LICA 6 - Everything you need to know about voltage regulators PDF

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Everything you need to know about voltage regulators...

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

CHAPTER – 6: 6 – Hrs IC Voltage Regulators: Fixed voltage regulators, Adjustable voltage regulators, switching regulators, linear voltage regulators IC 723, Design of low voltage regulator and high voltage regulator using IC 723. The function of a voltage reference/regulator is to provide a stable dc voltage VO starting from a less stable power source VI . The general setup is depicted in Fig 1. In the case of a regulator, VI is usually a poorly specified voltage, such as the crudely filtered output of a transformer and diode rectifier. The regulated output VO is then used to power other circuits, collectively referred to as the load and characterized by the current IO that the load draws from the regulator. In the case of a voltage reference, VI is already regulated to some degree, so the function of the reference is to produce an even more stable voltage VO to serve as a standard for other circuits.

a constant output of performance al coefficient. In the bility are also ves a measure of the rying input ypically an r quality than the is usually derived bridge rectifier, and a eference to the resulting from a change ΔVI at the input. Line regulation is expressed in millivolts or microvolts per volt, depending on the case. An alternative definition is with the units being percent per volt. As you consult the catalogues, you will find that both forms are in use. O

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

io (RRR), expressed in decibels as put ripple resulting from a ripple connection with voltage ount of ripple (usually 100-Hz t’s ability to maintain the conditions, or rs should behave like ideal voltage dless of the load current. The i –v e positioned at v = VO. A practical put impedance whose effect is a nce is expressed via the load er ampere, depending on the finition ses the above dependence in icient VO), gives a measure of the tput voltage VO under varying , microvolts per degree Celsius, or LINEAR REGULATORS: Figure2 shows the basic ingredients of a voltage regulator. The circuit uses the Darlington pair Q1-Q2, also called the series-pass element, to transfer power from the unregulated input source VI to the output load at a prescribed regulated voltage VO. (In MOS technology, the series-pass element is a power FET.) The feedback network R1-R2 samples VO and feeds a portion thereof to the error amplifier EA for comparison against a reference VREF. The amplifier provides the series-pass element with whatever drive it takes to force the error close to zero. The regulator is a classic example of series-shunt feedback, and it can be viewed as a noninverting op amp that has been equipped with a Darlington current booster to give 2

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

Fig.2 Simplified circuit diagram of a typical bipolar positive voltage regulator. Since the error amplifier provides currents on the order of milli-amperes and the load may draw currents on the order of amperes, a current gain on the order of 103 A/A is required. A single power BJT is usually insufficient, so a Darlington pair is used instead, whose overall current gain is β ≈ β1 ×β2. We observe that for an npn BJT to work in the forward-active region, where IC =β IB, the conditions vBE = VBE(on) and vCE ≥ VCE(sat) must hold. A low-power BJT has typically β ≈ 100, VBE(on) ≈ 0.7 V, and VCE(sat) ≈ 0.1 V; a power BJT may have β ≈ 20, VBE(on) ≈ 1 V, and VCE(sat) ≈ 0.25 V. If a power MOSFET is used as the series-pass element, then just one transistor suffices because the gate terminal draws virtually zero current. Protections: The reliable performance of a power BJT is critically affected by powerdissipation capabilities, current and voltage ratings, maximum junction temperature, and second breakdown, a phenomenon resulting from the formation of hot spots within the BJT, which cause uneven sharing of the total load among different regions of the device. The above factors define a restricted region of the iC-vCE characteristic, known as the safe operating area (SOA), within which the device can be operated without the risk of failure or performance degradation. Voltage regulators are equipped with special circuitry to protect the power stage against current overload, second breakdown, and thermal overload. Each circuit is designed to be inactive under normal operating conditions, but to become active as soon as an attempt is made to exceed the corresponding safety limits. Current overload protection is dictated by maximum power-rating considerations. 3

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

Since the power dissipated by the series-pass BJT is P ≈ (VI − VO)IO, we must ensure IO ≤ Pmax/(VI −VO) for safe operation. The protection scheme of Fig. 3, uses a brute force approach to keep IO below the limit Isc = Pmax/VI, which occurs when the output is short-circuited to ground, or VO = 0. As we know, the design equation is ss BJT within its SOA, its collector current must be ector-emitter voltage rises above a safety level, a likely e transients are present on the unregulated input line. mented with the Zener diode Dz, as shown in Fig. 2. cut-off, is designed to turn on as soon as VI rises above nt supplied by Dz will then turn on Q3 and divert current he series-pass BJT, as in the case of current overload. decouple the base of Q3 from the low-impedance emitter o limit the current through Dz, particularly in the spikes on the input line.

Fig.3 Output overload protection. Excessive self-heating may cause permanent damage to BJTs, unless junction temperatures are kept from rising above a safety level, usually 175 ◦C or less. The series-pass BJT is protected by sensing its instantaneous temperature and reducing its collector current in case of thermal overload. In the circuit of Fig.2 this protection is provided by Q4, a BJT mounted in close thermal coupling with the series-pass element. Temperature is sensed by exploiting the negative TC of VBE4. This BJT is designed to be in cut-off during acceptable thermal conditions, but to turn on as soon as the temperature approaches 175 ◦C. Once in conduction, Q4 will divert current away from the base of the series-pass BJT, reducing its conduction to the point of even shutting it off until the temperature drops to a more tolerable level. Efficiency: The efficiency of a regulator is defined as η(%) = 100PO/PI , where PO(= VO IO) is the average power delivered to the load, and PI (= VI II ) is that absorbed from the input source. The current II drawn from the VI source splits between the load and the control circuitry consisting of the bandgap reference, the error amplifier, 4

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

and the feedback network. Being only on the order of milli-amperes, the latter current is negligible compared to IO, so we approximate II ≈ IO and write

hereof, is available in monolithic form roducts to gain widespread popularity tors, and the μA7900 series of web). The μA78G series is similar to esistor pair of Fig.2 is omitted, and the ifier, referred to as the control pin, is al setting of VO. Called a four-terminal ally useful in remote sensing. As network right across the load and sure a regulated voltage of respective of any voltage drops across our-terminal version of the 7900 ed by three-terminal adjustable regulator and the LM337 negative wn examples. In the LM317 functional bandgap reference biased at 50 μA. The takes to keep the voltage at the output adjustment pin. Thus, connecting the 5 V. By the superposition principle, Eliminating VADJ gives The purpose of R1 and R2, besides setting the value of VO, is to provide a conductive path toward ground for the quiescent current of the error amplifier and the remaining circuitry in the absence of a load. The data sheets recommend imposing a current of 5 mA through R1 to meet this requirement. One can then verify that the effect of the 50-μA current becomes negligible, so VO = (1 + R2/R1)1.25 V. By varying R2, VO can be adjusted anywhere from 1.25 V and 35 V. A more recent adjustable regulator is the LT3080, which you can search online for its circuit schematic as well as useful application suggestions.

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

minal

ally w plified reduce or is p e thick as uired

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

Fig. 6 Typical circuit connection of the μA7805 voltage regulator. Power Sources: With the help of a few external components, a voltage regulator can, like a voltage reference, be configured for a variety of voltage source or current source applications, the main difference lying in the much higher currents available. A regulator is configured for a higher output voltage by raising its common terminal to a suitable voltage pedestal. In Fig.7a we have

Fig.7 configuring a regulator (a) as a power voltage source, and (b) as an adjustable power current source. The role of the op amp, which is powered from the regulated output to eliminate any PSRR and CMRR errors, is to prevent the feedback network from being loaded by the common terminal. However, if the current of this terminal is sufficiently small, as in the case of adjustable regulators such as the LM317 and LM337 types, then we can do without the op amp and the circuit simplifies to the familiar form of Fig. 7b. The IC 723 general purpose regulator overcomes the limitations of three terminal fixed voltage regulators. The IC 723 is a low current device, and can be employed for providing a load up to 10A or more by the addition of external transistors. The functional block of IC 723 is shown in fig.8 (a). Fig.8 (b) and (c) shows the pin diagram for a 14 pin DIP and 10 pin metal package for the device. Zener diode, the constant current source and a reference amplifier form one section of the IC. The constant current source helps in maintaining a fixed output voltage from zener diode D2. The error amplifier, series pass element Q1 and current limit transistor Q2 form the second section. The error amplifier compares the input voltages applied at Non-inverting (NI) and inverting (INV) input terminals. The error signal obtainable at the output of error amplifier devices the series pass element Q1.

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

(a)

n Metal

Fig.9 Functional block diagram of a low voltage regulator using IC 723. Fig. 9 shows the functional block diagram for a low voltage regulator using IC 723. This circuit arrangement is used for regulating voltages ranging from 2V to 7V, and hence it is called low voltage regulator. The output voltage is directly fed back to the INV input terminal. The non-inverting input(NI) is obtained across the potential divider formed by resistor R1 and R2. Hence, voltage at NI 8

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

mplifier amplifies the ending on the error signal, the mises the difference between e, the output voltage V0 is

INV terminal input goes positive. This drives the NPN her current is driven in to the e. This compensates for the any rise in load voltage gets

Fig. 10 Functional Block Diagram of a high voltage regulator using IC 723. The IC 723 can be used for designing a high voltage regulator for output voltages ranging from 7V to 37V. The circuit connection diagram is shown in fig. 10. The non-inverting input (NI) terminal is directly connected to Vref through R3. The inverting input (INV) terminal is connected to the junction of resistors R1 and R2 connected with the output V0. The resistor R3 is selected to be equal to R1 ││R2. Then the error amplifier acts as a non-inverting amplifier with a voltage gain of Av = 1 + R1 /R2. Therefor the output voltage for the circuit is Vo = Vref (1 + R1/R2) = 7.15(1 + R1/R2) Current Limit Protection: 9

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

The limitation of the regulator IC723 is that it has no built-in thermal protection and short circuit protection. Therefore, the current limit protection in regulator ICs is necessary for providing protecting against short circuit condition across the load. The low pass and high pass regulator circuits. An active current limiting circuit for IC 723 is shown in fig. 11(a). This circuit prevents the load current from increasing beyond a safe value. The operation of the circuit can be explained as follows:

Fig. 11 (a) Current Limiting circuit (b) Its output characteristics. The series pass element Q1, which is part f regulator circuit, is shown connected in series with a current limiting resistor RCL. The voltage drop across the resistance RCL can bias the transistor Q2 and turn it ON. Assume that the circuit can supply a maximum current IL(max). The output voltage remains constant for any value of ICL up-to the maximum current of IL(max). The output voltage remains constant for any value of ICL up to the maximum current ICL(max). In such normal load conditions. The voltage VCL across the resistor RCL (i.e. VCL = ICL x RCL) is insufficient to turn transistor Q2 ON. Therefore, Q1 supplies the current demanded by the load conditions at the fixed output voltage VL. Now consider that the load current IL increases. This leads to more current through RCL, and the voltage drop VCL increases too. This turns the transistor Q2 ON> Hence, any current, which is in excess of ICL is diverted away from the base of Q1. This effectively reduces the emitter current of Q1, and thus the load current reduces. Similarly, when the load current reduces, the drop across RCL drops, turning Q2 OFF and allowing Q1 to pass IL. The curve shown in fig. 11(b) shows the output characteristics of series pass voltage regulator using such a simple current limiting method. The transistor Q2 supplies an additional small amount of current to the load when the current limiting takes place.

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

7.15V) r low voltage ower and This, When the output is accidently shorted, the worst case power dissipation will occur. Then, the maximum allowable power dissipation, PD(max) = IL(max) Vi(max). Q.1 Referring Fig.12 (b), assume the output voltage VL = 20V. (i) What value of RCL is to be used for limiting the maximum current at 0.5A. (ii) With the value of RCL, find the output voltage from the regulator when RL = 100Ω. (iii) Comment on the operation of the circuit for RL =10Ω. Soln: Given IL(max) = 0.5 A. we know IL(max) = 0.7 / RSC Therefore RSC = 0.7 / 0.5 = 1.4 Ω. (ii) Given RL = 100 Ω IL = 20 / 100 = 0.2A. The maximum current obtainable from the circuit is given as 0.5A. Since IL < IL(max), the output voltage will stay at 20V. (iii) If load RL = 10 Ω, then IL = 20 / 10 = 2A. 11

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

Since IL of 2A > IL(max) of 0.5A, current limiting will happen. The output voltage will therefore be V0 = RL x IL(max) = 10 x 0.5 = 5V. Q.2 Design a +12V voltage regulator using LM723, with a current limiting value of 50mA. Solution: The basic circuit for getting +12V is shown in fig.10. The output equation is V0 = Vref (1 + R1/R2) Choosing an arbitrary standard value for R2 of 10KΩ, and solving for R1, we get R1 = (V0/Vref) R2 – R2 = (12/7.15)10 x 103 -10 x 103 = 6.78KΩ Fold back Current Limiting:

Fig. 13 (a) Fold back current limiting characteristics. In the simple current limiting, the maximum current limit was pre-set such that the power PD will never be more than the value set by PD = VL IL(max) and the resistance RCL was accordingly chosen. The net effect is that the regulator is underutilised. In such conditions, the current fold back method provides full protection to the device, in addition to allowing higher currents to the load.

Fig. 13(b) Fold back current limiting circuit. Fig. 13(a) shows the fold back current limiting characteristics in comparison with linear fold back method. The fold back limiting method reduces both the output current and voltage when IL(max) is reached. The fold back current limiting circuit is shown in fig. 13(b). This method of over-current protection is employed to reduce both the output load current and voltage, when the load resistance becomes smaller than what would draw a specified maximum current of IL(max) . In the current limit protected regulator, as IL(max) is exceeded, the output voltage of the regulator decreases. On the other 12

LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

hand, in fold back current limiting, as load resistance decreases beyond a certain minimum value, both load voltage and load current decrease, and when the load becomes a short circuit, they approach zero. The important advantages of fold back method of current limiting are (a) protecting the load from the over current operation and (b) protecting the regulator itself. The base of Q3 is connected to the voltage divided formed by R3 and R4. Applying KVL around the loop, we get VBE = VCL –VR3. The current limit transistor Q3 starts conducting only when its base to emitter voltage VBE is approximately 0.7V, that is, VCL must become sufficiently large to exceed the drop across R3 by a minimum of 0.7V. That is 0.7 = VBE = VCL – VR3. At this point, current limit statrs occuring. As the load resistance decreases, the load voltage drops, and VR3 also reduces. As a consequence, asmaller value of VCL is then required to maintain VBE of Q3 at 0.7V. Then as transistor Q3 starts conducting, transistor Q1 starts to turn OFF, and the load current decreases. The drop across R3 further reduces, increases the conduction of Q3 and reducing the conduction of Q1. The load current IL further reduces. This process continues until V0 becoms 0V and load current becomes a minimum. If the load resistance is brought to its nominal operating value, the circuit resums its normal regulation action. High Current Voltage Regulator:

Fig. 14 Low voltage regulator circuit with current boosting. The maximum current obtainable from IC 723 is 140 mA. For applications requring higher values, boost pass transistor Q1 can be added to the regulator as shown in fig. 14. The collector of Q1 is connected to unregulated dc supply. The output terminal V0 of regulator drives the base of Q1. Therefore, I0 = βboost transistor x I0(of 723). A darligton connected transistor pair can also be used in palce of Q1 as the pass transistor for obtaining much higher values of load currents.

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LICA, Chapter 6

Dhirendra Mehta

SEM – V B. Tech. (Mechatronics)

CODE: BTMA05002

Q.3 Design a continously adjustable power supply for the range of 2V to 5V with a current of 1A using IC LM723.

Fig.15 Adjustable voltage regulator using IC LM723 Solution: The adjustable voltage regulator for high current is shown in fig. 15. The output voltage is given by , In order to produce a 1A load current, an external pass transistor Q1 is employed. To obtain the desired voltage adjustment, resistor R1 is replaced with a series potentiometer/resistor combination (R1a, R1b) as shown in fig.15. Here, ...