Transformers and Semiconductor devices

Transformers

A transformer is used to change the amplitude of an AC signal. Unlike an amplifier a transformer is entirely passive, and there is no power gain.

Transformers can be loosely categorised into two classes: power and signal. Power transformers are used to convert AC voltages inside power supplies. Signal transformers are used for interstage coupling in amplifiers.

A simple transformer is shown, with N 1 turns on the primary and N2 turns on the secondary. With no load on the secondary an AC primary voltage ein is applied, which produces an alternating flux which is dependent on ein and N 1• If this flux in primary and secondary windings is the same we can say:

In the unloaded state, the flux rjJ induces a voltage in the primary winding which, in a perfect transformer, is equal to ein and no primary current flows.

If we now connect a load across the secondary winding, current will flow. This current opposes the flux from the primary, reducing the total flux in the core. At the primary, the induced voltage no longer equals ein and primary current flows, increasing the flux until the equation above is met again.

As there is no power gain, input power and output power must be equal, and for a perfect transformer:

The transformer is sometimes used as an impedance transformer. If we connect a load RL to the secondary, the impedance seen at the primary is given by:

The transformer can thus be used to match impedances for maximum power transference.

Transformers can also be used as current measuring devices in

AC power applications (shown below). Here a large AC current is passed through the primary of a transformer. This induces a flux which is cancelled by the induced secondary current. If the secondary is effectively short-circuited:

i.e. the secondary current is proportional to the primary current.

Current transformer

Such a device is called a current transformer, and typically has a 200: I ratio (i.e. 200 primary amps cause I amp to flow in the secondary). Often a single primary turn is used.

Current transformers must always have a secondary load connected, or dangerously high voltages are developed across the secondary coil (there being no secondary current with an open­ circuit secondary, to oppose the primary induced flux).

In the analysis above, we have assumed a perfect transformer with no losses. Practical transformers lose energy between primary and secondary. The first cause is due to I2R losses in the transformer windings. This is sometimes referred to as the copper loss.

The second cause is the hysteresis curve of the iron core. Each cycle around the curve loses energy equal to the area of the curve, and this energy again appears as heat. This is sometimes referred to as the iron loss.

If the core was a solid iron block, there would also be consider­a ble loss due to core eddy currents. These are reduced to negligible levels by the use of insulated laminations, shown earlier. At frequencies above about 25 kHz, eddy currents in laminations again become significant, and ferrite dust cores are used instead.

Semiconductor devices

The basic theory of semiconductor devices and the simple p-n junction are described earlier. These are the basis for many of the semiconductor devices described now.

Semiconductor diodes

A semiconductor diode is simply a p-n junction. The basic characteristics of a diode and the circuit symbol are shown below.

Diodes broadly fall into two classes: rectifier and signal diodes. Rectifier diodes are used in power supplies to convert AC to DC. Rectifier diodes carry large currents, have to withstand high peak inverse voltages, but generally work at low frequencies (usually 50 or60 Hz).

Signal diodes are used as logic elements or as demodulators in RF circuits. The currents and voltages are small, but these devices are required to operate at very high speed.

These differences are reflected in construction. A rectifier diode has to dissipate a considerable amount of heat, the energy absorbed being given by the mean forward current and the forward voltage drop. Rectifier diodes are usually bulky and often have stud mountings for heat sinks to assist cooling.

The speed of a diode depends on factors such as stray capacitance, and signal diodes tend to be very small devices.

Rectifier and signal diodes can be made from germanium or silicon. Germanium diodes have a low forward drop (about 0.2 V), but a junction temperature limit of 75°C. Silicon diodes have a forward voltage drop of about I V, but will operate up to nearly 200°C. The reverse leakage current is considerably lower for a silicon diode. In general, silicon diodes are preferred for rectifier diodes and logic applications, and germanium diodes or RF circuits.