基于UCC25600的8 引脚高性能谐振模式 LLC 控制器

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基于UCC25600的8 引脚高性能谐振模式 LLC 控制器

　　1 Features

　　Variable Switching Frequency Control

　　Programmable Minimum Switching Frequency With 4% Accuracy (3% Accuracy at Temperature Range: –20°C to 105°C)

　　Programmable Maximum Switching Frequency

　　Programmable Dead Time for Best Efficiency

　　Programmable Soft-Start Time

　　Easy ON and OFF Control

　　Overcurrent Protection

　　Overtemperature Protection

　　Bias Voltage UVLO and Overvoltage Protection

　　Integrated Gate Driver With 0.4-A Source and 0.8-A Sink Capability

　　Operating Temperature Range: –40°C to 125°C

　　SOIC 8-Pin Package

　　2 Applications

　　100-W to 1-kW Power Supplies

　　LCD, Plasma, and DLP® TVs

　　Adaptors, Computing, and ATX Power Supplies

　　Home Audio Systems

　　Electronic Lighting Ballasts

　　3 Description

　　The UCC25600 high performance, resonant mode controller is designed for dc-to-dc applications using resonant topologies, especially the LLC half-bridge resonant converter. This highly integrated controller implements frequency modulation control and complete system functions in only an 8-pin package. Switching to the UCC25600 will greatly simplify the system design and layout, and improve time to market, all at a price point lower than competitive 16-pin device offerings.

　　The internal oscillator supports the switching frequencies from 40 kHz to 350 kHz. This high-accuracy oscillator realizes the minimum switching frequency limiting with 4% tolerance, allowing the designer to avoid over-design of the power stage and, thus, further reducing overall system cost. The programmable dead time enables zero-voltage switching with minimum magnetizing current. This maximizes system efficiency across a variety of applications. The programmable soft-start timer maximizes design flexibility demanded by the varied requirements of end equipment using a half-bridge topology. By incorporating a 0.4-A source and 0.8-A sink driving capability, a low cost, reliable gate driver transformer is a real option.

　　The UCC25600 delivers complete system protection functions including overcurrent, UVLO, bias supply OVP, and overtemperature protection.

　　Due to high power density and high efficiency requirements, LLC topology is employed in many applications. The LLC resonant converter has many unique characteristics and improvements compared with hard-switch bridge topology and phase-shift full bridge. For example, LLC has a simple structure, and could achieve primary MOSFET zero voltage switching (ZVS) and the secondary rectifier zero current switching (ZCS) from no load to full load.

　　The UCC25600 device is an LLC-resonant half-bridge controller which integrates built-in, state-of-the-art efficiency boost features with high-level protection features to provide cost-effective solutions.

　　The half-bridge resonant converter is controlled by the nearly 50% duty-cycle variable frequency square-wave voltage. This allows the half bridge to be easily driven by the gate-driver transformer. Compared with a half-bridge driver device, a gate-driver transformer provides a simple and reliable solution, which:

　　Eliminates the need for gate driver power supply

　　Enables simplified layout

　　Prevents shoot-through due to the transformer coupling

　　Requires no latch up

　　The UCC25600 integrates two-gate drivers with 0.4-A source and 0.8-A sink capability to directly drive the gate-driver transformer.

For the LLC resonant converter, it is critical for the gate-driver signal to be precisely symmetrical. Otherwise, the resonant tank operation will be symmetrical. The load current distribution will be unbalanced for the output rectifiers, which in turn require the over-design of the power stages and thermal management.

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　　In UCC25600, the gate-driver output is precisely trimmed to have less than 50-ns mismatch. Although the gate-driver signal is quite symmetrical, it is still recommended to insert the dc blocking capacitor in the gate-driver transformer primary side to prevent transformer saturation during fast transients.

　　During light load condition, the resonant converter tends to increase its switching frequency and maintain the output voltage regulation. However, due to ringing caused by the transformer parasitic capacitor and the rectification-diode-junction capacitors, the energy could be directly transferred to the load through these capacitors. When this power becomes more than the load requires, the output voltage become higher than the regulation level. In this case, further increasing the switching frequency will not help the situation because energy transfer to the load is not through the power stage itself.

　　To prevent output overvoltage during this condition, the UCC25600 includes the burst-mode operation function. When the control loop demands switching frequency higher than 350 kHz, the gate driver is disabled and the power stage stops switching. When the output voltage drops, the control loop begins to demand switching frequency less than 330 kHz, the gate driver recovers and the power stage begins to deliver power again. This allows the output voltage to be regulated.

　　This burst mode can be easily disabled by limiting the maximum switching frequency to less than 350 kHz. In this way, the control loop will never demand a switching frequency higher than 350 kHz and burst mode operation will not occur.