Skip to main content

National Bird Day: Why Birds Matter to Us

National Bird Day is celebrated every year on 5 January . This day is observed to spread awareness about birds, their importance in nature, and the need to protect them. Birds are beautiful living beings and play a very important role in keeping our environment healthy. Origin of National Bird Day National Bird Day was first celebrated in the year 2002 . It was started by bird lovers and environmental groups to protect birds from dangers like deforestation, pollution, and illegal hunting. The main aim of this day is to teach people, especially students, why birds are important and how we can help save them. Why Is National Bird Day Celebrated Every Year? National Bird Day is celebrated every year because many bird species are disappearing due to human activities. Cutting trees, using plastic, pollution, and climate change are harming birds and their homes. This day reminds us that: Birds need protection Nature should be respected Everyone has a responsibil...
Post a Comment

Power diodes | Power electronics

Power diodes

Introduction of power diode

  • Power diode is a two layer, two terminal, p-n semiconductor diode.
  • The two terminal of diode are called anode and cathode.

Characteristic of power diodes

Circuit diagram for characteristic curve of power diode
  • When diode is positive with respect to cathode, diode is said to be forward biased.
  • With increase of Vs from zero value, initially diode current I is zero.
  • From Vs = 0 to cut-in voltage (threshold voltage or truth-on voltage), the forward diode current is very small.
  • After cut in voltage, the forward diode current increases rapidly and the diode is said to conduct.
  • For Si diode, the value of cut-in voltage is 0.7V
  • When diode conduct, there is a forward voltage drop of the order of 0.8 to 1V.
Characteristic curve of power diode
  • When cathode is positive with respect to anode, the diode is said to be reverse biased.
  • In reverse bias condition, a small reverse current, known as leakage current flow in the circuit and this current is of the order of µA or mA.
  • The value of leakage current increases slowly with the increase in reverse voltage up to breakdown or avalanche voltage.
  • At breakdown voltage, the diode is turned on in the reverse direction.
  • The reverse avalanche breakdown of a diode is avoided by operating the diode below specified peak repetitive reverse voltage VRRM.
  • PIV = peak inverse voltage of a diode.
  • The PIV of a diode is the maximum reverse voltage to which a diode may be subjected during its working. It is same as VRRM.

Diode reverse recovery characteristics

  • After the forward diode current decays to zero, the diode continuous to conduct in the reverse direction, due to the presence of stored charges in the two layers.
  • The reverse current flows for a time called reverse recovery time trr.
Reverse recovery characteristic of power diode

Reverse recovery time (trr)

  • Reverse recovery time is defined as the time between the instant forward diode current becomes zero and the instant reverse recovery current decays to 25% of its reverse peak value IRM.
  • trr = ta + tb
  • ta : The time between zero crossing of forward current and peak reverse current IRM. During this time the charge stored in depletion region is removed.
  • tb : It is measured from the instant of IRM to the instant where 0.25 (or 25%) IRM is reached. During this time the charge from the semiconductor layers is removed.
  • During trr, the stored or reverse recovery charge QR must be removed.

Softness factor or S factor

  • The ratio of  tb and ta is known as softness factor or S-factor.
  • The S factor is a measure of the voltage transient that occur during the time, diode recovers.
  • Usually the value of softness factor is found to be unity, which indicates low oscillatory reverse recovery process.
  • If softness factor is small, then diode has large oscillatory over voltages.
  • If S-factor is 1, the diode is called soft-recovery diode.
  • If S-factor is less than 1, the diode is called or fast recovery diode. or snappy recovery diode.
  • If vf is forward voltage drop across diode, and if is forward current
  • vf x if is power loss in diode
  • Fig (c) shows that major power loss in a diode occurs during tb
  • From fig (a), IRM = ta (di/dt)
  • Here IRM = Peak inverse current, and di/dt = Rate of change of reverse current
  • If we assume that reverse recovery characteristic of fig. (a) is triangular, then
  • QR = ½ IRM x trr ⇒ IRM = 2 (QR / trr)
  • ∵  IRM = ta (di/dt)  and  ta = trr 

  • Thus trr (reverse recovery time) and IRM (peak inverse current) depends on QR (storage charge) and di/dt (rate of change of current)
  • Since QR depends on if (forward diode current)
  • So trr and IRM also depends on if.
  • So a power electronics engineer must know the value of IRM, QR, S-factor and PIV to design the circuit in which power diodes are used.
To know more about this topic please click on https://youtu.be/TjZ4fGWCOZI
Labels:

Comments

Post a Comment

Popular posts from this blog

Electric field due to circular loop of charge | Electromagnetics

Electric field due to circular loop of charge Electric field The space around a charged particle in which another charge experience a force is known as electric field. The source of electric field is either a charge or a time varying magnetic field. If the value of electric field does not change with time, then it will be uniform electric field, otherwise it will be non-uniform electric field. Electric field due to circular loop of charge If λ is linear charge density, then the charge on d l dq = λ d l      ⇒     dq = (q / 2πa) d l Electric field at P due to charge dq Special cases When P lies at the centre of the loop i. e., r = 0, then E = 0 When P lies very far from the centre of the loop i. e., r >> a, then E = kq / r 2 In this case circular loop behaves as a point charge. To know more about this topic please click on the link  https://youtu.be/54MIe0Ow43w   or...
Post a Comment
Read more

आवेश तथा उसके गुण | Charge and its properties in Hindi

आवेश तथा उसके गुण आवेश क्या है कोई नहीं जानता कि आवेश क्या है, केवल हम यह जानते हैं कि आवेश क्या कर सकता है तथा इसके गुण क्या हैं ? आवेश के प्रकार आवेश दो प्रकार के होते हैं, धनावेश तथा ऋणावेश। धनावेश प्रोटॉन के कारण होता है तथा ऋणावेश इलेक्ट्रॉन के कारण। तीसरे प्रकार का कोई आवेश ब्रहाण्ड में विद्यमान नहीं है। आवेश का क्वांटीकरण आवेश सदैव पैकेट के रूप में होता है, सतत्‌ नहीं। किसी वस्तु को दिया गया आवेश सदैव एक न्यूनतम आवेश का पूर्ण गुणज होता है यह न्यूनतम आवेश इलेक्ट्रॉन के आवेश के बराबर होता है, जिसका मान e = 1.6*10 -19 C होता है। q = ne,        यहां n = 0, 1, 2, ... आवेश संरक्षण का नियम आवेश को न तो उत्पन्न किया जा सकता है तथा न ही नष्ट किया जा सकता है, परन्तु इसे निकाय के एक भाग से दूसरे भाग में स्थानान्तरित किया जा सकता है। किसी विलगित निकाय का आवेश सदैव संरक्षित रहता है। Σq i = नियत,           यहां q i = iवे कण का आवेश आवेश के गुण स्थिर विद्युत आव...
Post a Comment
Read more

गैसों का अणुगति सिद्धान्त तथा आदर्श गैस का दाब | Kinetic theory of gases and Pressure of an ideal gas in Hindi | Thermodynamics

गैसों का अणुगति सिद्धान्त तथा आदर्श गैस का दाब गैसों के गतिज सिद्धान्त की अभिधारणाएं एक गैस अत्यन्त छोटे, अदृश्य एवं पूर्णतः प्रत्यास्थ कणों से मिलकर बनी होती है, जो अणु   कहलाते हैं। एक शुद्ध गैस के सभी अणु समदृश होते हैं तथा ये सभी सम्भव दिशाओं में सभी सम्भव वेग से सतत्‌ रूप से गति करते रहते हैं। गैस जिस पात्र में भरी जाती है, वह उस पात्र की दीवारों पर दाब लगाती है। गैस के अणु किन्हीं दो क्रमागत टक्करों के मध्य सीधी रेखा में गति करते हैं। गैस के अणुओं का आकार किन्हीं दो क्रमागत टक्करों के मध्य तय की गई दूरी की तुलना में अनन्त सूक्ष्म होता है। ये टक्करें तात्क्षणिक होती हैं तथा टक्करों में गतिज ऊर्जा की कोई हानि नहीं होती है। अणु एक दूसरे पर कोई बल नहीं लगाते हैं। वे एक दूसरे पर बल केवल टकराने के दौरान लगाते हैं। इनकी सम्पूर्ण आणविक ऊर्जा, गतिज ऊर्जा होती है। गैस के अणुओं का कुल आयतन, उस पात्र के आयतन, जिसमें यह भरी है कि तुलना में नगण्य होता है। गैस में अन्तर-आणविक दूरी बहुत अधिक होती है, जिससे कि गैस के अणु उसके लिए उपलब्ध सम्पूर्ण स्थान में मुक...
Post a Comment
Read more