Notes

A Triac is equivalent to two thyristors connected back to back.
Thus, it is a bidirectional switching device, in contrast to the thyristor, which is a unidirectional device,
having a reverse blocking characteristic, preventing the flow of current from Cathode to Anode.
So, when it (triac) is in conduction mode, current flows in both directions (forward and reverse).
This switching device is called as TRIAC (TRIode AC switch)

The triac conducts in the positive direction from MT2 to MT1 when a positive pulse is applied at the gate (G) terminal with respect to MT1 and at the same time, the
positive voltage is applied between two terminals, MT2 (+) and MT1(−). Similarly, the triac
conducts in negative direction from MT1 to MT2 when a negative pulse is applied at the gate
(G) terminal with respect to MT1 and at the same time, the positive voltage is applied between
two terminals,MT1 (+) and MT2(−). the voltage between two terminals, MT2 and MT1 is negative now,

The triac is a low power device, used in voltage control circuits, used as light dimmers, speed
control for fan motors (single-phase), etc. Some of the advantages and disadvantages of the triac
vis-a-vis thyristor are given.
Advantages
1. Triacs are triggered by positive or negative polarity voltages applied at the gate terminal.
2. A triac needs a single heat sink of slightly larger size, whereas anti-parallel thyristor pair
needs two heat sinks of slightly smaller sizes, but due to the clearance total space required is
more for thyristors.
Disadvantages
1. Triacs have low / dtdv rating as compared to thyristors.
2. Triacs are available in lower rating as compared to thyristors.
3. Since a triac can be triggered in either direction, a trigger circuit for triac needs careful
consideration.
4. The reliability of triacs is lower than that of thyristors.


DIAC
A Diac is equivalent to two diodes connected back to back. Also, it is a bidirectional device, in
contrast to the diode, which is a unidirectional device, having reverse blocking characteristic,
preventing the flow of current from Cathode to Anode. So, when it (diac) is in conduction mode,
current flows in both directions (forward and reverse). This switching device is called as DIAC
The two terminals of the diac are designated as T1 and T2. These are similar to the terminals, A (Anode)
and K (Cathode), of the diode. The diac conducts, when the break-over voltage is reached in
either polarity across its two terminals. When T1 is positive with respect to T2, and if at that time
if the voltage,V12 exceeds VB01 (break-over voltage), the diac conducts in positive direction
from T1 to T2 . Similarly, when T2 is positive with respect to T1 , and if at that time if the
voltage,V21 exceeds VB02 (break-over voltage), the diac conducts in negative direction from
T2 to T1 . So, a diac can conduct in both directions (positive and negative), whereas a diode conducts
only in positive direction from Anode (A) to Cathode (K), if, at that time, the voltage,
Vak exceeds Vb0 (break-over voltage)


BJT
BJT is a current controlled device. Its operation is controlled by the base current.
The Power BJT is used as ON/OFF switch in the power converter circuits.
Power BJT operates in saturation and cutoff region when used as a switch. ie, when the device operates in saturation region it is in ON and when the device operates in cutoff region, it is in OFF state.
Sufficient base current is required to drive BJT in saturation.
Amount of carrier injected in base region determine storage time of BJT.
Storage time determines turn-on and turn-off times of the transistor.
There should be mechanism to control the amount of saturation so as to control storage time.
Saturation of the device should be avoided to ensure the fast switching.
It means that the base drive must ensure that the collector-to-emitter voltage is kept as low as possible ( to minimize losses), while not allowing the collector base junction to become forward biased.

pulse transformer
Pulse transformer designers usually seek to minimize voltage drop, rise time, and pulse distortion. Drop is the decline of the output pulse voltage over the duration of one pulse. It is cause by the magnetizing current increasing during the time duration of the pulse.

To understand how voltage doop and pulse distortion occurs, one needs to understand the magnetizing ( exciting, or no-load ) current effects, load current effects, and the effects of leakage inductance and winding capacitance. The designer also needs to avoid core saturation and therefore needs to understand the voltage-time product.

The magnetic flux in a typical A.C. transformer core alternates between positive and negative values. The magnetic flux in the typical pulse transformer does not. The typical pulse transformer operates in an “unipolar” mode ( flux density may meet but does not cross zero ).

A fixed D.C. current could be used to create a biasing D.C. magnetic field in the transformer core, thereby forcing the field to cross over the zero line. Pulse transformers usually (not always) operate at high frequency necessitating use of low loss cores (usually ferrites).