feature

Internal avalanche ruggedized sensor Low startup current (40uA max) at 240V AC and 0.4W load Precise fixed operating frequency (66kHz) Low EMI frequency modulation Pulse-by-pulse current limiting (adjustable) Overvoltage protection (OVP) Overload protection (OLP) Thermal Shutdown function (TSD) Auto-restart mode Undervoltage lockout (UVLO) with hysteresis Built-in soft-start (15ms)

application

Switching Power Adapters for VCRs, SVRs, Set-Top Boxes, DVDs and DVCDs Switching Power Supplies for Liquid Crystal Displays Related Application Description AN-4137: Offline Flyback Design Guidelines Converters Using Fairchild Power Switches (FPS) AN-4140: Offline Transformer Design Considerations Using Fairchild Flyback Converters for Power Switches AN-4141: Fairchild's Troubleshooting and Design Tips Power Switch Flyback Applications AN-4148: Sound Noise Reduction Technology Applications for FPS

illustrate

The FSCM0765R is an integrated pulse width modulator (PWM) and designed for high performance off-line switch mode power supplies (SMPS) using minimal external components. This device is an integrated high voltage power switching regulator combined with an avalanche ruggedness sensor and a current mode PWM control block. The PWM controller includes an integrated fixed frequency oscillator, undervoltage lockout, leading edge blanking (LEB), optimized gate driver, internal soft start, temperature compensated precision current source for loop compensation and self-protection circuitry. Compared with discrete MOSFET and PWM controllers, it can reduce overall cost, component count, size and simultaneously improve efficiency and productivity, as well as system reliability. This device is a basic platform suitable for low-cost designs of flyback converters.

Note:

Typical continuous power environment 50°C acclimation measurement in 1:1 non-ventilated enclosure

50 degree design for maximum practical continuous power environment in 2:1 open frame

3: 1230 VAC or 100/115 VAC with doubler.

Function description

1. Start-up: The figure shows the typical start-up circuit and application of the auxiliary winding of the FSCM0765 R transformer. Only the startup current (typically 25uA) is consumed before the FSCM0765R starts switching and the DC link supply current consumption is charged through the FPS (Icc) to the external capacitor (Ca) connected to the Vcc pin. When Vcc reaches the startup voltage at 12V (V startup), the FSCM0765R starts switching and the current consumed by the FSCM0765R increases to 3mA. Then, the FSCM0765R continues its normal switching operation with the power required by the device assisted by the transformer winding unless VCC is below the stop voltage of 8V (VSTOP). Guaranteed stable operation in the control IC, Vcc has under-voltage lockout (UVLO) with 4V hysteresis. Graph showing current consumed by FPS (Icc) and supply voltage (Vcc)

The minimum current supplied through the start-up resistor is given by

Where Vlinemin is the minimum input voltage, Vstart is the start-up voltage (12V), and Rstr is the start-up resistance. The startup resistor should be chosen so that the minimum value is greater than the maximum startup current (40uA). If not, the Vcc cannot be charged to the startup voltage and the FPS will not start.

2. Feedback control: FSCM0765R adopts current mode control, as shown in the figure. Optocouplers such as H11A817A) and shunt regulators such as KA431 are often used to implement feedback networks. Comparing the feedback voltage and the voltage through the Rsense resistor can control the switching duty cycle. When the KA431 exceeds the internal reference voltage of 2.5V, the H11A817A LED current increases, thereby reducing the feedback voltage and reducing the duty cycle. This activity occurs when the input voltage increases or the output load decreases.

2.1 Pulse-by-pulse current limit: Because the current mode employs control, the peak current through the sensor is determined by the inverting input of the PWM comparator (Vfb*) as shown. When the current through the photocell is zero, the current limit pin (#5) is left floating and the 0.9mA feedback current source (IFB) flows only through the internal resistor (R+2.5R=2.8k). The maximum cathode voltage and peak drain current at this diode D2 are 2.5V and 3A, respectively. The ground resistor (#5) on the current limit pin can be used. The limit level of current using an external resistor (RLIM) is given by:

2.2 Leading edge blanking (LEB): That is, the internal SESFEEFET is turned on, and there is usually a high level. The current spikes through the sensor network are caused by the primary side capacitor and the secondary side rectifier reverse recovery. Excessive voltage on the Rsense resistor can cause incorrect feedback operation control in current mode. To counteract this effect, the FSCM0765R uses a leading edge blanking (LEB) circuit. This circuit suppresses short-time PWM comparators that have been turned on.

3. Protection circuit: FSCM0765R has multiple protection functions such as self-overload protection (OLP), voltage protection (OVP) and thermal shutdown (TSD). Because these protection circuits are fully integrated in an integrated circuit with no external components, their reliability can be improved without increasing cost. Once a fault occurs, the switch is terminated and the sensor remains off. This causes Vcc to drop. When Vcc reaches UVLO the stop voltage is 8V, the current consumed by the FSCM0765R is reduced to the start-up current (typically 25uA), the current supplied by the DC link is an external capacitor (Ca) connected to the Vcc pin. When Vcc reaches the startup voltage of 12V, the FSCM0765R resumes normal operation. In this way, auto-restart alternately enables and disables power switching until the fault condition is removed (see diagram)

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, overloading during the load transition can activate the protection circuit. To avoid this undesired operation, the overload protection circuit is designed to activate after a specified time to determine whether it is a transient condition or an overload condition. Because of the pulse current limiting capability, the maximum peak current through the SESFET is limited, so the maximum input power is limited by a given input voltage. If the output consumes more than this maximum power, the output voltage (VO) is lower than the set voltage. This reduces the current through the optocoupler LED, which also reduces the current in the optocoupler transistor, which increases the feedback voltage (Vfb). If Vfb exceeds 2.5V, D1 is blocked and the 5.3uA current source (Idelay) starts charging CB slowly until Vcc. In this case, Vfb continues to increase until when the switching operation is terminated as shown. The shutdown delay is required to charge the CB from 2.5V to 6.0V with 5.3uA (Idelay). Generally speaking, a delay time of 10~50ms is used for most applications.

3.2 Over Voltage Protection (OVP): If the secondary side feedback circuit fails or a welding defect causes the feedback path to open, the current through the optocoupler transistor is almost zero. Then, Vfb climbs in an overload-like manner, forcing a preset maximum current to be supplied until the overload protection kicks in. Because the output voltage may exceed the rated voltage before the overvoltage is supplied with more energy than required, the load protection starts, causing the equipment on the secondary side. To prevent this, one employs an overvoltage protection (OVP) circuit. In general, Vcc is related to the output voltage and the FSCM0765R uses Vcc instead of directly monitoring the output voltage. If VCC exceeds 19V, the OVP circuit activates causing the switch to terminate operation. Avoid during normal operation, Vcc should be designed to be lower than 19V.

3.3 Thermal Shutdown (TSD): The sensor and control chip are built into one package. This enables the detection of sensory nets. When the temperature exceeds about 145°C, the thermal protection is triggered causing the FPS to shut down.

4. Frequency modulation: EMI reduction can be achieved by modulating the switching power supply. FM can reduce EMI by spreading the energy over a wider frequency range than the bandwidth measured by EMI test equipment. The amount of EMI reduction is directly related to the depth of the reference frequency. As shown the frequency changes from 63KHz to 69KHz in 4ms.

5. Soft start: FSCM0765R has an internal soft start to increase the circuit voltage of the inverter input of the PWM comparator plus the slow current start after the sensor. Typical soft-start time is 15ms. The pulse width is gradually increased to the power switching device to establish the correct operating conditions for the transformers, rectifier diodes and capacitors. The output voltage capacitor is gradually increased in order to smoothly establish the desired output voltage. It is also useful to reduce the auxiliary diode during startup to prevent saturation of the transformer.

6. Burst operation: In the standby state, the FSCM0765R enters the burst mode for light-load operation. The feedback voltage decreases as the load decreases. As shown when the feedback voltage is lower than VBL (300mV). At this point switching stops and the output voltage begins to drop at a rate depending on the backup current load. This causes the feedback voltage to rise. Switching continues once VBH (500mV) is passed. Then the feedback voltage drops and the process repeats. Burst Mode operation alternately enables and disables switching of the power sensor, reducing switching losses in standby mode.

feature

High efficiency (>81% at 85V AC input) Low power consumption in standby mode (<1W at 240Vac input and 0.4W load) Small number of components Improve system reliability through various protection functions Reduce EMI through frequency modulation Internal soft-start (15ms ) Key Design Notes Resistors R102 and R105 are designed to prevent overload protection from starting at low input voltage with a delay time of 50ms, C106 is 47nF. If fast triggering of the OLP is required, C106 can be reduced to 22nF.