DMT 121 – ELECTRONIC 1

DMT 121 – ELECTRONIC 1 CHAPTER 2 DIODE APPLICATIONS

Load Line Analysis The analysis of electronic circuits can follow one of the two paths : • Actual characteristic or approximate model of the device. • Approximate model will be always used in the analysis VD= 0.7 V

Load Line Analysis • The load line plots all possible current (ID) conditions for all voltages applied to the diode (VD) in a given circuit. E / R is the maximum ID and E is the maximum VD. • Where the load line and the characteristic curve intersect is the Q-point, which specifies a particular ID and VD for a given circuit. Fig. 2.1 Drawing the load line and finding the point of operation

Load Line Analysis The intersection of load line in Fig. 2.2 can be determined by applying Kirchhoff’s voltage in the clockwise direction, which results in: Fig. 2.2 Series diode configuration ID and VD are the same for Eq. (2.1) and plotted load line in Fig. 2.2 (previous slide). Set VD = 0 then we can get ID, where Set ID = 0 then we get VD, where

Example • For the series diode configuration of Fig. 2.3a, employing the diode characteristics of Fig. 2.3b, determine VDQ, IDQ and VR Fig. 2.3 (a) Circuit; (b) characteristics.

Solution • From the result, plot the straight line across ID and VD. • The resulting load line appears in Fig. 2.4. The Q points occurred at • VDQ 0.78 VIDQ  18.5mA • VR=IRR=IDQR=(18.5 mA)(1k) =18.5 V

Figure 2.4

Example • For the series diode configuration of Fig. 2.13, determine VD, VR and ID. Solution:

Example • Repeat example 2.4 with the diode reversed Solution: Open Circuit

Diode as Rectifier • Rectifier: An electronic circuit that converts AC to pulsating DC. • Basic function of a DC power supply is to convert an AC voltage to a smooth DC voltage.

Half-Wave Rectifier • The diode conducts during the positive half cycle. • The diode does not conducts during the negative half cycle.

Fig. 2.44 Half-wave rectifier. Fig. 2.45 Conduction region (0  T/2). Fig. 2.46 Nonconduction region (T/2  T). Sinusoidal Input: Half Wave Rectification

Average Value of Half Wave Output Voltage The average value of the half-wave rectified output voltage (also known as DC voltage) is The process of removing one-half the input signal to establish a dc level is called half-wave rectification

Example What is the average value of the half-wave rectified voltage? Solution: Vm/π = 15.9 V

Effect of Barrier Potential(Silicon diode) • Applied signal at least 0.7 for diode to turn on (Vk = 0.7V) • i ≤ 0.7 V  diode in open circuit and o = 0V • When conducting, Vk=0.7V ,then o= i – Vk this cause reduction in o, thus reduce the resulting dc voltage level. • Now Vdc 0.318 (Vm – Vk)

Example Draw the output voltages of each rectifier for the indicated input voltages.

Peak Inverse Voltage (PIV) • PIV=peak input voltage and is the maximum voltage across the diode when it is not conducting/reverse bias. • Can be found by applying Kirchhoff’s voltage law. The load voltage is 0V so the input voltage is across the diode at tp.

Peak Inverse Voltage (PIV) • Because the diode is only forward biased for one-half of the AC cycle, it is also reverse biased for one-half cycle. • It is important that the reverse breakdown voltage rating of the diode be high enough to withstand the peak, reverse-biasing AC voltage. • PIV=Vm OR accurately • PIV (or PRV)  Vm • PIV = Peak inverse voltage • PRV = Peak reverse voltage • Vm = Peak AC voltage Diode must capable to withstand certain amount of repetitive reverse voltage

Full-Wave Rectifier • A full-wave rectifier allows current to flow during both the positive and negative half cycles or the full 360°. • Output frequency is twice the input frequency. • VDC or VAVG = 2Vm/π

Full Wave Rectifier Half Wave Rectifier Full-Wave Rectification • The rectification process can be improved by using more diodes in a full-wave rectifier circuit. • Full-wave rectification produces a greater DC output: Half-wave: Vdc =0.318Vm =Vm/π Full-wave: Vdc =0.636Vm =2Vm/π

Example Find average value of the full-wave rectified voltage?

Full-Wave Rectification • Center-Tapped Transformer Rectifier • Requires • Two diodes • Center-tapped transformer VDC=0.636(Vm)

Full-Wave Center Tapped • Current flow direction during both alternations. The peak output is about half of the secondary windings total voltage. • Each diode is subjected to a PIV of the full secondary winding output minus one diode voltage drop PIV=2Vm(out)+0.7V During positive half-cycles, D1 is forward-biased while D2 is reverse-biased. During negative half-cycles, D2 is forward-biased while D1 is reverse-biased.

Full-Wave Rectification • Bridge Rectifier • Four diodes are required • VDC = 0.636 Vm

Full-Wave Bridge Rectifier • The full-wave bridge rectifier takes advantage of the full output of the secondary winding. • It employs four diode arranged such that current flows in the direction through the load during each half of the cycle. During positive half-cycle of the input, D1 and D2 are forward-biased and conduct current. D3 and D4 are reverse-biased. During negative half-cycle of the input, D3 and D4 are forward-biased and conduct current. D1 and D2 are reverse-biased.

Full-Wave Bridge Rectifier • The PIV for a bridge rectifier is approximately half the PIV for a center-tapped rectifier. PIV=Vm(out)+0.7V

Summary of Rectifier Circuits Vm = peak of the AC voltage = Vp In the center tapped transformer rectifier circuit, the peak AC voltage is the transformer secondary voltage to the tap.

Power Supply Filters and Regulators • In most power supply – 60 Hz ac power line voltage  constant dc voltage • Pulsating dc output must be filtered to reduce the large voltage variation • Small amount of fluctuation in the filter o/p voltage - ripple

Power Supply Filters and Regulators – Capacitor-Input Filter

Power Supply Filters and Regulators – Capacitor-Input Filter • Surge Current in the Capacitor-Input Filter: • Being that the capacitor appears as a short during the initial charging, the current through the diodes can momentarily be quite high. To reduce risk of damaging the diodes, a surge current limiting resistor is placed in series with the filter and load. The min. surge Resistor values: IFSM = forward surge current rating specified on diode data sheet.

Power Supply Filters and Regulators • Regulation is the last step in eliminating the remaining ripple and maintaining the output voltage to a specific value. Typically this regulation is performed by an integrated circuit regulator. There are many different types used based on the voltage and current requirements.

Power Supply Filters and Regulators • How well the regulation is performed by a regulator is measured by it’s regulation percentage. There are two types of regulation, line and load. • Line and load regulation percentage is simply a ratio of change in voltage (line) or current (load) as a percentage.

Capacitor Input Filter – Ripple Voltage • Ripple Voltage: the variation in the capacitor voltage due to charging and discharging is called ripple voltage • Ripple voltage is undesirable: thus, the smaller the ripple, the better the filtering action • The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter. Figure (a) and (b) Easier to filter -shorted time between peaks. -smaller ripple.

Capacitor Input Filter – Ripple Voltage Ripple factor: indication of the effectiveness of the filter [half-wave rectifier] Vr(pp) = peak to peak ripple voltage; VDC = VAVG = average value of filter’s output voltage. • Lower ripple factor  better filter • [can be lowered by increasing the value of filter capacitor • or increasing the load resistance] • For the full-wave rectifier: Vp(rect) = unfiltered peak.

Diode Limiters (Clipper) Clippers are networks that employ diodes to “clip” away a of an input signal without distorting the remaining part of the applied waveform. Clippers used to clip-off portions of signal voltages above or below certain levels.

Diode Limiters (Clipper)

Example What would you expect to see displayed on an oscilloscope connected across RL in the limiter shown in above figure.

Solution

Biased Limiters (Clippers) • The level to which an ac voltage is limited can be adjusted by adding a bias voltage, VBIAS in series with the diode • The voltage at point A must equal VBIAS + 0.7 V before the diode become forward-biased and conduct. • Once the diode begins to conduct, the voltage at point A is limited to VBIAS + 0.7 V, so that all input voltage above this level is clipped off. A positive limiter

Biased Limiters (Clippers) • In this case, the voltage at point A must go below –VBIAS – 0.7V to forward-bias the diode and initiate limiting action as shown in the above figure. A negative limiter

Modified Biased Limiters (Clippers)

Example Figure above shows combining a positive limiter with a negative limiter. Determine the output voltage waveform?

Solution

Summary Limiters (Clippers) In this examples VD = 0 In analysis, VD = 0 or VD = 0.7 V can be used. Both are right assumption.

Diode Clampers • A clamper is a network constructed of a diode, a resistor, and a capacitor that shifts a waveform to a different dc level without changing the appearance of the applied signal. • Sometimes known as dc restorers • Clamping networks have a capacitor connected directly from input to output with a resistive element in parallel with the output signal. The diode is also parallel with the output signal but may or may not have a series dc supply as an added elements.

Diode Clampers Positive clamper operation. (Diode pointing up – away from ground)

Diode Clampers Negative clamper operation (Diode pointing down – toward ground)

Diode Clamper • If diode is pointing up (away from ground), the circuit is a positive clamper. • If the diode is pointing down (toward ground), the circuit is a negative clamper

DMT 121 – ELECTRONIC 1