Resonant Controllers Aim To Build Efficient Supplies at Low Cost

As energy efficiency initiatives push for the replacement of cheap linear power supplies with more-efficient switchers, one chip maker has developed a family of controller ICs that address this challenge by exploiting digital and analog control techniques within a resonant topology. According to CamSemi, the Cambridge, England-based fabless semiconductor company, its C2470 controllers enable ac-dc supplies to achieve high efficiency, while costing the same or less than linears. Moreover, the company claims that power supplies based on its chips can achieve 7% to 10% better efficiency than competing flyback designs in the 6-W to 40-W range.

The C2470 family is based on advances in intelligent digital/analog control, coupled with a resonant single-switch topology. The mixed-signal control allows the use of a low-cost bipolar junction transistor for the switch rather than a more expensive MOSFET, which is typically used in popular switch-mode power supply (SMPS) designs.

At output power ratings of around 6 W and above, the company says supplies based on the C2470 become cheaper than linear power supplies. Moreover, the C2470 power supply designs can be significantly cheaper than existing flyback and ringing choke converter (RCC) designs.

The controllers employ both analog and digital control loops. A digital control scheme governs base drive to the bipolar transistor and prevents it from saturating. Meanwhile, an analog control loop is responsible for management of the load. The chip implements a forward converter topology, or more specifically a resonant discontinuous forward converter topology, which requires no freewheeling diode or output choke and requires no feedback circuit. No “Y” capacitor is required. One drawback of this forward converter approach is that it does not support autoranging or universal input.

However, the controllers’ resonant operation offers inherently low EMI. This characteristic reduces filtering requirements and enables the replacement of linear power supplies in noise-sensitive applications such as audio products and cordless phone chargers. In addition, zero-voltage switching faciltates high efficiency while also enabling the use of the low-cost bipolar switch.

According to the vendor, their approach allows manufacturers to secure operating efficiencies in excess of 80% and limit standby power consumption to 100 mW. In addition, the controllers provide undervoltage, overcurrent and overtemperature protection.

The first three members of the C2470 family are for the 6-W to 40-W power range and are available in volume for applications such as battery chargers, mini adapters, routers, cordless phones and audio systems. Pricing for a representative part, the SOT-23-6-packaged C2472PX2, is $0.45 each for 1000-piece quantities. Other packaging options include an SOP-8 and a PDIP-8. For more detailed product information and reference designs, or to request samples see www.camsemi.com