Introduction

In lighting applications with fluorescent lamps, the choice of the starter switch to be used is always very important for the designer: the cost, reliability, ruggedness, and ease to be driven must always be kept in mind. This is especially important in lighting circuits where the designer has to optimize the operating life of the fluorescent lamps by using the right starter switch.

In the large family of electronic switches, the thyristor must be considered as a low cost and powerful device for lighting applications. Thyristors can take many forms but they have certain features in common. All of them are solid-state switches that act as open circuits capable of withstanding the rated voltage until triggered. When they are triggered, thyristors become low-impedance current paths and remain in that condition (i.e. conduction) until the current either stops or drops below a minimum value called the holding level. Also, once a thyristor has been triggered the trigger current can be removed without turning off the device.

Silicon controlled rectifiers (SCRs) and triacs are both members of the thyristor family. SCRs are unidirectional devices while triacs are bi-directional. A SCR is designed to switch load current in one direction, while a triac can conduct load currents in either direction.

Structurally, all thyristors consist of several alternating layers of opposite p and n silicon, with the exact structure varying by device. The load is applied across the multiple junctions and the trigger current is injected at one of them. The trigger current allows the load current to flow through the device setting up a regenerative action which keeps the current flowing even after the trigger is removed.

These characteristics make thyristors extremely useful in control applications. Compared to a mechanical switch a thyristor has a very long service life and very fast turn on and turn off times. Because of their fast reaction times, regenerative action, and low resistance (once triggered) thyristors are useful as power controllers and transient over-voltage protectors -- in addition to simply turning devices on and off. Thyristors are used to control motors, incandescent and fluorescent lamps, and many other kinds of equipment.

Although thyristors of all sorts are generally rugged, there are several points to keep in mind when designing circuits using them. One of the most important parameters to respect is the device's rated limits on rate-of-change of voltage and current (dV/dt and di/dt). If these are exceeded, the thyristor may be damaged or destroyed.

Definitions

Ambient Sound Levels. Background noise generated by ballast and other equipment operating in a building.

Arc. Intense luminous discharge formed by the passage of electric current across a space between electrodes.

Ballast. An electrical device used in fluorescent and high-intensity discharge (HID) fixtures. It furnishes the necessary starting and operating current to the lamp for proper performance.

Electrode. Metal filament that emits electrons in a fluorescent lamp.

Fluorescent lamp. Gas-filled lamp in which light is produced by the interaction of an arc with phosphorus lining the lamp's glass tube.

Fluorescent light circuit. Path over which electric current flows to operate fluorescent lamps. Three major types of fluorescent lighting circuits are in use today, preheat, instant start (slimline) and rapid start.

Instant start (slimline). A class of fluorescent. Ballast provides a high starting voltage surge to quickly light the lamp. All instant start lamps have a single pin base and can be used only with instant ballast.

Rapid Start Lamps. Fluorescent lamps that glow immediately when turned on and reach full brightness in about 2 seconds.

Preheat Lamp. A fluorescent lamp in which the filament must be heated before the arc is created.

This application note is designed for preheat start lamp circuits, whose functionality is described below:

How The Lamp Works (Using the conventional glow-tube starter)

The above Figure illustrates a fluorescent lamp with the conventional glow-tube starter. The glow-tube starter consists of a bimetallic switch placed in series with the tube filament which closes to energize the heating filaments. It then opens to interrupt the current flowing through the ballast inductor thereby generating the high voltage pulse necessary for starting. The mechanical glow-tube starter is the circuit component most likely to cause unreliable starting.

The principle disadvantage of the conventional glow-tube starter is that it often has to open several times in the filament circuit to interrupt the current flowing through the ballast inductor in order to generate the high voltage necessary for turning-on the fluorescent lamp. Those interactions decrease the life of the lamp considerably and ,also, the lamp takes around 3 seconds to turn-on.

On the other hand, the following schematic diagrams show the electronics which can be substituted for the conventional glow-tube starter for fluorescent lamp applications of 20 W and 40 W using a diode, SCR, and a TVS or zener clipper(s):

click for larger image

The main reason why these two circuits are different is due to the different high voltage that must be generated. The inductor ballast for fluorescent lamps of 40 W provides a higher voltage than the inductor ballast for lamps of 20 W. Note: this applies equally well when conventional glow-tube starters are used: there is no general glow-tube starter that can operate for all kinds of fluorescent lamps.

The following plots show the voltage and current waveform in the electronic starter circuitry when the fluorescent lamps are turned-on:

Fluorescent Lamp of 20 W

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When the switch is turned-on, the voltage across the clipper (SA90A) is the same as the voltage of the ac Line (Vpeak=160 V), and since the clipper allows current-flow through itself only once its specified VBR is reached (100 Vpeak), the SCR (MCR100-8) turns-on and closes the circuit to energize the filaments of the fluorescent lamp. At this time the current through the circuit is around 1.2 Apeak, and once the lamp is warm enough its dynamic resistance reduces and permits current-flow through itself, which causes the voltage across the clipper to decrease to around 78 Vpeak. This makes the clipper turn off, since the voltage is less than the VBR of the device, and because the clipper turns off the SCR also turns-off and opens the circuit to interrupt the current flowing through the ballast inductor. That switched interruption generates the high voltage pulse necessary to start the lamp's arc. The time that the fluorescent lamp takes for turn-on is around 400 ms using the electronic starter, considerably faster than using a conventional glow-tube starter.

Fluorescent Lamp of 40 W

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The operation of the electronic starter circuit of 40 W is similar to that for 20 W, but the inductor ballast for 40 W generates a higher voltage than that for 20 W. The circuit for lamps of 40 W has two clippers and one snubber inside its control circuit. The current flowing through this circuit is around 2.1 Apeak and it appears around 550 ms -- the turn-on time, which is longer than that for 20 W.

In conclusion, the electronic starter circuits (for 20 and 40 W) are more reliable than the conventional glow-tube starters with a faster turn-on and a longer life. Pricing of an electronic solution is comparable to glow-tube starters.

It is also important to note that the ac voltage supply to the electronic starter circuits must be in the range of 115 Vrms to 130 Vrms for correct operation. If it is not within this voltage range there may be unreliable starting of the lamp. Also, extreme environmental temperatures could effect the functionality of the electronic starters but they can operate between 15ýC to 40ýC.

Note: This application note is also available from ON Semiconductor in a slightly different format as document AND8006/D. http://www.onsemi.com.

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