1998-06-25, BG9802,
Technical Backgrounder from Philips Semiconductors

Consumers are increasingly aware of the environment and demanding energy efficient equipment. To address this, equipment manufacturers have to use an IC with energy saving algorithms on it: the problem is that normal CMOS ICs cannot handle mains voltages directly. Philips Semiconductors' solution is to use a process that combines intelligence from digital circuitry with the ability to handle high voltages up to 750V. Called BCD PowerLogic™, it gets its name from the combination of Bipolar, CMOS and DMOS - all on the same chip. This process is the key to the new GreenChip™ range of Switched Power Mode Supplies that the company has just launched.

The demand for this technology has been rapidly increasing over the past couple of years at a rate of some 40% per annum, in response to the need for intelligent, power management to save energy. Uses include battery management for power plugs, shavers, mobile phones, and other mains recharging applications, as well as millions of units a year for lighting control in fluorescent lamps and for use in automotive applications.

The most important part of the process is the design of the DMOS (Double diffused MOS) parts that handle the high voltage. Philips invented the DMOS structure based on the 'Reduced Surface Field (RESURF)' principle in 1979*, and have been perfecting it ever since. It is the most critical part of the design as it must be able to handle the high voltages, which it does by having a long path (drift region) between the Drain and the Source that attenuates the high voltage down to around 20V. See figure 1. Normally with ICs, the designs can be shrunk to make them smaller but, in this case, shrinking the design would mean that this path reduces and so the voltage attenuation would be smaller. To counteract this, the diffusion profiles of the design have to be carefully optimised to ensure proper voltage attenuation while still realising minimum on-resistance and thus die area. Furthermore, in most applications, the products operate simultaneously at high voltages, high temperatures and high currents. As a result, these technologies must be able to meet stringent reliability requirements.

Once the critical DMOS design is done, the design of the CMOS and Bipolar parts is straightforward as these generally use all the same mask stages and can be optimised for maximum performance and minimum area.

Variety of structures
The BCD PowerLogic process enables ten different basic structures to be created, each with variants for handling different voltages:

• NPN transistor
• Lateral PNP transistor
• Buried Zener diode
• Low voltage NMOS/PMOS
• Low voltage lateral DMOST
• Medium voltage NMOS (DD-NMOS and ED-NMOS)
• High voltage LDMOST
• High voltage J-FET
• High voltage N-Well

Three main processes
The majority of products are made using the BCD750, the BCD650, or the BCD70 process. The BCD750 process allows high side operation of DMOS devices for example in TV RGB amplifiers and can handle up to 750V. The BCD650 is used for voltage from 650V down to 70V, after which the BCD70 is used, because it has been optimised for voltages below 70V. All three can handle currents of a few Amps. Should higher current capabilities be required, the BCD PowerLogic ICs are combined into a Multi-Chip Module (MCM) with a high power MOSFET created using Philips Semiconductors' TrenchMOS™ technology.

Fine-tuning
Critical parameters in the analog bipolar part of an ICs design can be fine-tuned during the final stages of production by use of Zener diodes. These provide a One Time Programmable (OTP) functionality in that they can be blown from a high resistance to low resistance ('Anti-Fuse') as required to adjust the performance of the circuit, such as trimming the offset voltage or oscillator frequency. This provides a neat solution that could only otherwise be achieved by using a much larger die area.

Philips Semiconductors, a division of Philips Electronics NV, headquartered in Eindhoven, The Netherlands, is the ninth largest semiconductor supplier in the world and the third largest supplier of discretes 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 systems labs.