AN871 APPLICATION NOTE

10/13

The major part of switches used in car are N-channel MOSFETS. For the high side switches or for the

upper arms of H-bridges as well, an additional specific circuit for the driver must be implemented: auxiliary

power supply, charge pump, bootstrap circuit etc... are needed and therefore increase the circuit complex-

ity and the cost.

We propose the diagram shown in Figure 11

High side switch.

Figure 11. Sensitive SCR can be used as upper arm of H-bridge. The driver, is this case, is

dramatically simplified.

The same logic level signal turns on the MOSFET (Q1) and generates a gate triggering current (through

the motor) to the opposite high side thyristor (T2). When the control signal drops to zero, the MOSFET

turns off and then the SCR opens.

CONCLUSION

The new generations of SCR’s and triacs bring solutions to the increasing demand for a low cost and re-

liable electronic switch in today’s car applications.

The particular characteristics of thyristor offer a powerful and cost effective solution in many applications.

The examples presented in this note concerning ignition circuits, door lock control and various motor

drives can give new ideas to the designer.

SCR’s and triacs are today fully compatible with automotive requirements in terms of quality and reliability.

The technology of these structures is in continuous evolution and a new step has been reached with the

integration of several "discrete" functions on the same silicon die (ASD™ Application Specific Discrete).

Some ASD products, dedicated to applications mentioned in this paper, are in development. This devel-

opment approach opens up a new prospect providing space saving and cost reduction combined with the

quality inherent in the planar technology.

M

+VBAT.

T1

T2

Q1

Q2

10V

0