Lecture 5: Diode, Rectifier and Capacitor Bo Wang Division of Information & Computing Technology Hamad Bin Khalifa University bwang@hbku.edu.qa 1
Why Rectifying? Voltage and current delivered from the transmission line is typically AC, while household appliances require steady constant DC signals for operation. Rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is called rectification. Circuitry that performs the opposite function, converting DC to AC, is called power inverter. Power generation Transmission line AC AC Rectifier DC User AC Transformer 1 (up conversion) Transformer 2 (down conversion) 2
Device for Rectification Diode: a two-terminal electronic component that conducts primarily in one direction; it has low (ideally zero) resistance to the current in one direction, and high (ideally infinite) resistance in the other (e.g. LED). Symbol: Anode (+) Cathode (-) Physical view B. Wang, 2017 Extreme macro photo of a Chinese diode of the seventies. CPEG - Lecture 5 3
Ideal Diode Begin with an ideal diode and look at its characteristics i Anode Cathode Reverse bias Forward bias + - v Equivalent circuit 0 v Anode i v + - v<0 à i=0 Cathode Anode + - v=0 à i>0 At reverse voltage, current is zero (no current); at forward voltage, current is infinite (or any current). i v Cathode 4
Real Diode Given a semiconductor PN junction we get a diode with the following characteristics Breakdown voltage Forward region Turn on voltage V ON Breakdown region Reverse region 3 regions Turn on voltage based on the built-in potential of the PN junction Reverse biased breakdown is mainly due to avalanche breakdown (on the order of several volts) 5
Diode Current The forward bias current is closely approximated by d i = I nvt S ev 1 V T = kt q where V T is the thermal voltage (25mV at room temp) k = Boltzman s constant = 1.38 x 10-23 joules/kelvin T = absolute temperature in Kelvin q = electron charge = 1.602 x 10-19 coulombs n = constant dependent on material between 1 and 2 (we will assume n = 1) I S = scaled current for saturation current that is set by dimensions V d = voltage across the diode. Notice there is a strong dependence on temperature We can approximate the diode equation for i >> I S i I S ev d nvt In reverse bias (when V d << 0 by at least V T ), then: i I S In breakdown, reverse current increases rapidly a vertical line 6
Rectification with Diodes Half-wave rectifier Ideal diode vs. real diode Ideal Real Only lets through positive voltages and rejects negative voltages Power delivered to the load only for half of the cycle Real diode induces a voltage drop of V ON due to its turn on voltage and the output signal period is less than half of a cycle After rectification (ideally:) u = 2V p π,u = V p à 2 u = V p π,u = V p 2 7
Rectification with Diodes à Without the loss of the negative half wave Full-wave rectifier 8
Rectification with Diodes à Without the loss of the negative half wave Full-wave rectifier (ideal and real diode) D1 D3 V p -2V ON Ideal D2 D4 Real Power to the load in both positive and negative cycles Power to the load fluctuates with time A real diode induces extra voltage losses of 2V ON Voltage fluctuates with time and needs to be stabilized After rectification(ideally): u = 2V p à π,u = V p 2 u = 2V p π,u = V p 2 9
Output Ripple Rectifier output ripple With C1 Without C1 Add a smoothing capacitor to reduce the output ripple 10
Capacitor (Review) A capacitor is a passive two-terminal electrical component that stores electrical energy in an electric field. A capacitor consists of two conductors separated by a non-conductive region. The non-conductive region can either be a vacuum or an electrical insulator material known as a dielectric (to reduce E). Symbol: C = εa d = Q V 11
Capacitor A capacitor is a charge storage element Ceramic capacitor Electrolytic capacitor Polyester film capacitor And more Analogy à Pool: as a water (charge) buffer to stabilize the water (current) flow Water in Current out C B. Wang, 2017 CPEG - Lecture 5 12
Capacitor Actions Without capacitor S 1 off 0V S 1 on S 1 off 0V + - + - + - S 1 off, lamp off With capacitor S 1 on, lamp on S 1 off, lamp off + - S 1 off C 0V + - S 1 on C ++++ ---- + - S 1 off C +++ +++ S 1 off, lamp off, V c =0 S 1 on, lamp on, V C = S 1 off, lamp gradually off, V C is discharged 13
Capacitor Network Capacitor in parallel C eq = i C i = C 1 + C 2 + + C n Capacitor in series 1 C eq = 1 i C i = 1 C 1 + 1 C 2 + + 1 C n 14
Homework1 Will be posted online tonight and is due on Sept. 25 th. 15