In 70W - 500W AC - DC power supply, the efficiency of switching power supply using LLC resonant conversion technology is higher than that of traditional topology switching power supply. Therefore, LLC resonant conversion technology is more and more widely used in power products.
The following figure shows a typical half bridge LLC resonant converter circuit:
The equivalent voltage transmission relationship is:
FR1 is the resonant frequency of the resonator
X is the normalized value of the switching frequency relative to the resonant frequency
K is the ratio of transformer excitation inductance to resonance inductance
Zr is the characteristic impedance of the resonator
Re is the equivalent load AC impedance
Q is the quality factor
According to the voltage transmission relation formula, the voltage transmission relation curve under different variable conditions can be drawn
Referring to the above voltage transfer ratio curve of LLC resonant converter, it can be concluded that LLC converter has the following characteristics:
1. When the converter is designed to work in the blue area (ZVS zero voltage switch), the primary switch of LLC converter realizes zero voltage on within the full load range, and the switching loss of the switch is reduced to almost zero. At the same time, the rectifier diode on the secondary side can realize zero current switching, and there is almost no problem of reverse recovery, so as to reduce the loss and the impact on EMI.
2. With the front stage PFC circuit, LLC converter can reliably stabilize the output voltage by adjusting the switching frequency in a wide load range. At the same time, LLC converter can also rely on its voltage gain regulation ability to provide longer holding time without increasing the input electrolytic capacitor capacity.
three Since LLC resonant converter can realize the soft switching of switching components with low switching loss, it creates conditions for improving the switching frequency and reducing the volume of magnetic components, so as to realize the miniaturization and high power density of switching power supply on the premise of maintaining high efficiency, and maintain the low temperature rise of components under the condition of fan free heat dissipation design, Thus, the service life and reliability of the power supply are ensured.
