feature

Variable frequency control half-bridge resonant converter topology with 50% duty cycle High efficiency through zero voltage switching (ZVS) Internal super FET diode with fast recovery type body (trr=120ns) Fixed dead time (350ns) optimized for MOSFETs Up to 300kHz operating frequency Frequency hopping frequency limit (programmable) Remote on/off control with control pin under light load conditions Over Current Protection (AOCP), Internal Thermal Shutdown (TSD)

application

Plasma TV and LCD TV Style PC and Server Adapter Telecom Power Audio Power

illustrate

The fsfr series is a family of highly integrated power switches designed for high-efficiency half-bridge resonant converters. Providing everything necessary to build a reliable and robust resonant converter, the fsfr series simplifies design and increases productivity while enhancing performance. The FSFR series features a fast recovery type body diode power mosfet, a high-side gate drive circuit, a precision current controllable oscillator, a frequency limit circuit, soft-start and built-in protection functions. The high-voltage side drive circuit has the ability to eliminate common mode noise to ensure stable operation and is immune to large noise. The mosfet's fast recovery body diode improves reliability conditions for abnormal operation while minimizing the effects of reverse recovery. The use of zero voltage switching (ZVS) technology greatly reduces switching losses with significantly higher efficiency. ZVS also significantly reduces switching noise, enabling small EMI filters. The fsfr series can be applied to various resonant converter topologies such as series resonant, parallel resonant converters and llc resonant converters.

Function description

1. Basic Operation: The FSFR series is designed to drive high-side and low-side mosfets with a 50% duty cycle. A fixed dead time of 350ns is introduced between successive conversions, as shown.

2. Internal Oscillator: The FSFR series uses a current controlled oscillator as shown. Internally, the voltage at the RT pin is regulated to 2V, and the charge/discharge current for the oscillator capacitor, CT, is obtained by replicating the current flowing from the RT pin using the current mirror (ictc). Therefore, the switching frequency increases with ictc.

3. Frequency setting: The figure shows the voltage gain curve of a typical resonant converter, where the gain is inversely proportional to the switching frequency. The output voltage can be adjusted by adjusting the switching frequency. The figure shows the configuration of the RT pin of a typical circuit, where an optocoupler transistor is connected to the RT pin to adjust the switching frequency.

The minimum switching frequency is determined as: Min Min 5.2 100() kf kHz RΩ=×(1) Assuming that the optocoupler saturation voltage transistor is 0.2V, the maximum switching frequency is determined as: maximum value, minimum maximum value 5.2 4.68() 100() kkf kHz ΩΩ=+× (2) Prevent excessive inrush current and overshoot when starting the output voltage, and increase the voltage gain of the resonant converter. Since the voltage gain of the resonant converter is inversely proportional to the switching frequency, soft-start is achieved by reducing the switching frequency to build up the voltage from the initial high frequency (f iss ) to the output. The soft-start circuit consists of connecting the RC series network on the RT pin. The fsfr series also has an internal soft-start of 3ms to reduce the current overshoot period during the initial period. The external soft-start circuit is connected. Therefore, the initial frequency of the soft-start is as follows:

5.1 Over-Current Protection (OCP): When the sensingPIN voltage drops to -0.6V, OCP is a trigger and Mosfets remain off. This protection has a shutdown. Time delay 1.5 microseconds to prevent premature breakage at the beginning 5.2 Current Over Current Protection (AOCP): If the secondary rectifier diode is short, a high current extremely high DT/DT can pass the maximum flow before the OCP or OLP is a trigger. trigger. When inducing loose voltage, keep delaying below -0.9v. This protection is Latch mode and reset when LVCC is below 5V. 5.3 Overload Protection (OLP): Overload is defined as due to unexpected abnormal events. In this case, a protection circuit should be triggered to protect the power supply. However, even if the power supply is under normal conditions, overload conditions can occur during load transitions. To avoid triggering the protection prematurely, the overload protection circuit should only trigger after a specified time to determine whether it is a temporary or real overload condition. The diagram shows a typical overload protection circuit. Overload protection can be implemented by sensing the voltage on the control pin of the resonant capacitor. Using the RC time constant, the shutdown delay can also be introduced. The control pins are as follows: 5.4 Over Voltage Protection (ovp): When the lvcc reaches 23V, the ovp is triggered. Use this protection to use up to fps when the transformer auxiliary winding is supplying VCC. 5.5 Thermal Shutdown (TSD): The mosfet and the control integrated circuit are integrated in one package, which is convenient for the control IC to detect the mosfet. Thermal shutdown is triggered if the temperature exceeds approximately 130°C.