The UCC25640x is a fully featured LLC resonant controller for AC/DC power supplies. The high level of integration of UCC25640x enables significant reduction of component count and solution size without compromising functionality. UCC25640x achieves very low standby power and low audible noise standby operation using an optimized burst mode. The device's novel control scheme offers excellent transient performance.
Many consumer applications, including large screen televisions, AC-DC adapters, industrial power supplies, and LED drivers, employ PFC + LLC power supplies because they offer improved efficiency and small size, compared with a PFC + flyback topology. A disadvantage of the PFC + LLC power supply system is that it has poor light load efficiency and high no-load power consumption because the LLC stage requires a minimum amount of circulating current to maintain regulation. To meet light load efficiency and standby power consumption requirements, traditionally an auxiliary flyback converter is used. The auxiliary flyback converter runs continuously to allow the main PFC + LLC power system to be shut down when the system enters low power or standby mode. UCC25640x contains a number of novel features that enable it to offer excellent light load efficiency and low no-load power. This will allow power supply designers to create systems that meet the stringent no-load power target without needing an auxiliary flyback converter.
UCC25640x uses a novel control algorithm, Hybrid Hysteretic Control (HHC), to achieve regulation. In this control algorithm, the switching frequency is defined by the resonant capacitor voltage, which carries accurate input current information. This allows the controller to monitor and correct the input current directly. Compared with traditional Direct Frequency Control (DFC), HHC makes the system close to a first order system if the frequency control portion is small. This enables excellent load and line transient response.
UCC25640x adopts an advanced burst mode to meet the stringent requirements on standby power consumption and audible noise level. At low output power levels UCC25640x automatically transitions into light-load burst mode. In burst mode, UCC25640x repetitively delivers a burst packet with a fixed number of switching pulses and a shut-off period. The shut-off time period between burst packets is terminated by the secondary regulator loop based on the FB pin current. The LLC equivalent load current level during the burst on period is a programmable value. For each burst packet, the switching frequency slowly ramps down at the first few switching periods and ramps up at the last few switching periods, to slowly ramp up and ramp down the resonant current during burst operation. This burst soft-on and soft-off can effectively help to minimize the audible noise during burst mode operation. In addition, UCC25640x operates in a low power mode during burst mode with a very low quiescent current and biased optocoupler operation with low current.
UCC25640x monitors the half-bridge switch node to determine the required dead-time for the gate signals. In this way the dead-time is automatically adjusted to provide optimum efficiency and robust operation. UCC25640x includes a slew rate detector with improved sensitivity of the switch node voltage for adaptive dead-time that makes its operation inherently robust compared with alternative parts.
UCC25640x includes high and low-side drivers that can directly drive N-channel MOSFETs in an LLC power stage. This allows complete and fully featured power systems to be realized with minimum component count.
UCC256402 and UCC256404 includes a high-voltage startup JFET to initially charge the VCC capacitor to provide the energy needed to start the PFC and LLC power system. Once running, power for the PFC and LLC controllers is derived from a bias winding on the LLC transformer. UCC256404 also includes the active X-capacitor discharge feature to discharge the remaining voltage on the X-capacitor of the EMC filters after unplugging the AC input. UCC256403 does not include the high voltage startup and active X-capacitor discharge features. It requires an auxiliary supply to power the VCC.
UCC25640x includes robust algorithms for avoiding the zero-current switching (ZCS) operation region. When ZCS operation is detected, UCC25640x overrides the feedback signal and ramps up the switching frequency until non-capacitive operation is restored, after which the switching frequency is ramped back down at a rate determined by the soft-start capacitor until control has been handed back to the voltage control loop.
Additional protection features of UCC25640x include three-level over current protection (OCP), output over voltage protection (OVP), input voltage under-voltage protection (UVP), gate driver under-voltage lock-out (UVLO) protection, and over temperature protection (OTP).
The key features of UCC25640x can be summarized as follows: