Lecture 3 Types of Power Diodes

Types of Power Diodes

1. COLLEGE OF ENGINEERING ROORKEE Established in 1998 Lecture 3: Types of Power Diodes Dr. Aadesh Kumar Arya Associate Professor & Head

2. Introduction Department of Electrical & Electronics Engineering 2 The diode totally stops any leakage current never considering any breakage of the junction barrier. But since things are not ideal in this world, we do face some problems considering non ideal behavior regarding the reverse leakage current in the diodes. So to adjust this, we associate a reverse recovery time with each diode, which marks the time required for the diode to recover from the reverse current. But the reverse recovery time acts as a bargain with the diode cost. A diode requiring lesser reverse recovery time will be more expensive than a simple diode with a larger reverse recovery time. Power Diode Applications:  As a rectifier Diode  For Voltage Clamping  As a Voltage Multiplier  As a freewheeling Diode

3. Department of Electrical & Electronics Engineering 3 Diodes are classified according to their reverse recovery characteristics The three types of power diodes are as under 1. General Purpose Diodes 2. Fast Recovery Diodes 3. Schottky Diodes

4. General Purpose Diodes Department of Electrical & Electronics Engineering 4 General-purpose diodes are two-terminal electronic components that allow current to flow in only one direction, from an anode (+) to a cathode. These simple semiconductors are PN junctions with a positive or P-region with positive ions and a negative or N-region with negative electrons These diodes have high reverse recovery time. It will be in the range of 25μs. .The diode current rating will be from 1A to several thousand amperes. The voltage rating will be from 50V to 5kV.

5. Applications Department of Electrical & Electronics Engineering 5 General-purpose diodes are often used in any application that does not require a specialized function or power requirements. Some of the applications include: Battery charging circuits Voltage regulators Switching applications C lamping circuits P ower supplies DC-DC converters AC-DC converters DC-AC inverters Signal rectifiers Oscillators Signal modulators / demodulators Cellular phones TV chassis Camcorders Motherboards electrical traction

6. Fast Recovery Diodes Department of Electrical & Electronics Engineering 6 Fast Recovery Diode is a semiconductor device, which possesses short reverse recovery time for rectification purpose at high frequency. A quick recovery time is crucial for rectification of high-frequency AC signal. Diodes are mostly used in rectifiers because they possess ultra-high switching speed. The major problem with the conventional diode is that they possess quite high recovery time. Due to which the rectification of high frequency is not possible with a conventional diode. These are used in high frequency circuits in combination with controllable switches where a small reverse recovery time is needed. At power levels of several hundred volts and several hundred amperes, these diodes have trr ratings of less than a few microsecond.

7. Department of Electrical & Electronics Engineering 7 As the name indicates, these diodes have very low reverse recovery time. It will be less than 5μs. They are mainly used in switching circuits like choppers, commutation circuits, switching mode power supplies etc. The current rating of the diode will be from 1A to several thousand amperes The voltage rating will be from 50V to 3kV. In this type of diode, (1) For low voltage ratings ( below 400V), the epitaxial process is used for the diode fabrication. (2) For diodes having voltage rating above 400V, diffusion technique is used for the fabrication of diodes.

8. Construction of Fast Recovery Diode Department of Electrical & Electronics Engineering 8 The fast recovery diode is constructed in the similar manner by which ordinary diode is constructed . The major difference in construction between these diodes and conventional diodes is the presence of recombination centres. In fast recovery diodes, Gold (Au) is added to the semiconductor material. This leads to augmentation in the numerical value of recombination centres due to which the lifetime (?) of charge carriers decreases. The semiconductor material used in these diodes is Gallium Arsenide (GaAs). And with the addition of Gold (Au) in the semiconductor material, the recovery time becomes less (about 0.1ns). On the other hand, the value of recovery time in case of silicon is 1-5 ns. Thus, it is evident that addition of materials like gold decreases the recovery time.

9. Applications of Fast Recovery Diode Department of Electrical & Electronics Engineering 9 1. Rectifier: These diodes are used in rectifier especially for high-frequency rectification. 2. Industrial and commercial areas: They are used in electronics circuits in various industries and automobile sector. 3. Radio signal detector: They are used in radio signal detectors to detect high-frequency RF waves 4. Analog and Digital communication Circuits: In analogue and digital communication circuit these diodes are extensively used for rectification and modulation purpose

10. Schottky Diodes Department of Electrical & Electronics Engineering 10 Schottky diode is a metal-semiconductor junction, which does not store charge carriers at the junction because it has no depletion layer. It finds its application where fast switching is needed. Significance of Schottky diode When a P-N junction diode is forward biased, it starts conducting, but when it is reverse biased, it stops conduction. However, this transition from conduction to insulation is not instant. Diode takes some time to reach a steady state of no conduction when it is reverse biased. This happens because during forward biasing charge carriers move across the junction and when it is suddenly reverse biased, some of the charge carriers are still at the junction, but they have not recombined yet.

11. Department of Electrical & Electronics Engineering 11 Thus, this phenomenon is called charge storage. Due to this charge storage, diode takes time while switching. The time taken by the diode to achieve a steady state of no conduction in reverse biasing is called reverse recovery time. The effect of reverse recovery time is negligible at frequencies below 10MHz, but at high frequencies, this effect is significant. These diodes are used where a low forward voltage drop (usually 0.3V) is needed in low output voltage circuits. In high frequency switching circuits like SMPS, the Schottky diodes are used (in those applications we cannot use the general-purpose diodes). In this, type of diodes, instead of P-N junction (semiconductor-to-semiconductor) metal to semiconductor junctions used. The Schottky diodes have very fast recovery time and low forward voltage drop. The current flow will be done by only majority carriers. So the time delay due to reverse recombination is avoided. The drawback with Schottky diode is they have low voltage ratings (around 100V) and forward current ratings (upto 300A).

12. Department of Electrical & Electronics Engineering 12 Advantages of Schottky diode 1. It possesses high switching speed. 2. Due to high switching speed, its reverse recovery time is very less as compared to other bipolar diodes. 3. The value of forward voltage in the case of this diode is also minimal comparatively other bipolar diodes Disadvantages of Schottky Diode 1. It possesses a significant value of leakage current. 2. The reverse breakdown voltage of these diodes is very small. Thus, even a small amount of reverse voltage can damage it. Applications of Schottky Diode 1. It is used as Schottky TTL in digital devices as these devices need fast switching. 2. A Schottky diode is the most significant component for digital computers, as the performance of digital computers is determined by switching speed of diodes.

13. Difference between Schottky Diode and PN Junction Diode Department of Electrical & Electronics Engineering 13

14. Why Schottky diode switching faster than bipolar Junction Diode? Department of Electrical & Electronics Engineering 14 In this diode, the electrons are the majority carriers on both sides of the junction ( i.e , in the N-type semiconductor as well as in the metal). So depletion layer is not formed near the junction. ( i.e no stored charge carriers in the junction) This scenario will give the following benefits: With reverse bias condition, there is no significant current from the metal to the semiconductor. Thus the time delay existing in the junction diodes due to hole – electron recombination is absent in Schottky diode. Hence, the Schottky diode can switch OFF faster than a bipolar diode.

15. How to test Diode? Department of Electrical & Electronics Engineering 15 We know that fact that resistance of diode in forward biased condition is low and the resistance of diode in reverse biased condition is high . Keep the multimeter in the ohmmeter section. If we measure the resistance of  a diode using the connections like red lead to anode and black lead(common) to cathode, a healthy forward biased diode will give low resistance. A high resistance reading in both directions indicates an open (defective device) condition, while a very low resistance reading in both directions will probably indicate a shorted device.    

16. Diode Protection Department of Electrical & Electronics Engineering 16 Snubber circuits are essential for diodes used in switching circuits as they can save a diode from overvoltage spikes, which may arise during the reverse recovery process. A common snubber circuit consists of a series RC connected in parallel with the diode . They have many different purposes, namely the reduction of power dissipation in power electronic switching networks.

17. Department of Electrical & Electronics Engineering 17 Series/parallel connections : necessary in high voltage and high current applications. Matching diode in terms of their reverse recovery properties is important in order to avoid large voltage imbalances between the diodes. A parallel RC snubber in parallel with each diode overcomes most of these problems