AN3168

Conclusion

Doc ID 17193 Rev 1

19/20

4

Conclusion

This application note has presented circuits that can be used to control SCR, Triacs or

ACS / ACST devices, as far as a non-insulated circuit is allowed.

It has been shown that a negative power supply is the best topology as it can be used for all

AC switches. Moreover if an SCR has to be controlled with a board using a negative supply

to trigger AC switches, a simple schematic has been presented to control the SCR with the

same supply.

Schematics for negative switched mode power supplies have been presented.

Even if the first reflex of a designer is to implement a positive power supply, negative power

supplies are as easy to implement as positive ones. There are also some benefits with a

negative supply topology such as, for example, the removal of output overvoltage protection

for non-insulated SMPS (buck-boost topology compared to buck topology), optotransistor

removal (for flyback converters) and the use of NPN transistors instead of PNP to amplify

the gate current.

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References

1.

“Latching current”, Application note AN303, STMicroelectronics.

2.

“QII and QIII Triac triggering with positive power supply”, Application note AN440,

STMicroelectronics.

3.

“VIPower: Low Cost Power Supplies Using VIPer12A in Non Isolated Applications”, N.

Aiello - F. Gennaro - R. Santori, Application note AN1357, STMicroelectronics, 2001.

4.

“Discontinuous Flyback transformer description and design parameters”, Technical

note TN0023, STMicroelectronics.

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Revision history

Table 7.

Document revision history

Date

Revision

Changes

09-Mar-2010

1

Initial release.