What is a BJT (Bi Polar Junction Transistor)?
A bipolar junction transistor (BJT), unlike a thyristor-like device, is a two-junction, self-controlled device where the collector current is under the control of the base drive current.
Why Darlington pair is used and what are its advantages and disadvantages?
The dc current gain of a power transistor is low and varies widely with collector current and temperature. The gain is increased to a high value in the Darlington connection,However, the disadvantages are higher leakage current. higher conduction drop. and reduced switching frequency.
How to control collector leakage current and enable base bias voltage?
The shunt resistances and diode in the base-emitter circuit help to reduce collector leakage current and establish base bias voltages.
What is avalanche breakdown (1st breakdown) and 2nd (second) breakdown in a BJT and how they destroy BJT?
Power transistors have an important property known as the second breakdown effect. This is in contrast to the avalanche breakdown effect of a junction, which is also known as first breakdown effect. When the collector current is switched on by the base drive, it tends to crowd on the base-emitter junction periphery, thus constricting the collector current in a narrow area of the reverse-biased collector junction. This tends to create a hot spot and the junction fails by thermal runaway, which is known as second breakdown. The rise in junction temperature at the hot spot accentuates the current concentration owing to the negative temperature coefficient of the drop, and this regeneration effect causes collapse of the collector voltage, thus destroying the device.
What happens when inductive load is turned off in BJT circuitry?
when an inductive load is turned off the base-emitter junction becomes reverse-biased, the collector current tends to concentrate in a narrow area of the collector junction. which causes the 2nd breakdown and BJT is destroyed after junction hotspot leading to thermal runaway of device.
What is a MOSFET, abbreviation and definition?
Unlike a BJT, a power MOSFET (metal-oxide semiconductor field-effect transistor) is a unipolar, majority carrier, "zero junction." voltage-controlled device.
Can you explain the working of a MOSFET?
If the gate voltage is positive and beyond a threshold value. an N-type conducting channel will be induced that will permit current flow by majority carrier (electrons) between the drain and the source.
What is the major advantage/disadvantage of MOSFET over BJT?
An important property of a MOSFET is that it does not have the second breakdown problem of a BJT. If localized and potentially destructive heating occurs within the device. the positive temperature coefficient effect of resistance forces local current concentration to be uniformly distributed across the total area. The conduction loss of a MOSFET is large for higher voltage devices. its turn-on and turn-off switching times are extremely small, causing low switching loss. The device does not have the minority carrier storage delay problem associated with a bipolar device.
Please name a few applications of MOSFET?
MOSFETs are extremely popular in low-voltage, low-power. and high-frequency (hundreds of kHz) switching applications. Application examples include switching mode power supplies (SMPS), brushless dc motors (BLDMs). stepper motor drives. and solid-state dc relays.
