5.6 Switching regulator-SMPS
The switching regulators are also called as switched mode regulators(Switched Mode Power Supply). Such a switching regulator requires an external transistor and a choke. The series pass transistor in such a regulator is used as a controlled switch and is operated in cut-off region or saturation region. Hence, the power transmitted across such a transistor is in the form of discrete pulses rather than a steady flow of current.
When the transistor is operated in the cut-off region, there is no current and it dissipates no power. When it is operated in the saturation region, a negligible voltage drop appears across it and hence dissipates very small power, providing maximum current to load.
In any case, the power dissipated in the transistor is very small. The entire power gets transmitted to the load,hence the efficiency of the switching regulators is always very high.
Principle of switching Regulator
The pulse width modulation is the basic principle of the switching regulators. The average value of repetitive pulse waveform is proportional to the area under the waveform. So switching regulators use the fact that, if duty cycle of the pulse waveform is varied, the average value of the voltage also changes proportionally. Basic switching regulator is shown in the figure to obtain the switching regulator.
Basic switching regulator
A voltage source Vin is a d.c. supply which is a battery. It has to satisfy the following requirements:
i. It has to supply required power.
ii.It must be high enough to satisfy the minimum requirements of the regulator.
iii.It must be large enough to supply sufficient dynamic range of line and load changes.
The switch is generally a transistor. The pulse generator output makes it ON and OFF. The pulse generator produces a required pulse waveform. The most effective range of pulse waveform frequency is 20kHz. The typical operating frequency range is 10 to 50 kHz. The filter F1 may be RC, RL or RLC. Most commonly used filter is RLC It converts the pulse waveforms obtained from the switch into a d.c. output voltage.
Block Diagram of SMPS
The figure shows the functional block diagram of basic switching voltage regulator, which uses transistor Q1 as a switch. The part R2/R1 + R2 of the output is fedback to the inverting input of error amplifier. It is compared with the reference voltage.
The difference is amplified and given to the comparator inverting terminal. The oscillator generates a triangular waveform at a fixed frequency. It is applied to the non-inverting terminal of the comparator.
The output of the comparator is high, when the triangular voltage waveform is above the level of the error amplifier output. Due to this, the transistor Q1 remains in cut-off state. Thus, the output of the comparator is nothing but a required pulse waveform.
Functional block diagram of switching regulator
The period of this pulse waveform is same as that of oscillator output say T. The duty cycle is denoted as δ = ton/T. This duty cycle is controlled by the difference between the feedback voltage and the reference voltage.
When Q1 is ON in saturation state, VCE(sat) for Q1 is zero. Hence, entire input voltage Vin appears at point A. Thus, the current flows through inductor L1. When Q1 is OFF, L1 continues to supply current to the load. The diode D1 provides the return path for the current.
The capacitor C1 acts to smooth out the voltage and the voltage at the output is almost d.c. in nature. The output voltage V0 of the switching regulator is a function of duty cycle and the input voltage Vin. Mathematically it is expressed as,
Thus when T is constant, output is proportional to ton. This method is called pulse width modulation (PWM). When ton is constant, the output is inversely proportional to the period T. i.e., proportional to frequency of the pulse waveform. This method is called frequency modulation as shown in Figure.
A high switching frequency allows small values of L1 and C1 and thus reduces size, cost and weight. It also reduces the ripple at the output. But the efficiency decreases and electrical noise increases.
On the other hand, low switching frequency improves efficiency and reduces noise but requires large filtering components. As a result of this, the range of operating frequency to get maximum efficiency is 10 to 50 kHz.
Pulse width modulation and average value
Types of Switching Regulators
The term switched mode regulator is used to describe a circuit which takes d.c. input (unregulated) and provides a single d.c output of the same or opposite polarity and of a lower or higher voltage. The switched mode regulators use an inductor and there is no input to output isolation.
Converters use transformer and may provide input to output isolation. There are three basic configurations of the switching regulators.
1.Step down or Buck switching regulator.
2.Step up or Boost switching regulator.
3.Inverting type switching regulator.
Applications of SMPS :
Various types of SMPS are used in the following variety of applications such as
1.Adjustable high voltage constant current sources.
2.Battery powered systems.
3.Telecommunication circuits.
4.Personal computers.
5.Printers.
6.Video games.
Step Down Switching Regulator using 78S40
The step down switching regulator can be obtained using μA 78S40. The output saturation voltage is defined as the switching element voltage for Q2 and Q1 in the Darlington pair with the collectors tied together. It is typically 1.1 V and 1.3 V.
The operation of μA 78S40 as a typical step down switching regulator, for an output voltage of 10 V is shown in the Figure. The input is 25 V and output is 10 V which ensures the step down operation of the regulator.
Step down switching regulator with μA 78S40
5.7 LM 380 power amplifier
Pin diagram of LM 380
Pin LM 380 DIP package
Figure shows the schematic diagram of LM 380 comprised of four stages namely:
1)PNP emitter follower Stage
2)Differential amplifier Stage
3)Common emitter Stage
4)Quasi-complementary emitter follower Stage
The internal circuit diagram is shown in Figure.
Internal circuit diagram of LM 380
The PNP transistors Q1 and Q2 for an emitter follower input stage drives PNP Q3-Q4, the differential amplifier.
The choice of PNP input transistors Q1 and Q2 allows the input to be referenced to the ground, that is, the input can be directly coupled to either the inverting ( pin 6) or the non-inverting (pin 2) terminals of the amplifier. The current in the PNP differential pair Q3 – Q4 is divided by the Q7, R3 and +VCC.
The current mirror of Q7 and Q8 and the associated resistors then establishes current through collector of Q9. The transistor Q5– Q6 acts as active load for the PNP differential pair.
The output of the differential pair is taken as the function of Q4 and Q6 transistors and is applied as an input to the common emitter voltage gain stage. The transistor Q9 with diodes D1 and D2 form the common emitter voltage gain stage where Q8 acts as a current source load.
The capacitor is a feedback capacitor and is used for frequency compensation to stabilize the amplifier against any type of oscillations.
The diodes D1 and D2 are used to develop a small voltage across the base emitter function of cross over Q11, so as to minimize the cross over distortion. The resistance R6 and R7 are used for current limiting.
The output stage is a quasi (false) complementary pair emitter follower formed by NPN transistor Q10 and Q12. In fact, the combination of PNP transistor Q11 and NPN transistor Q12 have the power capability of NPN transistor but the characteristics of a PNP transistor.
The resistors R1 and R2 provide a d.c return path for the input bias current so that the amplifier can be operated with either input terminal pin. The overall internal voltage gain of the amplifier is fixed at 50.
5.8 ICL 8038 function generator IC
Function generators are designed to provide the basic waveforms such as square wave, triangular wave and sine wave. They are also called as waveform generators.
The monolithic function generators provide these basic waveforms with a minimum number of external components reducing complexity, but increasing the reliability of the circuit. It is used in communication, telemetry, electronic music and testing and calibration in the laboratory.
In function generators, VCO (voltage controlled oscillator )generates the triangular and square waves. The triangular wave is passed through the wave sharper to generate a sine wave.
The sawtooth and pulse waveforms are generated by configuring the oscillator for highly asymmetric duty cycle.
5.8.1 Basic Principle
It is very easy to understand working principle of ICL 8038 function generator using its simplified block diagram, as shown in Figure.
Simplified block diagram of ICL 8038 function generator
The operation of ICL 8038 is based on charging and discharging of a grounded capacitor C. Charging, discharging rates of C are controlled by programmable current generators IA and IB respectively.
When switch is at position A, the capacitor charges at a rate determined by current source IA. Once the capacitor voltage reaches the value the switch position changes from position A to B. Now, capacitor starts discharging at the rate determined by the current sink IB.
Once the capacitor reaches VLT, the lower comparator (CMP2 triggers and sets the flip-flop output. This causes the switch position changes from position B to A and this cycle repeats. As a result, we get square wave at the output of flip-flop and triangular wave across capacitor. The triangular wave is then passed through the wave shaper to generate sine wave.
To allow automatic frequency control, currents IA and IB are made programmable through an external control voltage Vi. For equal magnitudes of IA and IB, output waveforms are symmetrical conversely, when two currents are unequal, output waveforms are asymmetrical. By making one of the currents much larger than other, we can get saw tooth waveform across capacitor and rectangular wave at the output of flip-flop.
Circuit diagram for ICL 8038
As shown in figure, transistors Q1 and Q2 form programmable current sources whose magnitudes are set by external resistors RA and RB. These current sources are driven by the emitter follower (transistor Q3).
It also compensates base-emitter voltage drops for Q1 and Q2 to ensure VRA = VRB = Vi. Thus, IA = Vi / RA and IB = Vi / RB. The current IA controls the charging rate of capacitor C.
The current IB is diverted to current mirror (Q4–Q5–Q6), where it undergoes polarity reversal as well as amplification by 2 due to the combined action of Q5 and Q6. The result is a current sink of magnitude of 2 IB, as shown in Figure.
The voltage across capacitor is applied to the schmitt trigger. The schmitt trigger shown in figure is similar to that of the IC555, with VUT =2/3 VCC and VLT =1/3 VCC.
Transistor Q7 acts as a switch. When output of flip-flop is high, Q7 saturates and pulls the bases of Q5 and Q6 low, thus shutting off the current sink. As a result, capacitor C starts charging at a rate set by current IA.
Once the capacitor voltage reaches 2/3 VCC(VUT), CMP1 triggers and clears the flip-flop, thus turning Q7 off. This enables current mirror to sink current equal to 2IB, so that net current flowing out of the capacitor is IL=2IB–IA. This causes capacitor to discharge.
Once capacitor voltage reaches 1/3 VCC (VLT). CMP2 triggers and sets the flip-flop and action repeats. It is important to note that net current flowing out of capacitor C should be positive i.e. 2 IB–IA > 0 discharging capacitor and hence 2 IB>IA.
Frequency of Output Waveform
The frequency of the output waveform can be determined as follows:
where T = TC + Td
The charging time Tc can be given as
The discharging time Td can be given as
Multiplying RA by numerator and denominator we get,
Duty cycle is given as
With RA = RB, duty cycle is 50% and we get symmetric waveforms. For symmetric waveforms i.e. RA = RB, the frequency of the waveform is given as,