Lecture-2.pptx
- 1. POWER ELECTRONICS AND CONVERTERS Southern School of Engineering and Technology DEPARTMENT OF ELECTRICAL ENGINEERING Dr. Muhammad Abrar
- 3. Power Semiconductor Diodes Diode is a simple Semiconductor device which is used for many purposes in electronic applications. Diode consists of two doped semiconductors, one of a P-type and other of N-type.
- 4. Review of Basic p-n Diode When Tetravalent semiconductor is doped with pentavalent impurities , then electrons are the majority carriers and N-type semiconductor is formed. Secondly, P-type with holes as the majority carriers is formed, when trivalent impurities are added to the tetravalent semiconductor material. After that, P-type semiconductor and N-type Semiconductor are diffused together.
- 5. Review of Basic p-n Diode By Diffusion, electrons from N-type region flow to P-type region and holes flow from P-type to the N- type region leaving behind negatively charged ions. In this process, electric field is created with opposite polarities to the respective (n-type or p- type) regions which blocks the further transfer of both carriers and as a result, a potential barrier is formed in between the semiconductors which is called Depletion region.
- 6. Review of Basic p-n Diode The PN junction can be used according to the need and in a controlled manner. It can be used in three modes, Zero bias , Forward Bias and Reverse Bias. In Zero Bias mode, it remains at its neutral behaviour. In Forward Bias mode, a potential difference is applied to the junction where P-type material is connected with the positive terminal and N-type is connected with the negative terminal of the Source.
- 7. Forward Bias mode When this arrangement is made , opposite polarity potential (w.r.t depletion region) breaks the potential barrier(inside) and flow of charges is started from both ends and as a result the current is observed.
- 8. Reverse Bias Mode In Reverse Bias, P-type is connected with negative terminal and N-type is connected with the positive terminal of the source.
- 9. Reverse Bias Mode This arrangement keeps the majority carriers of both materials away from breaking the depletion region and source potential(outside) attracts the majority carriers which results in increment of depletion region and a large voltage is observed.
- 10. 10 The Diode Characteristic I-V Curve
- 11. Power Diode Diodes are used in almost every electronic device both on small and large scales. However, diode needs some modification in high power applications. Diode undergoes some problems such as ability to conduct high current in forward bias and to hold high voltage and avoid breakdown in reverse bias. To overcome these problems, concept of Power diodes is introduced.
- 12. Power Diode Power diode is built with vertical orientation to provide extra cross- sectional area which is required to handle large amount of current. The other problem of controlling breakdown voltage in reverse bias is overcome by controlling the relative factors.
- 13. Power Diode Breakdown occurs due to the high electric field in potential barrier. Electric field and depletion width has inverse relation (E=V/d), so to control the break down at very high voltage, depletion width has to be altered, and to alter the depletion width, Doping of the material has to be changed.
- 14. Power Diode An extra layer of N-type material is introduced. Here, N-type in the middle region is less doped and the other P-type and N-type materials are normally doped. When the diode comes in reverse bias mode, it sees a large potential barrier across the lightly doped N-type region, which makes it able to withstand more voltage than normal. Extra layer used here, has to provide required majority carriers to maintain the current without reduction during the forward bias mode.
- 15. Reverse bias i-v characteristics of a power Diode.
- 16. Forward bias i-v characteristics of a power Diode.
- 17. Difference Between Normal and Power Diode 1- Design Simple diodes consist of a simple PN junction. They are smaller in size and lighter in weight, while power diodes are in modified form and they are larger and heavier normally. Power diodes are usually available in metal form. 2- Voltage Rating Simple diodes are used in normal low power components so they have a relatively low voltage ratings as compared to power diodes. Power diodes can have very high voltage ratings usually in thousands of volts.
- 18. Difference Between Normal and Power Diode 3- Current Rating Power diodes have a high hand in Current ratings over normal diodes , that can be in several hundred amperes. 4-Temperature Major probelm in handling high power applications is the generation of heat and temperature. Power diodes have the accessiblity of operation at higher temperature whereas normal diodes do not need this feature in high priority. 5- Cost Power Diodes are costly as compared to the simple diodes due to the additional components and features.
- 19. Summary A p-n junction diode is a minority carrier, unidirectional, uncontrolled switching device. A power diode incorporates a lightly doped drift region between two heavily doped p type and n type regions. Maximum reverse voltage withstanding capability of a power diode depends on the width and the doping level of the drift region. A power diode should never be subjected to a reverse voltage greater than the reverse break down voltage. The i-v characteristics of a forward biased power diode is comparatively more linear due to the voltage drop in the drift region.
- 20. Summary The forward voltage drop across a conducting power diode depends on the width of the drift region but not affected significantly by its doping density. For continuous forward biased operation the RMS value of the diode forward current should always be less than its rated RMS current at a given case temperature. Surge forward current through a diode should be less than the applicable surge current rating.
- 21. Summary During “Turn On” the instantaneous forward voltage drop across a diode may reach a level considerably higher than its steady state voltage drop for the given forward current. This is called forward recovery voltage. During “Turn Off” the diode current goes negative first before reducing to zero. This is called reverse recovery of a diode.
- 22. Summary The peak negative current flowing through a diode during Turn Off is called the “reverse recovery current” of the diode. The total time for which the diode current remains negative during Turn Off is called “the reverse recovery time” of the diode. A diode can not block reverse voltage till the reverse current through the diode reaches its peak value. Both the “reverse recovery current” and the “reverse recovery time” of a diode depends on the forward current during Turn Off, rate of decrease of the forward current and the type of the diode.
- 23. Summary Normal or slow recovery diodes have smaller reverse recovery current but longer reverse recovery time. They are suitable for line frequency rectifier operation. Fast recovery diodes have faster switching times but comparatively lower break down voltages. They are suitable for high frequency rectifier or inverter free- wheeling operation. Fast recovery diodes need to be protected against voltage transients during Turn Off” using R-C snubber circuit.