1996-10-15 , IC-426/590

Eindhoven - 15 Oct 1996 - A revolutionary new high-voltage BCD Powerlogic IC from Philips Semiconductors allows constant-voltage/constant-current DC supplies to be housed within universal slim-line power plugs that plug directly into 90 V to 280 V AC line supplies. Target applications for this new IC include fast-chargers for cellular and cordless telephones.

The TEA1401t Self-Oscillating Flyback Power Supply Controller derives feedback on output voltage and current from an auxiliary winding on the isolation transformer, eliminating the need for any current or voltage sensing circuitry on the secondary side of the transformer. It even includes an on-chip 650V/0.5A DMOS FET and snubber circuit to switch the rectified AC line voltage. As a result, the only other components required in an electronic power plug capable of charging 10 W to 20 W loads are an AC line supply filter and rectifier, a miniature transformer and a few passive components and diodes. Previous solutions have required the use of a large number of active and passive discrete components on both the primary and secondary side of the transformer, leading to much bulkier power-plug designs.

"The TEA1401t not only allows manufacturers to produce cheaper and smaller assemblies for electronic power plugs that can be sold worldwide," said Product Marketing Manager Bas Fransen", it will also provide end-users of portable equipment such as GSM phones with a lightweight slim-line charger they can take with them anywhere in the world."

Depending on the load impedance presented by the connected appliance, the TEA1401t generates either a constant-voltage or a constant-current output -allowing the power plug to power the appliance directly or re-charge its internal batteries. Actual output voltage and current limits are programmed by the turns ratios of the transformer and by two resistors connected to the TEA1401t.

An important feature of this new IC is its unusually steep foldback characteristic, which automatically reduces the output current to a tenth of its normal limit value whenever the output voltage falls below a specified level. Because it allows the power plug to be effectively shut down simply by shorting its output, this foldback characteristic not only provides short-circuit protection, but also allows a connected appliance to control the power plug intelligently without the need to route a dedicated control signal to the primary side of the transformer. Features such as automatic duty cycle control, initiated from within the connected appliance by a microcontroller or a fast-charge controller such as Philips' TEA1102, become very simple to implement.

Although the TEA1401t operates exceptionally well in stand-alone mode for most applications, the accuracy of its voltage and/or current regulation can be further enhanced by providing instantaneous feedback from the secondary side of the transformer via an opto-coupler. Sensing the voltage and current within the connected appliance and routing a feedback signal to the power-plug via a 3-wire cable not only achieves the highest possible regulation, it also minimises the number of components required in the power-plug and hence reduces size even further.

Self-oscillating at frequencies up to 145 kHz and incorporating an on-chip snubber circuit for its DMOS switching transistor, the TEA1401t features excellent EMC performance. Only simple filtering of the rectified AC line supply is required to meet EMC regulations worldwide. For maximum energy efficiency several steps have been taken to minimise power dissipation, including powering of the TEA1401t from the auxiliary transformer winding instead of the rectified AC line supply immediately after start-up. Other features of this unique IC include over-voltage, over-current and over-temperature protection, plus soft start-up capabilities.

Philips Semiconductors, a division of Philips Electronics NV, headquartered in Eindhoven, The Netherlands, is the eleventh largest semiconductor supplier in the world. Philips Semiconductors' innovations in digital audio, video, and mobile technology position the company as a leader in the consumer, multimedia and wireless communications markets. Sales offices are located in all major markets around the world and are supported by regional customer application labs.