Primary side flyback controller
Fairchild's FL7732, a highly integrated pulse width modulation (PWM) controller, has several features that enhance the performance of low power flyback converters. The FL7732's patented topology enables the most simplified circuit design, especially for LED lighting applications. By using a single-stage topology with primary-side regulation, LED panels can be implemented with minimal external components for minimal cost; no bulk input capacitors and feedback circuitry are required. To implement high power factor and low total harmonic distortion (THD), an external capacitor is used for constant on-time control. Figure 1 shows a typical application circuit for the FL7732 controller.
Figure 1: Typical Application Circuit for the FL7732 Controller
Constant current regulation is also an important feature of LED illumination. The precision constant current control in the FL7732 precisely regulates the output current compared to changes in the input and output voltages. The output current can be evaluated using the peak leakage current of the MOSFET and the discharge time of the inductor current because the output current is equivalent to the diode current under steady-state conditions. The output current estimator uses the inductor discharge time and switching period to obtain the current peak through the peak detection circuit and calculate the output current. This output current information is compared to the internal precision reference information to derive the error voltage, which determines the duty cycle of the MOSFET in constant current mode. Using Fairchild's innovative TRUECURRENT® topology, the constant output current is precisely controlled to:
In general, the primary side regulation prefers the DCM mode of operation because it allows for better output regulation. The operating frequency is changed according to the output voltage to ensure that it operates in DCM mode, making it more efficient and simpler to design. In order to maintain the DCM over a wide range of output voltages, the frequency in the linear frequency control should vary linearly depending on the output voltage. The output voltage is sensed by the auxiliary winding and a voltage divider resistor connected to the VS pin. When the output voltage drops, the secondary diode conduction time increases, and the linear frequency control feature makes the switching period longer, thus ensuring that the converter remains in the DCM mode of operation over a wide output voltage range. Under full load conditions, the frequency control also reduces the primary current rms for better power efficiency.
The FL7732 also provides protection features such as open LED, shorted LED and over temperature protection. An important feature when the LED is shorted is that the current limit level is automatically reduced, minimizing the output current and protecting the external components. The FL7732 also features frequency hopping in the oscillator for better electromagnetic interference (EMI) performance.
Latest Super Junction MOSFET
In high-voltage MOSFET technology, charge balancing technology is the most significant way to achieve a reduction in on-resistance. This technology is derived from a super-junction structure that has a deep P-type columnar structure compared to the conventional structure of conventional planar process technology. The columnar structure effectively limits the electric field in the lightly doped epi region. Thanks to this P-type columnar structure, the N-type epi resistor can be significantly reduced while maintaining the same level of breakdown voltage. In addition to low on-resistance, the second-generation SuperFET® technology enables less stored energy in the output capacitor. In low power applications, such as LED lighting, this energy value is more important because energy is consumed each time it is turned on.
SuperFET® II technology was evaluated with a 20W rated power LED lighting panel. The panel was originally developed using Fairchild's 60V N-Channel MOSFET, FDD5N60NZ and FL7732 devices. Among the main components, the FDD5N60NZ is a planar process technology MOSFET with an on-resistance of 2OΩ. At the same cost, SuperFET II technology provides 0.9OΩ on-resistance and lower stored energy in the output capacitor. With these superior electrical characteristics, SuperFET II technology delivers excellent boost system efficiency. Figure 2 shows the results of an efficiency test using various AC inputs.
Figure 2: MOSFET system efficiency
SuperFET II technology delivers optimum efficiency over the entire input range and is a significant improvement over FDD5N60NZ's planar process technology. In addition, SuperFET II technology is more efficient than competitive super-junction MOSFETs, especially at high input voltages. This is a good example of how stored energy in the output capacitor affects system efficiency. Since the competitive super-junction MOSFET has the same on-resistance as the SuperFET II MOSFET, the previous efficiency difference between the two is considered to be from the switching loss. As shown in Figure 3, competitive super-junction MOSFETs store more energy in the output capacitor due to increased drain supply voltage. This means that at higher input voltages, the MOSFET will consume more energy during turn-on. In Figure 2, the device level characteristics are consistent with the evaluation board test results.
Figure 3 Storage energy in the output capacitor
in conclusion
LED lighting power supplies require high power factor, high efficiency, isolated secondary ends to meet safety standards, and require fewer components due to space constraints. The FL7732 and SuperFET II MOSFETs provide a complete solution to these requirements.
Light reflectors in China with resonable price and good quality,We hope to establish cooperative relationship with you.
Light Reflector,Aluminum Light Reflector,Street Light Reflector,Energy Save Light Reflector
Yangzhou Huadong Can Illuminations Mould Manufactory Co., Ltd. , https://www.light-reflectors.com