Series regulator with Current Pre-regulator

Series regulator with Current Pre-regulator

The circuit of fig. 1 is an improved version of series voltage regulator discussed in previous lecture. Besides Q₁ being replaced with a current regulator circuit. The function of D₂, R₆, R₇, and Q₃ is to establish and maintain a constant I₁.

Fig. 1

The circuit works this way :- I₁ is the collector current of Q₃, and hence it is also approximately equal to I_E3. The voltage at the base of Q₃ relative to V₁ is held at a constant level by D₂; current through R₆ is selected to keep D₂ in breakdown and to yield the proper temperature coefficient. Should I₁ rise, I_E3 will also rise, increasing the voltage across R₁. This reduces V_EB3, which in turn reduces I_E3 and I₁. Thus I₁ is regulated and remains fairly constant even if there are changes in the unregulated input.

One disadvantage of this circuit is that a larger input voltage is required to supply the various voltage drops between V_i and V_o. In this case V_i must supply V_o plus the two V_EB drops of Q₁₁ and Q₁₂(which takes us to point A), plus the collector-base bias for Q₃ (which takes us to the base Q₃), plus the Zener voltage for D₂.

  Power Supply Using IC Regulator (Three-Terminal Regulator)

Monolithic integrated circuits have greatly simplified the design of a wide variety of power supplies. Using a single IC regulator and a few external components, we can obtain excellent regulation (on the order of 0.01%) with good stability and reliability and with overload protection.

IC regulators are produced by a number of manufacturers. The IC regulator improves upon the performance of the Zener diode regulator. It does this by incorporating an operational amplifier. In this section, we present basic design considerations for IC regulators. These techniques are useful in the design of power supplies for a variety of low power applications. We consider the internal theory of operation of these and other three-terminal voltage regulators in the current section. These products vary in the amount of output current. The most common range of output current is 0.75 A to 1.5 A (depending on whether a heat sink is used).

Fig. 2

The functional block diagram of fig. 2 illustrates the method of voltage regulation using this series regulator. The name series regulator is based on the use of a pass transistor (a power transistor) which develops a variable voltage which is in "series" with the output voltage. The voltage across the pass transistor is varied in such a manner as to keep the output voltage constant.

A reference voltage, V_REF, which is often developed by a Zener diode, is compared with the voltage divided output, v_out. The resulting error voltage is given by

The error voltage v e is amplified through a discrete amplifier or an operational amplifier and used to change the voltage drop across the pass transistor. This is a feedback system which generates a variable voltage across the pass transistor in order to force the error voltage to zero. When the error voltage is zero, we obtain the desired equation by solving equation (Equ-1) for v_out .

Thermal shutdown and current-limit circuitry exists between the error amplifier and the pass transistor. This circuitry protects the regulator in case the temperature becomes too high or an inadvertent short circuit exists at the output of the regulator.

The maximum power dissipated in this type of series regulator is the power dissipated in the internal pass transistor, which is approximately (V_{S max} - V_out) I_{L max}. Hence, as the load current increases, the power dissipated in the internal pass transistor increases. If I_Load exceeds 0.75 A, the IC package should be secured to a heat sink. When this is done, I_Load can increase to about 1.5 A.

We now focus our attention on the 78XX series of regulators. The last two digits of the IC part number denote the output voltage of the device. Thus, for example, a 7808 IC package produces an 8V regulated output. These packages, although internally complex, are inexpensive and easy to use.

There are a number of different voltages that can be obtained from the 78XX series 1C; they are 5, 6, 8, 8.5, 10, 12, 15, 18, and 24 V. In order to design a regulator around one of these ICs, we need only select a transformer, diodes, and filter. The physical configuration is shown in fig. 3(a). The ground lead and the metal tab are connected together. This permits direct attachment to a heat sink for cooling purposes. A typical circuit application is shown in fig. 3(c).

(a)	(b)
(c)

Fig. 3

The specification sheet for this IC indicates that there must be a common ground between the input and output, and the minimum voltage at the IC input must be above the regulated output. In order to assure this last condition, it is necessary to filter the output from the rectifier. The C_F in fig. 3(b) performs this filtering when combined with the input resistance to the IC. We use an n:1 step down transformer, with the secondary winding center-tapped, to drive a full-wave rectifier.

The minimum and maximum input voltages for the 78XX family of regulators are shown in Table-1.

Type	Min	Max
7805	7	25
7806	8	25
7808	10.5	25
7885	10.5	25
7810	12.5	28
7812	14.5	30
7815	17.5	30
7818	21	33
7824	27	38

Table - 1

We use Table -1 to select the turns ratio, n, for a 78XX regulator. As a design guide, we will take the average of V_max and V_min of the particular IC regulator to calculate n. For example, using a 7805 regulator, we obtain

The center tap provides division by 2 so the peak voltage out of the rectifier is 115 √2 / 2n = 16. Therefore, n = 5. This is a conservative method of selecting the transformer ratio.

The filter capacitor, C_F, is chosen to maintain the voltage input range to the regulator as specified in Table 8.1.

The output capacitor, C_Load, aids in isolating the effect of the transients that may appear on the regulated supply line. C_Load should be a high quality tantalum capacitor with a capacitance of 1.0 µF. It should be connected close to the 78XX regulator using short leads in order to improve the stability performance.

This family of regulators can also be used for battery powered systems. Fig. 3(c) shows a battery powered application. The value of C_F is chosen in the same manner as for the standard filter.

The 79XX series regulator is identical to the 78XX series except that it provides negative regulated voltages instead of positive.

Example-1

Design an IC circuit regulator to generate a 12 V output into a load whose current varies from 100 mA to 500 mA. The input is 115 V rms at 60 HZ.

Solution:

We use the circuit of fig. 3(b) with a 7812 regulator. The center-tapped transformer and full-wave rectifier must produce a minimum voltage of at least 14.5 V and a maximum voltage of no more than 30V. This information is obtained from Table 8.1. The input peak voltage is 115 √2 or 163 V. The center-tapped secondary divides this by 2 to yield 81.5V. Let us choose the mid-point between 14.5 and 30, or 22.25 V to select the transformer ratio. This yields a transformer ratio of 81.5 / 22.25 or 3.68.

Since V_{s min}= 14.5 V (from Table 8.1) and V_{s max}= 22.3 V, we have

We have used the fact that

ΔV = 22.3 – 14.5 = 7.8 V

and

R_load (worst case) = 29Ω

The value of C load is determined by the types of variations that occur in the load. A typical selection for this application is a 1.0 µF high quality tantalum capacitor.