Ignition Coil

In order to generate as high a voltage as possible at the secondary it is necessary to produce as high a voltage as possible at the primary. This is because the secondary voltage is just the turns ratio times by the primary voltage. There are two ways to do this.

1. Apply a high voltage to the primary. 2. Switch a current flowing through the primary off very quickly.

The first method is limited by the fact that the current in the primary will rise very quickly with a large applied voltage. If the current is allowed to increased to a very large value, the transformer core will saturate and power will be wasted. The current will rise even quicker once the coil's core is saturated and it is likely that the switching device (transistor) will blow. It is necessary to design a drive circuit to take these factors into consideration. Higher frequencies andor lower duty cycles are necessary to stop this fast rise of current.

The second method is limited by how much of a voltage spike the switching transistor will take (ie its Vce or Vcb rating). It is also limited by the speed at which a transistor can switch off (they do take an incredibly small but finite time to switch off).

The circuits I have designed so far have used a 35V to 50V supply to drive an ignition coil. I am currently designing a circuit to drive a flyback transformer but this is more of a challenge to get real impressive performance because of the high frequency of operation. I have so many things on the go that I have numbered my circuits. The ignition coil driver that I am working on at the moment is EHT3. The flyback-based design is EHTA.

Flyback Circuits.

The horizontal deflection (flyback) circuit inside a television or computer monitor uses a transistor as a 'switch', usually applying a fixed dc voltage to the primary of a transformer. This transistor can be known by several names: HOT (for horizontal output transistor), line output transistor, flyback transistor. These days it is almost always an NPN type transistor with the emitter grounded.

The switching frequency of the transistor in UK televisions is 15625Hz. This frequency is derived as follows: There are 25 pictures displayed on the screen per second. Each picture is interlaced, so there are 50 vertical scans per second. Each picture has 625 lines, so each vertical scan has (625/2)=312.5 lines. So the number of horizontal scans per second is 50 x 312.5 = 15625.

On some computer monitors the horizontal frequency is 15750Hz. This frequency is derived as follows: There are 30 pictures displayed on the screen per second. Each picture is interlaced, so there are 60 vertical scans per second. Each picture has 525 lines, so each vertical scan has (525/2)=262.5 lines. So the number of horizontal scans per second is 60 x 262.5 = 15750.

This transistor is of a special design. The voltages at the collector will be very high when the current to the transformer primary is switched off. The transistor is therefore manufactured to withstand this high voltage. There are two specifications that are used to define the high voltage characteristics of this type of transistor: Vce and Vcb.

The Vce rating of a transistor is the voltage that the transistor can withstand with the emitter grounded, but the base left unconnected. In this mode of operation, the high voltage at the collector can cause a tiny current to 'leak' through to the base. At voltages approaching the maximum this causes the transistor to start to switch on. This can then lead to destruction.

The Vcb rating of a transistor is the voltage that the transistor can withstand with the emitter and base grounded. This value is often greater than the Vce value. This is because any current 'leaking' from the collector to the base flows straight to ground instead of through the base region.

The design of these high voltage transistors usually leads to a low gain. So large currents are needed in the base circuit to switch a given current in the collector. When an increasing current is supplied to the base of a transistor, the transistor allows a multiplied current to flow through the collector (The transistor does this by altering its own collector to emitter resistance hence the name Transistor i.e. Transfer Resistor). The word 'allows' in the previous sentence is quite important, because in some circuits the amount of current flowing through the collector will be limited by the circuit. The transistor will reduce its collector to emitter resistance to a very low value in an attempt to allow the collector current to equal the base current times the gain. But eventually the collector to emitter resistance will become a minimum, the collector emitter voltage will be very low (typically 0.4V) and in this state the transistor is said to be 'saturated'.

Engineers design the flyback circuit so that when the horizontal output transistor is switched on it is in the 'saturated' state. They do this because the power dissipated in a saturated transistor is small. They do this also because it ensures that virtually the full supply voltage is applied to the transformer primary. To make sure that the transistor is saturated they use large base currents.

Diodes for the rectification of very high a.c. voltages are not as readily available as other types. It is therefore common practice to series connect diodes of lower reverse voltage rating. Because the resistance of these diodes under reverse conditions varies somewhat, it is standard practice to connect a resistor of a certain value in parallel with each diode so that the reverse voltage is evened out over the chain. The capacitance of the individual diodes under reverse conditions also varies, so it is also common practice to connect a capacitor over a certain value in parallel with each diode in the chain. The value of the resistors must be small enough to make the resistance of a reverse biased diode negligable in comparison, but large enough prevent upset to the rectifying action. The value of the capacitor must be large enough to make the capcitance of a reversed biased diode negligable in comparison, but small enough to act as an open circuit at the frequency of a.c. being rectified.

An alternative (but often more costly) approach is to use a series chain of avalanche diodes. Avalanche diodes act like zeners if their reverse voltage gets too high, and so are not damaged (providing the reverse current is not too high). Examples of suitable avalanche diodes are BYV96E (1kV, 1.5A) and BYW96E (1kV, 3A). A further advantage of avalanche diodes is that they are fairly fast devices, and so could be used in high frequency rectification.

Whichever method is used, potting of the resultant chain is essential to prevent corona. Corona losses can form a significant loss in any high voltage circuit, and in a series diode chain can lead to certain diodes being put under more stress than others.