FSQ0170RNA , FSQ0270RNA, FSQ0370RNA include an integrated current mode pulse width modulator (PWM) and avalanche resistant 700V sense FET. It is specifically designed for high performance off-site switched mode power supplies (SMPS) with minimal external components. Integrated PWM controller features include: a fixed frequency oscillation, under-voltage lockout (UVLO) protection resulting in leading edge blanking (LEB), optimized gate turn-on/turn-off drivers, thermal shutdown (TSD) protection, Temperature compensated precision current source loop compensation and fault protection circuits. Compared to discrete MOSFET and controller or RCC switching converter solutions, FSQ0170RNA, FSQ0270RNA, FSQ0370RNA reduce total component count, design size and weight, while improving efficiency, productivity and system reliability. These devices provide a basic platform well suited for the design of cost-effective flyback converters, such as auxiliary power supplies in personal computers.

block diagram

Function Description

1. Startup: In previous generations of Fairchild Power Switches (FPS?), an external resistor at the VSTR pin was required for the DC input voltage line. In this generation, the start-up resistor is replaced with an internal high-voltage current source, and 10ms after switching off the switching supply voltage V CC, cloud 12V above. The source turns back on if VCC falls below 8V.

figure 2

2. Feedback control: In the 700VFPS series using current mode control, as shown in Figure 3, an opto-magnetic coupler (such as H11A817 A) and a shunt regulator (such as KA431 ) are usually used to implement a feedback network. Comparing the feedback voltage with the voltage across the R SENSE resistor value, plus the bias voltage, makes it possible to control the duty cycle of the switch. When the reference pin voltage of the shunt regulator exceeds the internal reference voltage of 2.5V, the LED current of the optocoupler increases, and the feedback voltage VFB is pulled down, thereby reducing the duty cycle. This usually happens when the input voltage increases or the output load decreases.

image 3

3. Leading edge blanking (LEB): When the internal value of the SenseFET is turned on, the reverse recovery of the primary side capacitor and secondary side rectifier diode usually causes a high current spike through the SenseFET. An overvoltage SENSE resistor connected to R can cause incorrect feedback operation in current-mode PWM control. To counteract this effect, the described FPS employs leading edge blanking (LEB) circuitry. This circuit suppresses the conduction of the inductive FET after a relatively short time (tLEB) of the PWM.

4. Protection circuit: The FPS has several protection functions such as Overload Protection (OLP), Overvoltage Protection (OVP), Undervoltage Lockout (UVLO) and Thermal Shutdown (TSD). Since these protection circuits are fully integrated in the IC with no external components, reliability increases cost if not improved. Once a fault occurs, the switch is terminated and the value SenseFET remains off. This makes VCC drop. When VCC reaches the UVLO stop voltage, V stops (typically 8V), the protection is reset and the internal high voltage current source charges VCC through the V capacitor STR pin. When VCC reaches the UVLO startup voltage, V starts (12V typical), the FPS resumes its normal operation. In this manner, auto-restart can alternately enable and disable the switch-valued SenseFET of the power supply until the fault is removed. 4.1 Overload Protection (OLP): Overload is defined as a sudden event in which the load current exceeds a predetermined level due to an overload. 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, the OLP circuit can be activated during load transitions. To avoid this abnormal operation, the OLP circuit is designed to be activated after a specified time to determine if it is a transient condition or a true overload condition. In conjunction with the I PK current limit pin (if used), the current in the current mode feedback path is limited at the value when the maximum PWM duty cycle of the SenseFET is obtained. If the output consumes more than this maximum power, the output voltage (VO) is reduced by the rated voltage below. This reduces the current through the optocoupler's LED, which also reduces the current in the optocoupler's coupling transistor, thereby increasing the feedback voltage (VFB). If VFB exceeds 3V, the feedback input diode is blocked and the 5µA current source (I delay) begins to slowly sink ?FB up to VCC. Under this condition, VFB increases until it reaches 6V, then the switching action ends, as shown in Figure 4. The turn-off delay time is the time required to charge the C FB from 3V to 6V with a 5µA current source.

Figure 4

4.2 Thermal Shutdown (TSD): The value of the SenseFET and the control IC are integrated, making it easier to control the temperature of the SenseFET detected by the IC. Thermal shutdown is activated when the temperature exceeds approximately 140°C. 4.3 Over Voltage Protection (OVP): In the event of a fault in the secondary side feedback circuit, or a resulting weld defect opening the feedback loop, the current through the optocoupler transistor becomes almost zero (see Figure 14) . The VFB climbs up in a similar manner in the event of an overload, forcing a preset maximum current to be supplied to the switching power supply until the overload protection is activated. Because excess energy is supplied to the output, the output voltage can exceed the rated voltage before the overload protection activates, resulting in breakdown of the secondary side of the device. To prevent this, an overvoltage protection (OVP) circuit is used. In general, VCC is proportional to the output voltage and the FPS uses VCC instead of directly monitoring the output voltage. If VCC exceeds 19V the overvoltage protection circuit is activated, resulting in termination of the switching operation. To avoid normal non-active operation during OVP, VCC should be designed to be lower than 19V.

5. Soft-start: The FPS has an internal soft-start circuit that slowly increases the value of the SenseFET's current upwards after startup, as shown in Figure 5. The typical soft-start time is 10ms when the value of the SenseFET, which progressively is currently being allowed in the startup phase. The pulse width for power switching devices is gradually increased to establish correct operating conditions for transformers, inductors and capacitors. The voltage on the output capacitor is gradually increased to successfully establish the desired output voltage. This also helps prevent transformer saturation and reduces stress on the secondary diode during startup.

Figure 5

6. Burst operation: To minimize power consumption in standby mode, the FPS enters burst mode operation. The feedback voltage will decrease with load, as shown in Figure 6, and the device automatically enters burst mode when the feedback voltage is below VBURH (typically at 600mV). Swapping continues until the feedback voltage falls below V (typically 400mV). At this point, the switch stops working and the speed at which the output voltage begins to drop depends on the standby current load. This will cause the feedback voltage to rise. Once VBURH is passed, the switch resumes. The feedback voltage is then dropped and the process repeats. Burst mode of operation alternately enables and disables switching value of the SenseFET and reduces switching losses in standby mode.

Image 6

7. Adjust Peak Current Limit: As shown in Figure 7, the combined 2.8kΩ internal resistor is connected to the PWM comparator on the non-inverting lead. A receive pair of external resistors on the current limit pins are formed in parallel with a 2.8kΩ resistor, when the internal diodes are biased by the 900 μA main current source.

Figure 7