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Controller ICs for low-cost energy-efficient PSUs

Cambridge Semiconductor— CamSemi— designed its C2470 family of ac/dc power-supply controller ICs to provide very efficient and very low-cost off-line power supplies for consumer and other products in the 6 to 40W power range. With these devices, you can build a power supply that has switching-PSU (power-supply- unit) efficiency of over 80%, for no more than the cost of cheap linear supply.

CamSemi is a startup fabless chipmaker working to develop a novel power-transistor structure and other IP targeted at providing effi cient off-line power conversion. The power-device technology is called PowerBrane: it uses the concept of building power devices on much-thinned silicon wafer—a membrane of silicon, hence the name. One of the limitations of conventional devices is that breakdown occurs where electric fi eld lines concentrate as they pass into the silicon substrate underlying the active, diffused layers that form the transistor. Take away the substrate and you remove the problem, is CamSemi’s approach.

PowerBrane remains workin- progress: meantime, the company has “broken out” the controller part of its technology platform and is now offering it as a stand-alone series of ICs. CamSemi’s CEO David Baillie acknowledges that the company is making this move to get a revenue stream going while preparing the innovative power switches for market. The company plans to eventually integrate the controller technology on the same silicon as PowerBrane transistors. For the present, you use a C2470 controller to drive a discrete bipolar transistor.

The chip is a mixed-signal device that employs an architecture that CamSemi calls RDFC (resonant discontinuous forward converter). It operates at a switching frequency of 50 to 100 kHz, permitting the use of small ferritecore transformers, which— compared to the iron-core transformer present in conventional linear supplies—is one of the factors behind the company’s claims for the low bill-of-materials cost of a complete supply. As a forward- converter design, it has no feedback from secondary to primary, which further cuts component count and simplifi es isolation requirements. This means that regulation is limited in accuracy: 10 to 12%, the company claims. This is good enough for the ac/dc requirements of many products that run from simple off-line supplies, Baillie says. The same is true, he adds, of the fact that the architecture is suitable for operation from a single ac input voltage—the transformer turns-ratio fi xes the output voltage for a given input. There are many low to medium power applications that are “static” and don’t need universal input, Baillie explains. The C2470 chips adjust their switching frequency to the resonant frequency of the transformer winding/capacitor pairing, ensuring effi ciency: you do not need an output choke, “Y” capacitor, or freewheel diodes. Further, the circuit exploits the transformer’s leakage inductance and other parasitic components to reduce the discretecomponent count, and you can use a low-cost bipolar lighting transistor as the main switch. The mixed-signal chip incorporates a base-drive circuit to turn the bipolar on and off correctly. The chip automatically switches operating modes to maintain effi ciency over its load range—you can achieve standby dissipation of 100 mW for the supply, CamSemi states. One of the modes is start-up: once stable switching establishes, a separate transformer winding supplies power to the controller, but, prior to that, a resistor- divider chain provides power. The chip has protection modes and will automatically restart after a short-circuit condition ends. Switching is zero-voltage and “soft”, implying low switching losses and low conducted EMI. CamSemi will provide design support via Web-based design guides and hardwaredesign kits. An SO23-6 part, the C2472PX2, costs $0.45 (1000). Please click here for diagram

Cambridge Semiconductor, www.camsemi.com.