Power supply design: centralized control makes power supply design simpler

When I designed my first AC/DC power supply for an LCD TV several years ago, I added a lot of extra protection circuitry to ensure the power supply met regulations for safety and energy conservation standards, etc. Figure 1 shows a simplified block diagram of an LCD TV power supply from those years ago.

I applied a bleeder resistor to ensure that the x-capacitors in the EMI filter are discharged to a voltage level that is safe for humans within a certain time and meets the EN60950 safety standard. In standby mode, I applied an additional auxiliary power supply to meet Energy Star requirements. The power supply also requires external input undervoltage protection (UVP) and DC/DC on/off hysteresis circuits to ensure survival during AC on/off cycles and other critical tests.

Figure 1: Simplified LCD TV power supply block diagram from 10 years ago

With so many circuits on the power supply, it is difficult to reduce the overall bill of materials cost, and printed circuit board routing becomes more difficult. Due to advances in power electronics and semiconductor technology, we are now able to integrate many of the external circuits that I added 10 years ago into a single IC. For example, the UCC29950 is a centralized controller capable of driving a continuous conduction mode PFC circuit and LLC resonant converter with integrated input UVP, x-capacitor discharge, and DC/DC on/off hysteresis. In addition, the centralized control scheme allows the IC to collect information from the PFC and LLC stages and enter or not enter standby mode.

The most effective way to increase power density is to increase the switching frequency. The volume of magnetic components will be greatly reduced at high frequencies, but the increase in frequency will increase the switching loss of the switch tube, affecting the efficiency of the converter. High-frequency operation will greatly reduce the size of passive components, such as transformers and inductors. However, the resulting switching loss has an adverse effect on high-frequency operation, seriously restricting the continuous increase in switching frequency. In order to reduce switching loss and rectification loss and improve the working efficiency of switching power converters, resonant soft switching technology is proposed. The LLC resonant converter has a simple circuit structure, can achieve zero voltage (ZVS) conduction of the primary main switch tube and zero current (ZCS) shutdown of the secondary rectifier tube, and the design is relatively simple. At the same time, the current waveform is sinusoidal, the switching loss and noise can be greatly reduced, and the interference of electromagnetic radiation is effectively reduced.

    The UCC29950 provides all control functions for an AC-DC converter with a CCM boost power factor correction (PFC) stage and an LLC converter stage. The controller is optimized for ease of use.
The proprietary CCM PFC algorithm enables the system to achieve high efficiency, smaller converter size, and high power factor. The integrated LLC controller enables a high-efficiency DC-DC conversion stage with soft switching for low EMI noise. The integration of PFC control and LLC control in the combined controller allows the control algorithm to utilize information from both stages.
The controller includes control circuitry for startup using a depletion mode MOSFET with internal device power management that minimizes external component requirements and helps reduce system implementation costs.
To further reduce standby power, an X-Cap discharge circuit is integrated. The UCC29950 implements a full set of system protection features, including AC line loss, PFC bus undervoltage PFC and LLC, overcurrent, and thermal shutdown.

characteristic:

    ●High efficiency PFC half-bridge resonant LLC combination controller
●Continuous conduction mode (CCM) boost power factor correction
●Supports self-biased or auxiliary (external) biased operating modes
●PFC loop fully internally compensated
●PFC stage design divided into 3 simple steps (design voltage feedback, current feedback and power stage)
●Fixed 100 kHz PFC frequency with jitter for easy EMI compliance
●True input power limit, independent of line voltage
●Fixed LLC frequency operating range from 70 kHz to 350 kHz
●Dead time of LLC half-bridge power stage varies over load range to extend ZVS range
●Three-level LLC overcurrent protection
●Hiccup mode operation for continuous overload and short-circuit power protection●
Low standby power consumption by actively controlling the high-voltage start-up MOSFET and X-Cap discharge function
●Built-in soft-start and inverter sequencing for simplified design
●AC line blackout protection with fault indicator
●PFC bus overvoltage and undervoltage protection
●Overtemperature protection
●External gate driver, scalable with power level
●SOIC-16 package

When the UCC29950 is applied, the power supply can have low power consumption at no load even without an additional auxiliary power supply. Therefore, the power supply block diagram will be much simpler (see Figure 2), but still provide the same functionality as shown in Figure 1.

Figure 2: Simplified UCC29950 “no standby” power supply block diagram

A power supply with centralized control can greatly reduce the number of components and circuit cost. Because the controller integrates many protection functions, reliability is improved and the chance of catastrophic failure of the power supply is reduced.