feature

Current-mode PWM control with fixed operation

Frequency (300kHz)

Pulse-by-Pulse Current Limit

Overload protection

Over voltage protection

Thermal shutdown

Built-in auto-restart circuit

Line Brownout Detection and Sleep On/Off Function

Internal High Voltage Sensor (QFET)

Supports forward or reverse topology

application

DC-DC converter

illustrate

The FS6X1220R is designed for offline DCDC converters with minimal external components. This device is a current mode PWM controller with a high voltage power sensor in a package. The PWM controller includes an integrated fixed frequency oscillator, line undervoltage lockout, sleep on/off function, thermal shutdown protection, overvoltage protection, pulse-by-pulse current limiting, and temperature-compensated sources of precision current loop compensation. Compared to discrete MOSFET and PWM controller solutions, the FS6X1220R can reduce overall cost, parts count, size and weight while increasing efficiency, productivity and system reliability. The device is ideal for applications with DC-DC converters up to 40W output power

Pin Description

Absolute Maximum Ratings

(Ta=25°C unless otherwise specified)

notes:

1.Tj=25°C to 150 °C

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

3.L=4.7 mH, start-up Tj=25°C

Function description

1. Startup: In order to ensure the stable operation of the control chip, Vcc has under-voltage lockout (UVLO) and 6V hysteresis. The graph shows the relationship between supply current (Icc) and supply voltage (Vcc). The startup current is 60 microamps until Vcc reaches 15V, usually by the DC link through the startup resistor. When Vcc reaches 15V, the control IC starts to work and the operating current increases to 10mA, as shown in the figure. Once the control IC starts operation unless Vcc is below the stop voltage of 9V.

2. Feedback control: FS6X1220R adopts current mode control. The voltage at the comparison feedback pin is pulse-width modulated (PWM) with the current-sense voltage. Figure 2 illustrates a simplified PWM block. This feedback voltage determines the sensory network. The feedback network is usually implemented using optocouplers and TL431. The collector of the optocoupler transistor is connected to the feedback pin and the emitter is connected to the ground pin. When the reference pin of TL431 exceeds the internal reference voltage of 2.5V, the current of the optocoupler diode increases, pulling down the feedback voltage.

3. Protection circuit: In addition to pulse-by-pulse current limiting, FS6X1220R has 3 self-protection functions; overload protection (OLP), overvoltage protection (OVP) and thermal shutdown (TSD). Because these protection circuits are integrated into the integrated circuit, reliability can be improved without the need for external components. If these fault conditions occur, the FS6X1220R enters an automatic restart operation. In the event of a fault, the switching terminal and MOSFET remain off, which results in a reduction in Vcc. When Vcc reaches 9V, the protection reset supply current is reduced to 60uA. Then, Vcc starts to increase the resistor by starting the current supplied. When Vcc reaches 15V, the FS6X1220R resumes its normal operation after troubleshooting. In this manner, auto-restart alternately enables and disables switching power MOSFETs until the fault condition is eliminated as shown.

3.1 Pulse-by-pulse current limit: As shown in the figure, the drain current of the power MOSFET is controlled by the pulse width modulation comparator (Vfb*) input by the inverter. Assuming the 0.9mA current source is only through the internal resistor (28R+R=2.9k), the cathode voltage of diode D2 is about 2.6V, because when the feedback voltage (VFB) exceeds 2.6V, the maximum voltage D2 of the cathode is movable, Therefore, the maximum value is 0.1V, limiting the peak leakage current of the power MOSFET.

3.2 Overload Protection (OLP): Overload means that the load current exceeds the preset value due to abnormality. 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 whether it is a transient or overload condition. Because of the pulse current limiting capability, the maximum peak current through the SMPS is limited and therefore the maximum input power is limited. at a given input voltage. If the output consumes more than this maximum power, the output voltage (VO) decreases. lower than the set voltage. This reduces the increase in Vfb through the optocoupler diode and also reduces the optocoupler transistor current. If Vfb exceeds 2.6V, D1 is blocked and the 5µA current source starts to charge the CFB compared to when the 0.9mA current source charges the CFB. In this case, Vfb continues to increase until it reaches 7.5V, and the switching operation is terminated as shown. Shutdown Delay Time The time it takes for Cfb to charge from 2.6V to 7.5V (5) a. When CFB is 10NF (103), T12 is about 98MS. When CFB is 0.1 μF (104), T12 is about 98Ms. These values are sufficient to prevent shutting down SMPS under brief circumstances.

3.3 Over Voltage Protection (OVP): In the secondary side feedback circuit at fault, or the feedback loop is disconnected due to solder defects, the current through the optocoupler transistor is almost zero. The ventricular fibrillation then rises in a manner similar to an overload situation, forcing the secondary side to preset a maximum current until overload protection is activated. Because the energy supplied to the output exceeds the required overload voltage before the overload may exceed the rated voltage protection is activated, causing the secondary side of the device. To prevent this situation, over-voltage protection (OVP) circuits are employed. In general, Vcc is proportional to the output voltage and the FS6X1220R uses Vcc instead of directly monitoring the output voltage. If VCC exceeds 25V, the OVP circuit is activated causing switching to terminate. In order to avoid accidental activation of OVP operation under normal conditions, Vcc should be properly designed to be lower than 25V.

3.4 Thermal Shutdown (TSD): The thermal shutdown circuit induces junction temperature. The threshold has been set at 160°C. Threshold (160°C) power MOSFETs are disabled when the junction temperature is above this temperature.

4. Line UVLO and sleep mode Three operations define the mode according to the probe voltage; normal operation mode voltage lock mode and sleep mode are shown in Figure 5. When this pin voltage exceeds 2.55V, the FS6X1220R operates in normal mode. When the voltage on this pin is less than 2.55V, the line undervoltage lockout mode is entered to terminate the switching operation. When this pin is less than 1.8V, enter sleep mode. In sleep mode, the reference voltage generation circuit including the shunt regulator is disabled and only 300 microamps of operating current is required.

Typical Application Circuit

1. DC-DC converter application circuit (flyback)

2. Transformer schematic

3. Winding Specifications

4. Electrical Characteristics

5. Core wire and spool

Color: EPC 19

Spool: EPC 19

Acoustic emission (mm2) (22.7)

6. Demo Circuit Parts List