STSA1805 and STN851: Low Voltage NPN
Power Bipolar Transistors for Emergency Lamps

STSA1805 Product Page   
STN851 Product Page

New improved safety rules have caused a dramatic upturn in the sales of emergency lamps. The greater availability and the reduced price of these lamps have led to their widespread use in private homes, replacing the traditional candle or classic electric hand-held torch.

ST, already a world leader for high voltage Power Bipolar transistors in the lighting market, is now proposing a new low voltage device housed in TO-92 and SOT-223 suitable for emergency lamp applications.

Thanks to their exceptional low saturation voltage and high gain, STSA1805 and STN851 can also be used in both automotive applications, such as dashboard and direction control, and in consumer applications like CD readers, DVD players and alarm systems.

One of the most important features of an emergency lamp is the battery duration. To maximize the battery life it is necessary to minimize all the losses in the circuit due to active components. With regard to the losses related to a pair of STSA1805 or STN851, the application benefits from the low saturation voltage of these devices.

STSA1805 and STN851 are manufactured using a new base island planar technology bringing cost and size reduction (same electrical performance with 30% less silicon) but keeping a high level of efficiency.

ST’s devices exhibit a very high h_FE value, which allows a power saving at the base drive required.

Circuit Description

The emergency lamp can be schematised in:

	The bridge and filter
	Monitor circuit
	Battery pack with trickle charging
	DC/AC inverter

Features

	Low saturation voltage
	Exceptional high gain performance
	TO-92 and SOT-223 packages
	Low current base driving

STSA1805: Application Core

A pair of STSA1805 or STN851 is used in emergency lamp applications up to 60W and is physically placed in the DC to AC inverter (normally in Push Pull current feed configurations). This inverter topology is also known as a resonant version of the power inverter developed by George Royer. This inverter provides a high output voltage (before conducting the lamp needs a peak voltage of ~500V) with a low input voltage. This voltage amplification is due to the right winding ratio between the primary and the secondary transformer.

Turn-on waveforms of the STSA1805 showing a low base current required for driving

The voltage on W2 or W3 is generated by transistors that work alternately, when one is conducting the other is turned off and vice-versa. C1 is designed to fix the right frequency normally in the range of 25 to 50kHz.

Other Applications

	Voltage regulation
	Relay drivers
	High efficiency low voltage switching applications

Main Characteristics

P / N	VCES (V)	IC (A)	VCE(sat) (mV) Ic =5A Ib=200mA					P tot (W)	Package	Competition Similar P/N
STSA1805*	150	5	<600					1.1	TO-92	Zetex - ZTX851
STN851	150	5	<500					1.6	SOT-223	Zetex - FZT851

*available in Ammopack