SLUS846C September 2008 – June 2015 UCC25600
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The UCC25600 device is a high performance, resonant-mode controller designed for DC/DC applications using resonant topologies, especially the LLC half-bridge resonant converter.
The soft-switching capability, high efficiency, and long holdup time make the LLC resonant converter attractive for many applications, such as digital TV, ac-to-dc adapters, and computer power supplies. Figure 14 shows the schematic of the LLC resonant converter.
The LLC resonant converter is based on the series resonant converter (SRC). By using the transformer magnetizing inductor, zero-voltage switching can be achieved over a wide range of input voltage and load. As a result of multiple resonances, zero-voltage switching can be maintained even when the switching frequency is higher or lower than resonant frequency. This simplifies the converter design to avoid the zero-current switching region which can lead to system damage. The converter achieves the best efficiency when operated close to its resonant frequency at a nominal input voltage. As the switching frequency is lowered, the voltage gain is significantly increased. This allows the converter to maintain regulation when the input voltage falls low. These features make the converter ideally suited to operate from the output of a high-voltage, boost PFC pre-regulator, allowing it to hold up through brief periods of ac line-voltage dropout.
Due to the nature of resonant converter, all the voltages and currents on the resonant components are approximately sinusoidal. The gain characteristic of the LLC resonant converter is analyzed based on the first harmonic approximation (FHA), which means all the voltages and currents are treated as a sinusoidal shape with the frequency the same as the switching frequency.
According to the operation principle of the converter, the LLC resonant converter can be drawn as the equivalent circuit shown in Figure 15.
In this equivalent circuit, the Vge and Voe are the fundamental harmonics of the voltage generated by the half-bridge and the voltage on the transformer primary side, respectively. These voltages can be calculated through Fourier analysis. The load resistor Re is the equivalent resistor of the load, and it can be calculated as:
Based on this equivalent circuit, the converter gain at different switching frequencies can be calculated as:
In this equation VDC/2 is the equivalent input voltage due to the half-bridge structure.
|NORMALIZED GAIN||RESONANT FREQUENCY||QUALITY FACTOR||NORMALIZED FREQUENCY||INDUCTOR RATIO|
Following the definitions in Table 3, the converter gain at different switching frequencies can be written as:
Because of the FHA, this gain equation is an approximation. When the switching frequency moves away from the resonant frequency, the error becomes larger. However, this equation can be used as a design tool. The final results need to be verified by the time-based simulation or hardware test.
From Equation 16, when the switching frequency is equal to the resonant frequency, fn = 1 and converter voltage gain is equal to 1. Converter gain at different loads and inductor ratio conditions are shown in Figure 16 through Figure 19.
Based on its theory of operation, the LLC resonant converter is controlled through pulse frequency modulation (PFM). The output voltage is regulated by adjusting the switching frequency according to the input and output conditions. Optimal efficiency is achieved at the nominal input voltage by setting the switching frequency close to the resonant frequency. When the input voltage drops low, the switching frequency is decreased to boost the gain and maintain regulation.
The UCC25600 resonant half-bridge controller uses variable switching frequency control to adjust the resonant tank impedance and regulate output voltage. This 8-pin package device integrates the critical functions for optimizing the system performance while greatly simplifying the design and layout.
Resonant half-bridge converter relies on the resonant tank current at MOSFETs turn-off to achieve soft switching and reduce switching loss. Higher turn-off current provides more energy to discharge the junction capacitor, while it generates more turn-off loss. Smaller turn-off current reduces turn-off loss, but it requires longer time to discharge MOSFETs junction capacitors and achieve soft switching. By choosing an appropriate dead time, turn-off current is minimized while still maintaining zero-voltage switching, and best system performance is realized.
In UCC25600, dead time can be adjusted through a single resistor from the DT pin to ground. With internal 2.25-V voltage reference, the current flow through the resistor sets the dead time.
To prevent shoot through when the DT pin accidentally connects to ground, a minimum 120-ns dead time is inserted into the 2 gate driver outputs. Any dead-time setting less than 120 ns will be limited to 120 ns.
With variable switching frequency control, UCC25600 relies on the internal oscillator to vary the switching frequency. The oscillator is controlled by the current flowing out of the RT pin. Except during soft start, the relationship between the gate signal frequency and the current flowing out of the RT pin can be represented as:
Because the switching frequency is proportional to the current, by limiting the maximum and minimum current flowing out of the RT pin, the minimum and maximum switching frequency of the converter could be easily limited. As shown in Figure 20, putting a resistor from the RT pin to ground limits the minimum current and putting a resistor in series with the opto-coupler limits the maximum current.
The frequency limiting resistor can be calculated based on following equations.
This design example describes the HPA341 EVM design and outlines the design steps required to design a 300-W LLC resonant half-bridge converter, which provides a regulated output voltage nominally at 12 V at maximum 300 W of load power, with reinforced isolation of AC-DC off-line applications between the primary and the secondary, operating from a DC source of 390 V.
|DESIGN PARAMETER||TARGET VALUE|
|Output voltage||12 V|
|Rated output power||300 W|
|Input DC voltage range||375 V to 405 V|
|Typical efficiency at full load||91%|
|Switching frequency||85 kHz to 350 kHz|
|Resonant frequency||130 kHz|