Features

Monolithic 700V sensor power switch Precision fixed operating frequency: 250mW no-load power consumption at 130kHz 265Vac Pulse Current Limit Overload Protection (OLP) and Internal Thermal Protection Hysteresis Shutdown (TSD) Auto-Restart Mode No Auxiliary Winding Required

application

Set Top Box and DSL Power Home Appliances, IH Cookers, Auxiliary Power Supplies Related Resources AN-4137 - Offline Flyback Design Guide for Converters Using fps AN-4141 - Troubleshooting and Design Tips for Fairchild Power Switch (FPSTM) flyback applications: AN-4147 - RCD Shock Absorber Design Guide Flyback AN6075 - (Flyback) - AN-6075 - Compact Green Mode Adapter Using FSQ500L , Low Cost

illustrate

The fsq500n is a set-top box, DSL, and home appliance auxiliary power supply specially designed for low cost and small size. This device incorporates a current mode pulse width regulator (PWM) with a sensor. Features of this integrated pwm controller include: a fixed oscillator, undervoltage lockout (uvlo) protection, overload protection (OLP), leading edge blanking (LEB), an optimized gate switch driver, hysteretic thermal shutdown (TSD) ) protection, temperature compensated source of precision current loop compensation. When combined with linear power supplies, the FSQ500N reduces overall size while increasing efficiency, productivity and system reliability. This device provides a basic platform for cost-effective flyback converters.

Function description

1. Start-up and VCC rules: At start-up, an internal high voltage current source provides internal bias and charges an external capacitor (CA) connected to the VCC pin, as shown in Figure 15. An inline high voltage regulator (HV/REG) is located at the drain and the VCC pin regulates VCC to 6.5V and provides operating current. Therefore, the fsq500n requires no auxiliary bias winding.

2. Feedback control: FSQ500N adopts current mode control, as shown in the figure. Optocouplers as FOD817A) and shunt regulators (eg KA431 ) are often used to implement feedback networks. Combining the feedback voltage with the voltage through the RSENSE resistor controls the switching duty cycle. When the reference pin regulator voltage exceeds the internal reference voltage voltage of 2.5V, the optocoupler LED current increases, pulling down the feedback voltage to shorten the duty cycle. This usually occurs when the line input voltage increases or the output load current decreases. 2.1 Pulse-by-pulse current limit: Due to the current mode control, the peak current sensefet through is subject to a pulse width modulated comparator (VFB*) as shown. Assuming a current source of 225 microamps only flows through the internal resistor (8R+R=12KΩ), the cathode voltage of diode d2 is about 2.7v. When the feedback voltage (VFB) exceeds 2.7V, the maximum voltage at the cathode of D2 is clamped at this voltage, clamping VFB*. Therefore, the peak current through the sensor is limited. 2.2 Leading Edge Blanking (LEB): When the internal sensor has been turned on, a high current spike occurs through the sensor network, and recovery is caused by the reverse of the primary side capacitor and the secondary side rectifier. Too high a voltage on the RSENSE resistor can cause erroneous feedback operation for current mode PWM control. To counteract this effect, the FPS employs leading edge blanking (LEB) circuitry. This circuit suppresses the time after the short-circuit sensefet of the PWM comparator is turned on (tleb=250ns).

3. Protection circuit: fsq500n has two self-protection functions: overload protection (olp) and thermal shutdown (TSD). The olp is implemented as an automatic restart mode, and the trigger is not switched during TSD. Once an overload condition is detected; switching is terminated, the sensor remains off, and the HV/REG turns off. This causes VCC to drop. When VCC is lower than the under-voltage lockout (uvlo) stop voltage of 5.0V, the protection is reset and the startup circuit charges the VCC capacitor. When VCC reaches the startup voltage of 6.0V, the FSQ500N resumes normal operation. If the fault condition still does not exist after removal, sensefet and hv/reg remain off and VCC drops to VSTOP again. In this way, autorestart can alternately enable and disable switching until the fault state is eliminated, as shown. Because these protection circuits are fully integrated in the integrated circuit with no external components, reliability is improved without increasing cost.

3.1 Overload Protection (OLP): Define overload due to unexpected abnormal events. In this case, the protection circuit should trigger to protect the switching power supply. However, even when the smps is operating under normal conditions, the overload protection circuit can trigger during load transitions. To avoid accidental operation in this situation, overload protection circuits are designed to trigger after a specified time to determine whether the situation is a temporary or a true overload. Due to the pulse current limiting capability, the maximum peak current through the SENSEFET is therefore limited to the maximum input power. 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, it also reduces the optocoupler transistor current and thus increases the feedback voltage (vfb). If VFB exceeds 2.7V, D1 is blocked and the 5µA current supply starts charging the circuit breaker slowly until VCC. Under this condition, VFB continues to increase until it reaches 4.5V, when the switching operation is terminated, as shown. The shutdown delay is 5µA the time it takes to charge CB from 2.7V to 4.5V. Typically, a delay of 10~50ms is a typical delay application in most cases. This protection is implemented in automatic restart mode.

3.2.Thermal shutdown ("TSD": The sensefet and control IC in one package make it easy to control the IC to detect abnormality in temperature sense When the temperature is over ~140 degrees, thermal shutdown trigger. When to trigger, delay current damaged, the switching operation stops, and VCC is set to 5.7v via the internal high voltage current source, from 6.5v, as shown in the figure. Since the CTV signal is disabled from the switch sensor, there is no switching until the connection temperature drops enough. If the connection temperature is lower than the typical 60 degrees low. The signal is cleared and VCC is set to 6.5V again. While VCC increases from 5.7v to 6.5v, the soft-start function transitions the sense to no voltage and/or current stress

4. Soft-start: Soft-start time is increased by external vcc capacitor (ca), non-inverting input voltage of pwm comparator and slow current after sensor start-up. Before vcc reaches vstart, the CA is charged by the current ICH-IStart, where ICH and as shown. VCC reaches vstart and all internal blocks are activated so that the current consumption inside the IC becomes the IOP. Therefore, CA is charged by the current ICH-IOP, which makes the slope of VCC increase slowly. VCC is shifted by a negative voltage of 6.0V, then VCC-6.0V is the terminal of the input PWM comparator for one of the inputs. The leakage current follows VCC-6.0V instead of VFB* because of the low-dominant nature of the pwm comparator. Soft-start time can be long or short, select ca, as shown in the figure. During TS/S, disable IDELAY to avoid unwanted olp. Typically, ts/s is about 4.6 ms with 27 microF of Ca.

The peak switching device of the power drain current is stepped up to establish the correct operating conditions of the transformers, inductors and capacitors. The output voltage capacitor is gradually increased to smoothly establish the desired output voltage. It also helps prevent transformer saturation and reduces stress on the secondary diode at startup. 5. Burst operation: Put standby mode, FPS into burst mode operation. During burst mode operation, ifb (burst) is reduced by half ifb (normal). When the load decreases, the feedback voltage decreases. As shown in the figure, the device operates when the feedback voltage is lower than VbUrl (750mV). At this point, switching stops and the output voltage begins to drop depending on the rate at which the backup current is loaded. This causes the feedback voltage to rise. Switching resumes once Vburh (800mV) is passed. The feedback voltage then drops and the process repeats. Burst Mode alternately enables and disables the power switching sensefet, reducing switching losses in standby mode.