feature

Precise fixed operating frequency

FS7M0680 (66kHz)

Pulse-by-Pulse Current Limiting

overcurrent protection

Overload protection

Overvoltage protection (minimum 25V)

Internal thermal shutdown function

Undervoltage Lockout with Hysteresis

Internal high voltage sense FET

Latch Mode

soft start

illustrate

The Fairchild Power Switch (FPS) product family is designed for offline switching power supplies with minimal external components. The Fairchild Power Switch (FPS) consists of a high-voltage power sensor and a current-mode PWM controller. This PWM controller includes an integrated fixed oscillator, undervoltage lockout, leading edge blanking block, optimized gate switch drivers, thermal shutdown protection, overcurrent temperature compensation, precision current source protection loop compensation, and fault protection circuitry. With discrete MOSFET and PWM controller or Ring Choke Converter (RCC) solutions, Fairchild Power Switches (FPS) can reduce overall cost, parts count, size and weight and improve efficiency, productivity and system reliability. It has simple applications and is ideal for low-cost designs for flyback converters or forward converters.

Absolute Maximum Ratings

Note:

1.Tj=25°C to 150°C

2. Repeat rating: pulse width limited by maximum junction temperature

3.L=24mH, VDD=50V, RG=25Ω, start-up Tj=25°C

4.L=13μH, start Tj=25°C

General application

Generally speaking, FPS consists of several functional parts: under-voltage lockout circuit (UVLO), reference voltage, oscillator (OSC), pulse width modulation (PWM) module, protection circuit and gate drive circuit. The minimum current required to initiate FPS startup is 80 microamps. This current can be supplied by the DC link block capacitor (DC start) or directly from the AC line (AC start). The DC start assumes a wide input voltage range (85-265V) and uses the minimum input to calculate the maximum value of RSTED. The voltage is as follows

The calculated maximum input voltage for Rstart maximum power dissipation is as follows:

AC Start When the starting current is supplied directly from the AC line through a single rectifier diode, the maximum value of RSTART is calculated with the minimum input voltage as follows:

The calculated maximum input voltage for Rstart maximum power dissipation is as follows:

The current supplied through the start resistor is the Vcc capacitor. When Vcc is higher than the threshold voltage, the FPS starts the built-in switching operation of the power MOSFET. Then, the current required by the control IC is suddenly increased to 7mA, which makes the FPS run at the current provided by Rstart. Therefore, after the FPS starts, the auxiliary winding transformer should provide every second. Since the startup time can be delayed if it is too large. This operation is shown in the figure

2. Although in normal operation, it should be set to the voltage protection during overload (OVP) inactive condition. At full load, about 18~20V is suitable for VCC, and at no load, about 13~14V is suitable. The protection FPS not only has a pulse-by-pulse current limiting circuit, but also some self-protection circuits. These protection circuits are fully integrated and require no external components. After the protection circuit is activated, the FPS completely stops the SMPS (latch mode protection), until the power-on reset circuit activates the input power by removing and restoring, or automatically restarts the switching power supply (automatic restart mode protection).

These two operations are user-selected operations, so the user can select the appropriate device mode according to the shutdown situation. The principle of operation and application protection are described below

The pulse-by-pulse current limit graph shows the FPS. Since the FPS adopts peak current mode control, the current through the power MOSFET is limited by the inverting input voltage of the PWM comparator (Vfb*). Assuming that the 0.9mA current source only flows through the internal resistor (2.5R+R = 2.8k) and the diode forward voltage drop is 0.7V, the anode voltage of diode D2 is about 3.2V, because when the feedback voltage (Vfb) D1 When blocked beyond 3.2V, the maximum voltage at the anode of D2 is therefore, the maximum value of VFB* is about 0.7V, which determines the maximum current through the power MOSFET. Overload Protection Overload means that the load current exceeds a pre-set level because of abnormal conditions. In this case, the protection should activate the circuit to protect the switching power supply. However, even when the SMPS is in normal operation, the overload protection circuit can switch on load. To avoid this unnecessary operation, overload situations should be distinguished from normal load transition situations. As a solution to this problem, the overload protection circuit in the FPS is designed to activate after a specified time to determine if it is a transient condition or an overloaded condition. The protection circuit will allow switching off the switching power supply load condition to last longer than a preset time only when exceeded. The detailed working principle of this is shown in the figure. Because in a pulse current limiting circuit, the maximum current through the FPS is limited, hence the maximum value. 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 diode, which reduces the optocoupler transistor current and increases Vfb.

If Vfb exceeds 3.2V, D1 is blocked, and Cfb begins to charge the CFB slowly with the current source at 5µA compared to 0.9mA. The ventricular fibrillation continued to increase until it reached 7.5V, at which point the FPS turned off. Delay shutdown time is Cfb from 3.2V to 7.5V, 5μA. When Cfb is 10nF(103), t2 is about 86Ms, and when CFB is 0.1μF(104), T2 is about 86Ms. These values are sufficient to prevent. SMPS will not shut down under most transient conditions. Increasing the CFB for longer delay times can cause problems because CFB is an important parameter in determining the response speed of a switching power supply. To solve this problem, an auxiliary capacitor can be placed in series with the Zener diode. Used in parallel with a circulating fluidized bed. The breakdown voltage of the Zener diode should be between 3.9~4.7V. When Vfb is lower than the Zener voltage, the system dynamics are determined by the CFB. When Vfb exceeds the Zener voltage, the delay time is determined by the auxiliary capacitor. By using a large auxiliary capacitor, the delay time can be extended without sacrificing dynamic response. The Over Voltage Protection (OVP) circuit FPS has a self-protection function against faults such as open circuit or short circuit in the feedback circuit. When the feedback terminal is open due to secondary side feedback circuit failure or solder defect, the current through the optocoupler transistor becomes almost zero.

Then, the vibrating fluidized bed continues to increase to a preset maximum value. Current flows through the primary side until the overload protection circuit is activated. Because the maximum current is delivered to the secondary side, the secondary side voltage becomes much higher than the rated voltage. If there is no overvoltage protection circuit the secondary side will be damaged. To prevent this, the FPS has an overvoltage protection circuit (to prevent abnormal feedback circuit). In general, Vcc is proportional to the output voltage, and the FPS uses Vcc instead of directly monitoring the output voltage to detect overvoltage conditions. If VCC exceeds 24 V, the FPS activates the OVP circuit. Therefore, VCC should be properly designed to be below 24 volts during normal operation to avoid accidental activation of OVP.

Even though the FPS has OLP (overload protection) and pulse-by-pulse current limiting characteristics, these are not enough to protect the FPS from short-circuiting when the secondary diode is shorted or loaded. Therefore, the FPS has an internal OCP (Over Current Protection) circuit as shown in Figure 4. When the gate power MOSFET, OCP's power-on signal block is enabled and monitors 1us current through the sense resistor. The voltage across the resistor is compared to a preset OCP level. If the sense resistor voltage is higher than the OCP level for more than 200ns within the allowable comparison time of 1us, a reset signal is applied to the latch, causing the SMPS to shut down. Here, after the 200ns delay, the additional delay of 100ns is the time required for the protection circuit to operate. When the soft-start operation is initiated, the voltage-inverting input of the PWM comparator saturates to its maximum value. In this case the power MOSFET current is at its maximum and the maximum allowable power is delivered to the secondary side. until the output voltage is established. It should be noted that the entire circuit is critical when the SMPS provides maximum power to the secondary side during startup. emphasize. By using the soft-start function, this stress can be relieved. The figure shows how the soft-start circuit is implemented. At startup, the soft-start capacitor Cs pin 5 begins to charge through the internal resistor (Rss), forcing the comparator to invert the input voltage to slowly increase and also to slowly increase the duty cycle. When the voltage of CS reaches about 3.2V, the PNP transistor is turned off, and Cs continues to charge to 5V through Rss. The voltage-inverting input of the comparator then follows the feedback voltage at pin 4, not the voltage at CS. When the SMPS is turned off to protect the circuit, the CS is internally discharged through a resistor that allows the CS to charge from 0V when the switching power supply is operating.

3. Application Notes for Using FPS - Flyback Application (70W)

Transformer Specifications

2. Winding Specifications

4. Core wire and spool

Core: EER 2834

Spool: EER2834