Lecture Notes 3 Uncontrolled PSDs Prepared by Dr. Oday A Ahmed Website: https://odayahmeduot.wordpress.com Email: 30205@uotechnology.edu.iq Scan QR
Contents of this Lecture: Power Diode Characteristics Reverse Recovery Definition Diode Types Power Diode Power odes are made of silicon p-n junction with two terminals, anode and cathode. Diode is forward biased when anode is made positive with respect to the cathode. Diode conducts fully when the ode voltage is more than the cut-in voltage (0.7 V for Si). Conducting ode will have a small voltage drop across it. Diode is reverse biased when cathode is made positive with respect to anode. When reverse biased, a small reverse current known as leakage current flows. This leakage current increases with increase in magnitude of reverse voltage until avalanche voltage is reached (breakdown voltage). Fig.3.1 shows V-I Characteristics of ode. Ideal ode Fig.3.1 1
Forward Voltage Drop: The forward voltage drop is due to the forward resistance of the junction. forward volt drop is across the junction Reverse Leakage Current Thermal agitation does break some of the bonds in the crystal, resulting in minority carriers, which permit a small reverse current flow, i.e. leakage current. NOTE: The less abundant charge carriers are called minority carriers; Reverse Recovery Characteristics The current in a ode in forward-biased mode is due to the net effect of majority and minority carriers. Once a ode is turned off dude to reverse the polarity, the forward current reduced to zero and then continue to conduct due to the minority carriers that remain stored in the pn-junction and the bulk semiconductor materials. Fig.2 shows the effect of minority carriers on the turn off characteristics of the power ode. Reverse recovery time Reverse recovery Charge trr: time between 0 to 25% of Ι rrm Fig.3.2 Due to the minority carriers, which remain stored in the pn-junction (depletion region), and represents the time between the zero crossing and the peak reverse current, Ι rrm. Due to the charge stored in the body of the ode (bulk regions) and represents the time between Ι rrm and 25% of Ι rrm 2
The charge carriers (holes & electrons) require a certain time to recombine with opposite charges and to be neutralized; this time is called the reverse recovery time trr of the ode. From Fig.3.2, one can found the following relationships: t rr = t 2 + t 3 I rrm 2Q rr t rr = t 2 I rr = t 2 then Q rr = 1 2 I rrmt 2 + 1 2 I rrmt 3 = 1 2 I rrmt rr For Fast recovery t 3 t 2 t 2 = t rr t rr = 2Q rr Hence, I rrm = 2Q rr The fast decay of negative current creates an inductive drop that adds with the reverse blocking voltage V R as illustrate in Fig.3.3. Hence, the blocking voltage across the ode increases to: 3
V rrm = V rr + V R where, V rr is reverse recovery voltage due to the fast decay in the negative current and equal to: V rr = L There are two types of reverse recovery characteristics of junction odes: Soft recovery and Fast recovery where, the softness factor, SF is the ratio of t 2 /t 3. The storage charge that stored in the ode due to changeover from forward to blocking mode is depend on the forward current value. During t rr the ode behaves like a short circuit and cant blocking the reverse voltage lead to reverse current flow. t rr is very important in switching applications Example 1: A power ode has the following specifications: forward current 50A, reverse blocking voltage = 100V, the reverse recovery time of a ode is t rr = 3µs and the rate of fall of the ode current is /= 30A/ µs. determine The storage charge The peak reverse current The maximum reverse voltage due to reverse recovery if internal stray inductance is 10µH. Solution; t rr = 2Q rr Q rr = 1 2 t rr 2 4
Q rr = 1 2 30 106 (3 10 6 ) 2 = 135μC I rrm = 2Q rr = 2 (135 10 6 ) 30 10 6 = 90A It can be seen that due to reverse recover charge, the rated of the ode current is exceeded. Maximum reverse voltage equal to: V rrm = V rr + V R V rr = L = 10 10 6 30 10 6 = 300V V rrm = 300 + 100 = 400V Hence, due to reverse recover charge the reverse blocking voltage exceeng the rated of the ode. This voltage may be destructive and can be softened by a resistance-capacitance snubber, which will be scussed later. The recovery current causes aional loss (switching loss) in the ode; this can be known by multiplying ode current times the ode voltage shown in Fig.3.3. For high frequency applications rectifier power ode can not be used, this is due to the long reverse recovery time of theses odes. Increasing switching frequency (i.e. high sudden changing of polarity across the ode when working at high frequency) result in increasing the / that will lead to high over shoot voltage across the ode. This will also lead that the charge carriers (holes & electrons) require a longer time to recombine with opposite charges and to be neutralized. As shown in example 1, longer t rr results in increase the recovery charge stored that result in exceeng the rated current and voltage of the ode. The larger the active junction area, the larger the charge fference. Therefore, devices in the same family with larger e sizes, represented by higher current ratings, will have a larger reverse recovery charge. 5
Based on the ode reverse recovery characteristics power ode are classified into: Standard Recovery (General) Diodes Fast Recovery Diodes Schottky Diodes Silicon Carbide Diodes. For high frequency rectifier applications, Fast recovery and Schottky Diodes are generally used because of their short reverse recovery time and low voltage drop in their forward bias contion General Purpose Diodes The odes have high reverse recovery time of about 25 microsecs (µsec). They are used in low speed (frequency) applications. e.g., line commutated converters, ode rectifiers and converters for a low input frequency up to 1 KHz. Diode ratings cover a very wide range with current ratings less than 1 A to several thousand amps (2000 A) and with voltage ratings from 50 V to 5 KV. These odes are generally manufactured by ffusion process. Alloyed type rectifier odes are used in welng power supplies. They are most cost effective and rugged and their ratings can go up to 300A and 1KV. Fast Recovery Diodes The odes have low recovery time, generally less than 5µs. The major field of applications is in electrical power conversion i.e., in free-wheeling ac-dc and dcac converter circuits. Their current ratings are from less than 1 A to hundreds of amperes with voltage ratings from 50 V to about 3 KV. For high voltage ratings, greater than 400V they are manufactured by ffusion process and the recovery time is controlled by platinum or gold ffusion. For less than 400 V rating epitaxial odes provide faster switching speeds than ffused odes. Epitaxial odes have a very narrow base wih resulting in a fast recovery time of about 50 ns. Schottky Diodes A Schottky ode has metal (aluminium) and semi-conductor junction. A layer of metal is deposited on a thin epitaxial layer of the n-type silicon. They are mostly used in low voltage and high current dc power supplies. The operating frequency may be as high 100-300 khz as the device is suitable for high frequency application. 6
Silicon Carbide SiC Schottky Barrier Diode (SBD) SiC (Silicon Carbide) is a compound semiconductor comprised of silicon (Si) and carbon (C). Compared to Si, SiC has Ten times the electric breakdown field strength. Three times the bandgap. Three times the thermal conductivity. No reverse recovery time Ultrafast switching behavior A typical comparison between fferent types of odes is shown in the table below: Standard Recovery Diodes Upto 5000V & 3500A Reverse recovery time High trr ~=25µs. Typically used in rectifiers at power frequencies i.e., at 50Hz or 60 Hz. Fast Recovery Diodes Upto 3000V and 1000A Reverse recovery time Low trr 5µs. Typically operating at higher frequencies as freewheeling odes. Schottky Diodes Upto 100V and 300A Reverse recovery time Extremely low. trr is typically around few ns Typically operating at higher frequencies as freewheeling odes. Silicon Carbide Diodes. Upto 600V and 200A have extremely fast switching behaviour with ultra-low trr Typically operating at higher frequencies as freewheeling odes. V F = 0.7V to 1.2V V F = 0.8V to 1.5V V F = 0.4V to 0.6V V F <0.5V Self-assessments; 1. What is the fference between leakage current and reverse recovery current in the power ode? 2. Why the reverse recovery characteristics does not consider in signal ode? 3. What is the effect of ode revere recovery on the operation of the ode? 4. What are the fferences between General propose ode and Fast-recovery and Schottky odes? 7