is a CRT monitor application, this device has the function of synchronizing to minimize screen noise. The mosfet is turned on to synchronize with the external synchronization signal as shown in the figure. To reduce voltage stress on the secondary side rectifier, double pulse prevention is also included. The switch of the mosfet is to turn off the mosfet and suppress it for 5us in order to eliminate the double pulse situation.

5. Soft-start: The FPS has an internal soft-start circuit that slowly increases the voltage and induced FET current at the inverting input of the pwm comparator during startup. Typical soft-start time is 15ms. The pulse width is the correct operating condition for power switching devices to step up transformers, inductors, and capacitors. The voltage on the output capacitor is also gradually increased in order to successfully establish the desired output voltage. This also helps prevent transformer saturation, reducing secondary diodes during startup. 6. Burst operation: To minimize power consumption in standby mode, the fses0765rg adopts burst operation. Once the FSES0765RG enters burst mode, the effective switching frequency and all output voltages are reduced. Figure shows a typical circuit with feedback forcing the fses0765rg into burst operation. During normal operation, the application image turns on the signal transistor Q1, which turns on R3 and D1 from the feedback network. Therefore, only VO1 is affected by the feedback circuit working properly, by the R1 and R2 components. In standby mode, the picture on signal is disabled, and transistor Q1 is turned off, which connects R3 and D1 to the reference pin for the KA431. Then, when the reference voltage KA431 pin is greater than 2.5V, the current increases through the optocoupler, which increases the current through the photodiode. This lowers the feedback voltage (VFB) and forces the FPS to stop switching until VCC drops to 8.5V. When VCC reaches 8.5V, the FPS starts switching to a switching frequency of 50kHz with a peak leakage current of 0.6A until VCC reaches 9V. When VCC reaches 9V, the switching operation is terminated again until VCC drops to 8.5V.