13/02/2016. Diode Applications

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1 Diode Applications Introduction to diode circuits DC and AC diode circuits Diode applications Clippers Clampers Limiters Peak rectifiers Voltage multipliers Voltage regulators (with Zener diodes) Rectifiers Properties of electrical signals Average value (DC) Effective value (RMS) Half-wave rectifier Full-wave rectifier 1

2 Diode Approximations In Forward Bias: Silicon Diode: V D(ON) = 0.7V Germanium Diode: V D(ON) = 0.3V In Reverse Bias: Both diodes act like opens V D = source voltage and I D = 0A Diode in DC Series Circuit: Forward Bias The diode is forward biased. V D = 0.7V (or V D = E if E < 0.7V) V R = E V D I D = I R = V R /R 2

3 Diode in DC Series Circuit: Reverse Bias The diode is reverse biased. V D = E V R = 0V I D = I R = 0A Diode in any DC Circuit Solve this circuit like any Series/Parallel circuit, knowing V D = 0.7V (or up to 0.7V) in forward bias and as an open in reverse bias. V D1 = V D2 = V D(ON) = 0.7V V R = 9.3V Diodes are used in parallel to limit current: I R = E V D = 10V - 0.7V = 28mA R 0.33k I D1 = I D2 = 28mA/2 = 14mA What will happen if D 2 is replaced by a Germanium diode? 3

4 Clipper Diode Circuits Positive Half Cycle (using ideal diode model) Negative Half Cycle (using ideal diode model) The diode only conducts when it is in forward bias, therefore only half of the AC cycle passes through the diode. 4

5 Let us also plot the voltage transfer characteristics (VTC), i.e., v o vs. v i for the clipper circuit above. We already found that Thus, diodes clip a portion of the AC wave. The diode clips any voltage that does not put it in forward bias. That would be a reverse biasing polarity and a voltage less than 0.7V for a silicon diode. 5

6 Variations of the Clipper Circuit By adding a DC source to the circuit, the voltage required to forward bias the diode can be changed. Changing Output Perspective By taking the output across the diode, the output equals to the input voltage when the diode is not conducting. A DC source can also be added to change the diode s required forward bias voltage. 6

7 Clipper Examples Series clipper circuits A parallel clipper circuit and its output Series Clipper Circuits Summary 7

8 Parallel Clipper Circuits Summary Clampers (DC Restorers) A diode in conjunction with a capacitor can be used to clamp an AC signal to a specific DC level. Shifts the signal in the direction of the diode arrow by (V p -V 0 ) Discharge constant: 5 >> T / 2 (T is the period of the input signal and capacitor is discharging fully in five time constants, = R C ) e.g. = R C > 5T 8

9 Operation in a clamper circuit Input signal Diode on and the capacitor charging to V volts Determine v o with the diode off. Output signal Operation in a clamper circuit -2 A clamper circuit and its output 9

10 Variations of Clamper Circuits The input signal can be any type of waveform: sine, square, triangle wave, etc. You can adjust the DC camping level with a DC source. Variations of Clamper Circuits A clamper circuit and its output 10

11 Summary of Clamper Circuits Diode Limiter (Dual-Diode Parallel Clipper) Input Output vs Input Output 11

12 Peak Rectifier Input When a load R L is connected, the discharge constant = R L C should have a very high value compared to the half period of the signal, otherwise ripples are observed at the output voltage, i.e. R L should have a very large value. Output Voltage Multiplier Circuits 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 12

13 Voltage Doubler This half-wave voltage doubler s output can be calculated as V out = V C2 = 2V m V m = peak secondary voltage of the transformer. Operation of a Voltage Doubler Circuit The 1 st capacitor charges up to V m during the positive half of the cycle, then the 2 nd capacitor charges up to V m in the same polarity as the 1 st capacitor, finally the output is the sum of the voltages across both capacitors: V out = 2V m 13

14 Voltage Tripler and Quadrupler Circuits By adding more diode-capacitor networks the voltage can be increased. Zener Diode The Zener is a diode operated in reverse bias at the Zener Voltage (V z ). 14

15 Zener Calculations Determine the state of the Zener: if V i V Z, then the Zener is biased on ; the Zener is at V Z if V i < V Z, then the diode is biases off ; V Z = V i For V i V Z : The Zener voltage The Zener current I Z = I R -I L The Zener Power P Z = V Z I Z For V i < V Z : The Zener acts like an open. Load Resistance in a Zener Circuit The size of the load resistor affects the current in the Zener. R L is too large Not enough current through the Zener and it is biased off. The minimum current for a Zener is given as I ZK in the data sheets. R L is too small Too much current in the Zener and it avalanches and is quickly destroyed. The maximum current for a Zener is given as I ZM in the data sheets. 15

16 Finally, we arrive at the following equations where where Exercise (Midterm 1, ) In the figure, V s is an unregulated voltage that varies between 6V and 7V while the Zener diode voltage is V Z = 5V. The load resistor R L can have a value from 100 to (i.e. open circuit). Also you can take I Zmin 0 A. a) Find the maximum value of R 1 so that the load voltage V L would be still kept constant at 5V for all values of R L and V s. b) Provide a symbolic expression for the maximum power dissipated by the Zener diode. c) Determine the minimum value of R 1 so that the power dissipated by the Zener diode does never exceeds 1W for all values of R L and V s. 16

17 Other Zener Regulators A single Zener diode can limit one side of a sinusoidal waveform to the zener voltage while clamping the other side to near zero. With two opposing zeners, the waveform can be limited to the zener voltage on both polarities. Zener limiter circuits Zener Diode Parameters The basic parameters of a zener diode are: a) Obviously, the zener voltage must be specified. The most common range of zener voltage is 3.3 volts to 75 volts, however voltages out of this range are available. b) A tolerance of the specified voltage must be stated. While the most popular tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are available. A test current (I z ) must be specified with the voltage and tolerance. c) The power handling capability must be specified for the zener diode. Popular power ranges are: 1/4, 1/2, 1, 5, 10, and 50 Watts. 17