DMT 121 ELECTRONIC DEVICES CHAPTER 2 DIODE APPLICATIONS

At the end of this class, students should be able to:- Understand the concept of load-line analysis and how it is applied to diode networks. Explain the process of rectification to establish a DC level from a sinusoidal AC input.

Load Line Analysis VD= 0.7 V 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

Fig. 2.1 Drawing the load line and finding the point of operation 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. Point of operation of a circuit Fig. 2.1 Drawing the load line and finding the point of operation

Fig. 2.2 Series diode configuration 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

Fig. 2.3 (a) Circuit; (b) characteristics. 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 V IDQ  18.5mA VR=IRR=IDQR=(18.5 mA)(0.5k) = 9.25 V

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.

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

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) Vi ≤ 0.7 V  diode in open circuit and Vo = 0V When conducting, Vk=0.7V ,then Vo= Vi – Vk  this cause reduction in Vo, 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 inverse 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 Rectification Half Wave Rectifier Full Wave Rectifier 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?

Turns ratio, n = Nsec/Npri V(sec) = nV(pri) (in RMS value) Transformer Coupling Turns ratio, n = Nsec/Npri V(sec) = nV(pri) (in RMS value) Vp(sec)=√2 x V(sec)

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

PIV: Full-wave Rectifier Center-Tapped Transformer PIV can be shown by applying KVL for the reverse-biased diode. PIV across D2: 1 2 3 Substitute 4 to 2: PIV=2Vp(out) + 0.7 V 4

Example Show the voltage waveforms across each half of the secondary winding and across RL when a 100V peak sine wave is applied to the primary winding. What minimum PIV rating must the diodes have.

Solution 1. 2. PIV = 49.3 V

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 4 diodes 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.

PIV: Full-wave Rectifier Bridge Transformer Vp(out)=Vp(sec) – 1.4 V PIV=Vp(out) + 0.7 V

Example The transformer is specified to have a 12 Vrms secondary voltage for the standard 120 V across the primary. Determine the peak output voltage for the bridge rectifier. Assuming the practical model, what PIV rating is required for the diodes?

Solution 1. Vp(out) = 15.6 V 2. PIV = 16.3 V

Summary of Rectifier Circuits Ideal VDC Practical (approximate) VDC PIV Half-Wave Rectifier VDC = 0.318(Vm) = Vm/π VDC = 0.318(Vm)-0.7 PIV=Vm Full-Wave Bridge Rectifier VDC = 0.636(Vm) =2 Vm/π VDC = 0.636(Vm)-2(0.7) PIV=Vm+0.7V Center-Tapped Transformer Rectifier VDC = 0.636(Vm)-(0.7) PIV=2Vm+0.7V 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 Filtering is the process of smoothing the ripple from the rectifier.

Power Supply Filters and Regulators – Capacitor-Input Filter The capacitor input filter is widely used. A half-wave rectifier and the capacitor-input filter are shown.

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. A voltage regulator can furnish nearly constant output with excellent ripple rejection. 3-terminal regulators are require only external capacitors to complete the regulation portion of the circuit.

Power Supply 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 regulation: how much the dc output changes for a given change in regulator’s input voltage. Load regulation: how much change occurs in the output voltage for a given range of load current values from no load (NL) to full load (FL) Line regulation Load regulation

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.

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 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 Ripple factor: indication of the effectiveness of the filter Vr(pp) = peak to peak ripple voltage; VDC = VAVG = average value of filter’s output voltage [half-wave rectifier]

Example Determine the ripple factor for the filtered bridge rectifier with a load as indicated in the figure above.

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 Limiter/Clipper A diode limiter is a circuit that limits (or clips) either the positive or negative part of the input voltage.

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) A positive limiter 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.

Biased Limiters (Clippers) A negative limiter 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.

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.

Summary Limiters (Clippers)

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.

Clamper A clamper (dc restorer) is a circuit that adds a dc level to an ac signal. A capacitor is in series with the load. Positive clamper – the capacitor is charged to a voltage that is one diode drop less than the peak voltage of the signal. Vout = Vp(in) – 0.7 V Negative clamper Vout = -Vp(in) + 0.7 V Start with forward-bias!

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

Negative clamper operation (Diode pointing down – toward 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

Diode Clamper (Square Wave) Diode ‘OFF’ state Diode ‘ON’ state -V - Vc - Vo = 0; Vc = V Vo = -2 V V – Vc = 0 ; Vc = V; Vo = 0.7 V but ideal Vo = 0V Output

Summary of Clamper Circuits

Voltage Multipliers Voltage multiplier circuits use a combination of diodes and capacitors to step up the output voltage of rectifier circuits. Voltage Doubler Voltage Tripler Voltage Quadrupler

Voltage Doubler Vout = VC2 = 2Vm This half-wave voltage doubler’s output can be calculated by: Vout = VC2 = 2Vm where Vm = peak secondary voltage of the transformer

Half-Wave Voltage Doubler Positive Half-Cycle D1 conducts D2 is switched off Capacitor C1 charges to Vp Negative Half-Cycle D1 is switched off D2 conducts Capacitor C2 charges to Vp Vout = VC2 = 2Vp

Full-Wave Voltage Doubler Positif Half-Cycle D1 forward-biased → C1 charges to Vp D2 reverse-biased Negative Half-Cycle D1 reverse-biased D2 forward-biased → C2 charges to Vp Output voltage=2Vp (across 2 capacitors in series

Voltage Tripler and Quadrupler

Voltage Tripler Positive half-cycle: C1 charges to Vp through D1 Negative half-cycle: C2 charges to 2Vp through D2 Positive half-cycle: C3 charges to 2Vp through D3 Output: 3Vp across C1 and C3

Output: 4Vp across C2 and C4 Voltage Quadrupler Output: 4Vp across C2 and C4

The Diode Data Sheet The data sheet for diodes and other devices gives detailed information about specific characteristics such as the various maximum current and voltage ratings, temperature range, and voltage versus current curves (V-I characteristic). It is sometimes a very valuable piece of information, even for a technician. There are cases when you might have to select a replacement diode when the type of diode needed may no longer be available. These are the absolute max. values under which the diode can be operated without damage to the device.

The Diode Data Sheet (Maximum Rating) Symbol 1N4001 1N4002 1N4003 UNIT Peak repetitive reverse voltage Working peak reverse voltage DC blocking voltage VRRM VRWM VR 50 100 200 V Nonrepetitive peak reverse voltage VRSM 60 120 240 rms reverse voltage VR(rms) 35 70 140 Average rectified forward current (single-phase, resistive load, 60Hz, TA = 75oC Io 1 A Nonrepetitive peak surge current (surge applied at rated load conditions) IFSM 30 (for 1 cycle) Operating and storage junction temperature range Tj, Tstg -65 to +175 oC

The Diode Data Sheet (Maximum Rating)

Zener Diodes The zener diode – silicon pn-junction device-designed for operate in the reverse-biased region Zener diode symbol Schematic diagram shown that this particular zener circuit will work to maintain 10 V across the load

Zener Diodes Breakdown voltage – set by controlling the doping level during manufacture When diode reached reverse breakdown – voltage remains constant- current change drastically If zener diode is FB – operates the same as a rectifier diode A zener diode is much like a normal diode – but if it is placed in the circuit in reverse bias and operates in reverse breakdown. Note that it’s forward characteristics are just like a normal diode. 1.8V – 200V

Zener Diodes The reverse voltage (VR) is increased – the reverse current (IR) remains extremely small up to the “knee”of the curve Reverse current – called the zener current, IZ At the bottom of the knee- the zener breakdown voltage (VZ) remains constant although it increase slightly as the zener current, IZ increase. IZK – min. current required to maintain voltage regulation IZM – max. amount of current the diode can handle without being damage/destroyed IZT – the current level at which the VZ rating of diode is measured (specified on a data sheet) The zener diode maintains a constant voltage for value of reverse current rating from IZK to IZM

Zener Diodes (Zener Equivalent Circuit) Since the actual voltage is not ideally vertical, the change in zener current produces a small change in zener voltage By ohm’s law: Normaly -Zz is specified at IZT Zener impedance

Zener Diodes (Temp Coeff & Zener Power Dissipation and Derating) As with most devices, zener diodes have given characteristics such as temperature coefficients and power ratings that have to be considered. The data sheet provides this information

Zener Diodes Applications Zener diode can be used as Voltage regulator for providing stable reference voltages Simple limiters or clippers

Zener Regulation with Varying Input Voltage As i/p voltage varies (within limits) – zener diode maintains a constant o/p voltage But as VIN changes, IZ will change, so i/p voltage variations are set by the min. & max. current value (IZK & IZM) which the zener can operate Resistor, R –current limiting resistor

Zener Regulation with a Variable Load The zener diode maintains a nearly constant voltage across RL as long as the zener current is greater than IZK and less than IZM When the o/p terminal of the zener diode is open (RL=∞)-load current is zero and all of the current is through the zener When a load resistor (R) is connected, current flow through zener & load RL, IL, IZ The zener diode continues to regulate the voltage until IZ reaches its min value , IZK At this point, the load current is max. , the total current through R remains essentially constant.

Zener Limiting Zener diode also can be used in ac applications to limit voltage swings to desired level (a) To limit the +ve peak of a signal voltage to the selected zener voltage - During –ve alternation, zener arts as FB diode & limits the –ve voltage to -0.7V (b) Zener diode is turn around -The –ve peak is by zener action & +ve voltage is limited to +0.7V (c) Two back-to-back zeners limit both peaks to the zener voltage ±7V -During the +ve alternation, D2 is functioning as the zener limiter – D1 is functioning as a FB diode. -During the –ve alternation-the roles are reversed