Supply circuits Voltage rectifier and regulator circuits

Transcription

1 Supply circuits Voltage rectifier and regulator circuits Prepared by: Józef Maciak Agnieszka Zaręba Jakub Walczak I. Design of half wave and full wave rectifiers Due to its I-V characteristic, the simplest half wave rectifier is a diode. Fig. 1a shows a diagram of a half wave rectifier with a diode, and Fig. 1b shows an idealized I-V characteristic along with the input and output voltage transients. The positive half wave of the signal is passed through by the diode, whereas the negative one is blocked. In fact, the positive half wave of the output signal has a little bit smaller amplitude since a portion of the input voltage is lost across the non-zero resistance of the real diode. Moreover, the diode-based rectifier has a limited range of signal frequencies it can process, due to frequency limitations of the semiconductor device itself. Also, a full wave rectifier may be built using diodes. The simplest circuit, composed of 4 diodes in a form of a diode bridge is known as the Graetz bridge (Fig. 2a). In this circuit diodes work in pairs: for the positive half of the input signal diodes D1 and D2 are forwardly biased, and for the negative half diodes D2 and D3 are forwardly biased. Supply circuits. Voltage rectifier and regulator circuits 1

2 Fig. 1. a) Diagram of a half wave rectifier with a diode; b) The I-V characteristic of an ideal diode along with the corresponding voltage transients. Fig. 2. a) Diagram of a full wave rectifier with a Graetz bridge; b) The input and output voltage transients; c) Symbol of a Graetz bridge; II. Frequency limitations of a semiconductor p-n diode The diode AC parameters depend on the signal waveform and frequency. For sinusoidal voltage signals with small enough amplitudes (so called small signal operation), the section of the diode I-V characteristic corresponding to the signal peak-to-peak value may be approximated by a straight line. Then, the diode operation is described by small signal parameters defined by derivatives (hence, another name for the parameters is differential parameters). Depending on the ratio of the signal frequency Supply circuits. Voltage rectifier and regulator circuits 2

3 to the rate of nonstationary states within the p-n junction, the small signal parameters may be divided into the small frequency, medium frequency, and high frequency parameters. The small frequency differential parameters are static or quasi-static parameters being real numbers. However, the medium and high frequency parameters are dynamic parameters being complex numbers containing imaginary terms. Three types of p-n diode capacitance affect the dynamic operation and have to be considered in the equivalent model: - Differential capacitance of the depletion layer, related to the charge of exposed dopant ions; - Differential diffusion capacitance, related to collecting excess electronhole pairs in next-to junction regions of the diode; and - Capacitance of the diode casing. Out of the three, the diffusion capacitance is the most significant one for the p-n diode when operating at medium/high frequencies or with pulse signals, and it limits the use of the p-n diode in the range of higher frequencies. Reloading this capacitance after a sudden change of the polarization from the forward to the reverse direction requires a relatively long time (storage time, t s, recovery time t r, see: Laboratory 2: switching the p-n diode). Much better dynamic operation presents the Schottky diode. This type of diode is based on the rectifying metal-semiconductor junction, within which the charge stored is much smaller, and therefore the storage times are up to 1000 times smaller than in p-n diodes. Figure 3 shows oscilloscope screens of input voltage transient (sinusoidal) and output voltage transients (half wave, output of the circuit of Fig. 1a), comparing the two types of the diodes, i.e., the p-n diode (first column) and the Schottky diode (second column) in dynamic operation at Supply circuits. Voltage rectifier and regulator circuits 3

4 Electronics 1 - Laboratory 4: Supply Circuits four frequencies from 100 to Hz. For the p-n diode, excess carriers collect in the base during the forward half wave of the input pulse. After reversing the pulse, the carriers are gradually removed by a significant discharging reverse current that decreases the mean value of the rectified current. This effect becomes stronger along with an increase of the frequency. When the duration of the half wave becomes comparable with the lifetime of the excess carriers, the diode remains open (is still conducting) during the whole half wave of the reverse polarization, and thus the rectifying operation of the diode stops. As can be seen, for the Schottky diode merely a slight difference in the output voltage waveform only can be observed at a frequency as high as 100 khz. For both diodes one can see the voltage drop across the diode resistance, the voltage drop being smaller for the Schottky diode. p-n diode Schottky diode Fig. 3. Comparison of the operation of a p-n diode and a Schottky diode employed in a half wave rectifier and operating at different frequencies. Supply circuits. Voltage rectifier and regulator circuits 4

5 III. DC voltage regulators The simplest voltage regulator may be built using a Zener diode (Fig. 4) operated in the reverse direction. For semiconductor diodes, exceeding some critical value of the reverse bias (the breakdown voltage, V BR ) generates a rapid increase of the reverse current - the breakdown. Generally, this may lead to destruction of the diode. However, the Zener diodes are dedicated and designed intentionally for the operation in this range of the I-V characteristic. Fig. 4. a) Diagram of a voltage regulator with a Zener diode; b) I-V characteristic of the Zener diode; changes of the voltages and the currents have been indicated. In the circuit of Fig. 4a, after exceeding a well defined threshold voltage in the reverse direction (so called Zener voltage), the diode reverse current increases rapidly. However, the device is not getting damaged due to the presence of the current-limiting resistor R. Moreover, event large Supply circuits. Voltage rectifier and regulator circuits 5

6 changes of the diode current, I Z, are associated with merely slight changes of the diode voltage, V Z. One can assume that the voltage across the diode is constant being equal to the Zener voltage V Z. On the other hand, the circuit of Fig. 4a may be treated as a voltage divider: the negligible change of the output voltage V out is a result of dividing the change of the input voltage V in between the resistances R and R Z (R Z - the resistance of the reversely biased Zener diode): V out = V in R Z RZ + R The most important parameter of the regulator is the output voltage stabilization coefficient (also: line regulation or input regulation), defined as a ratio of output voltage change to input voltage change: S V V = V out in RZ = R + R From the above expression it is clear that in order to obtain good voltage stabilization, i.e., a small value of the stabilization coefficient, S V (typically ), the resistance R should be much bigger than the diode resistance R Z. Typically R Z ranges from several to several tens of ohms and depends on the current flowing through the diode. So, increasing the R resistance, one can increase the stabilization but, simultaneously, decreases the output current of the regulator. This is a significant limitation of the circuit of Fig. 4a. Z The regulator circuit may be improved by applying a negative loop feedback. Fig. 5 shows diagram of a regulator circuit with a negative loop feedback that comprises the following functional blocks: Supply circuits. Voltage rectifier and regulator circuits 6

7 - Comparator amplifier circuit (so called error amplifier with the transistor Tw); - Reference voltage source (Zener diode); - Output voltage divider (so called sampling circuit); - Control transistor Tr (emitter follower). Fig. 5. Diagram of a voltage regulator with a feedback loop The above circuit tracks the difference between the calibrated value of the output voltage (here: (10/11) V out stab ) and the reference voltage (the Zener diode voltage, here: V Z = 5.6V). This difference is input to the error amplifier. In this circuit, the error signal polarizes the base-emitter junction of a BJT transistor Tw (voltage V BEQ ). A change of V BEQ results in a change of the collector current I CQ, this in turn controlling the base potential of transistor Tr. In consequence, one obtains a stabilization of the output Supply circuits. Voltage rectifier and regulator circuits 7

8 voltage. In the circuit of Fig. 5: V BEQ 0.7 V = (10/11) V out stab V. Therefore, V out stab 7 V. In a regulator with a negative feedback loop, the output current is the emitter current of a BJT, so higher output currents are possible then in the circuit of Fig. 4a. Questions: 1. Give an example of a half wave rectifier. Explain its principle. Draw the circuit diagram. 2. Give an example of a full wave rectifier. Explain its principle. Draw the circuit diagram. 3. Give an example of a voltage regulator with Zener diode. Explain its principle. Draw the circuit diagram. Supply circuits. Voltage rectifier and regulator circuits 8