IC Regulated Power Supply Tutorial

  
Designing using 78 and 79 voltage monolith regulators


When we apply a DC voltage to the input of a voltage regulator, the output will take a continuous voltage with ripple much smaller than that at the input of the regulator, also the output voltage of the regulator is stable and does not vary with changes in the input voltage. Monolithic integrated circuits (IC) voltage regulators offer very good stabilization (regulation), have a small volume, they are cheaper than the stabilizing circuits using discrete components, and are quite easy to use.

One of the most widely used series of integrated voltage regulators providing predetermined constant output voltage, is the series 78XX regulators for positive voltage (supply positive voltage at the output), with output voltages of 5V, 6V, 8V, 12V, 15V and 24V, and series 79XX for negative voltage regulators (provides negative voltages at the output) with output voltages-5V,-6V,-8V,-12V,-15V and -24V. The last two digits indicate the output voltage of the regulator. Example: The voltage regulator 7805 provides +5 V at the output, the 7812 provides +12 V, the 7815 +15 V, the 7824 +24 V, the 7905 -5V, the 7908 -8V, etc.


Figure 1.


The series of 78XX and 79XX can provide an output current to about 1A, and have specific protective circuits within the circuit to prevent the destruction of the integrated circuit, in case of overheating, output over-current, or overvoltage at input. In any of these cases the IC stops operation. The regulators 78/79 manufactured in plastic and metal enclosures. Except the series 78XX and 79XX that provide stable voltages with a maximum output current of 1A or so, we have also the series 78LXX which provides positive voltages in the integrated output from 2,6 V up to 12V at 100mA and the 78MXX providing positive voltages from 5V to 24V at 500mA. Also 79LXX series provides negative voltages from -2,6V to -12V at 100mA, and the series 79MXX that provides negative voltages from -5V to -24V at 500mA.


Figure 2. Various regulators with voltage terminals and pin-outs.
 

BASIC CONNECTION OF REGULATORS



Figure 3. Negative Voltage Regulator Connection with voltage terminals.


As seen from Figure 1, the use of regulators is very simple. The three terminals are called regulator input(INput), common(COMmon) and output(OUTput). Figure 2 shows various regulators together with their terminals. The voltage that we want to stabilize is applied between the input terminals and stabilized voltage obtained between the output terminals. The common terminal is connected to earth. Figure 3 shows the assembly of the regulator when we want a negative voltage at its output. The capacitor Cin needed when the regulator is not located near the filter feed and reduces the sensitivity of the regulator of impulse noise. The value should be 0,33uF or more, and is of tantalum or mylar, or other capacitor which exhibits low internal impedance at high frequencies. The capacitor Co is used to maintain a low output impedance at high frequencies. The output impedance of all integrated voltage regulators increases at high frequencies.


Figure 4. Complete regulated power supply.

The value of Co should be larger than 0,1uF and ceramic or polyester. To operate the integrated circuit as a voltage regulator, the voltage applied to the input should be at least 2,5V higher than the stabilized voltage that we want to take at the output of the regulator (2V, for the case where the output voltage is 5V). Example: The voltage at the input of 7805 should not be less than 2+5 = 7V, whereas in 7815 should not be lower than 2,5+15 = 17,5V. Also, the voltage at the input of the regulator must not exceed a certain value, for series 78XX and 79XX this value is approximately 35V, except for 7824 and 7924 which is approximately 40V. Figure 4 shows a complete circuit of a power supply, which provides a stable output voltage, the value of which depends on the IC. The values of capacitors Cin and Co given by the manufacturer of IC, while the value of the capacity of the electrolytic capacitor C is calculated with mathematical or empirical as we will explain. Although the series of regulators 78/79 provide a stable output voltage, by using the circuit in Figure 5 we can increase the voltage at the output that we receive. The output voltage Vo is given by the relationship:




Where Vout(REG) is the stable output of the regulator ie. 5V for 7805, 15V for 7815 etc. and the current Iq is the bias current flowing through the common terminal and its value is given by the manufacturer. Iq = 4,5mA for series 78XX and 79XX and IQ = 3,5mA for series 78LXX and 79LXX. Example. With the regulator 7805 and R = 1Kohm the voltage drop across the R is:


Figure 5. The resistor R increases the output voltage.


Figure 3. Negative Voltage Regulator Connection with voltage terminals. VR = 1Kohm x 4,5mA = 4,5V

And the output voltage is:

V0 = 5V +4,5V = 9.5V.


Figure 6. The output voltage can vary by changing R2.


Using the circuit of Figure 6 we can also increase the output voltage. The output voltage is given from the relation:



Usually the current Iq is very small compared to the current that circulates in R1 (i.e. Vout(reg/R1) and the above relationship is approximately:





R2 can be variable, so we can adjust the output voltage. The output current can be increased by using the circuit of Figure 7. When the current flowing through the regulator exceeds 1A (78/79) or the 100mA (78L/79L) the resistor R has such a value that the current flowing through, causes a voltage drop about 560mV and the transistor is conductive and allows to pass through this additional current. The circuit of Figure 7 may be modified as shown in Figure 8 which provides protection against short circuit by using the transistor T and the resistance R.


Figure 7.


Using two voltage regulators (Figure 9) (one positive and one negative 78XX 79XX) we have a double power supply that provides negative and positive Voltages. To be able to use different IC voltage regulators we should have a knowledge of the sizes to determine the properties of a practical voltage regulator, these figures given by manufacturers and are the following:

Input Voltage Range
Is the range of voltages That can be applied to the input. The input voltage must be 2.5 V above the voltage output in order to operate the regulator. Also the voltage in input should not exceed a value of about 35V for series 78/79.

Ripple Rejection
Is the quotient of the voltage wave at the input of regulator to the voltage ripple at the output of regulator and is measured in dB, and is of 60 dB (1000 to 1). That is, when the voltage ripple at the input of the regulator is 1V, the significant ripple in the output is 1mV.

Line Regulation
The quotient of the change in output voltage caused by a change in input voltage to the change of the input voltage. For most regulators is less than 1%. So when the input voltage changes from minimum to maximum value causes a change in output voltage that is fewer than 1%.

Load Regulation
Load Regulation is the change in output voltage of a regulator for a change of output current.

Output voltage
The voltage at the output terminal relative to the earth. Usually ranges +5% of the stated value.

Temperature Coefficient
The change of output voltage for a change in temperature of the regulator is expressed in mV/°C.

Short Circuit Current
The output current of the regulator when the output is short-circuit with the earth.

Output voltage noise
The rms noise voltage at the output of the regulator.

Power Consumption

The maximum total power consumption that may consume one regulator operating with preset limits. The maximum power that can be consumed by the regulator 78/79 without heatsink when the ambient temperature is (TA) 25°C is about 2W, and when we use heatsink is approximately 14W. Also for the correct design of a power supply we should use the appropriate AC transformer and heatsink, if the power supply provides sufficiently large currents, and filter capacitor.



Figure 8.




Figure 9. Dual Power Supply


Choosing Transformer

The choice of a transformer for a power supply with regulator depends on the following values.

1. Output Voltage and current.

2. Maximum input voltage that can be applied to the regulator.

3. Low voltage differential input/output that can be applied to the regulator, eg in the case of 7808 the output voltage is 8V and the minimum voltage at the input must be:

8+2,5 = 10,5V, the difference of voltages is:

10,5V-8V = 2,5V.

4. Power P consumed in the regulator, is given by:

P = (Vin - Vout) x Iout

where, Vin = the voltage at the input of the regulator, V0ut = the output voltage of the regulator, and Iout = the output current of the regulator.

5. Maximum peak voltage at the secondary of transformer (VM).

6. The apparent power of transformer in volt-amps (VI). Example. In power supply circuit of Figure 10 using the 7815 to provide a constant voltage of 15V at the output of the power supply, and a maximum output current in 1A.


Figure 10.


The minimum voltage applied to the regulator to function is:

Minimum input voltage = output voltage + 2,5V = 17,5V

The minimum required AC voltage from the secondary of the transformer is equal to:

Vin(DC) / 1,41 = 17.5V / 1,41 = 12,4V

Lets Suppose that we have a transformer with a secondary voltage of 18V, because this voltage is alternating, the voltage across the filtering capacitor will be about:

18V X 1.41 = 25,4V

Hence the constant voltage of 25,4V applied to the input of the regulator, is within the minimum and maximum voltage that can be applied to the regulator. The apparent power VA (volt-amps) that the transformer must have is given by:

Apparent Power(VA) = Vin x Iout

where Vin = input voltage of the regulator and Iout is the output current of the regulator, in our example the apparent power of the transformer should be at least:

25,4V X 1A = 25,4 W.

The power consumption in the regulator is given by the known relation:

P = (Vin - Vout) X Iout, So:

P = (25.4 -15) x 1 = 10,4W

Since the power consumption exceeds 2W (2W is maximum power that can be consumed by the regulator when the ambient temperature is 25 ° C without heat sink) the regulator should be mounted to a suitable heatsink thermal resistance with silicone grease between the metal surfaces and sink regulator. As we see the voltage (Vin - Vout) to the regulator should be small, so to have low power consumption. We need to do the calculation of thermal resistance of the heat sink. For the calculation of the thermal resistance of the heat sink from the relation:



where 0hs = thermal resistance of the heat sink, 0sa = Thermal resistance of the base of heatsink, 0sa = Thermal resistance of heatsink - environment. Tj = maximum contact temperature, Ta = ambient temperature, P = power. In the case of regulators of series 78/79, Tj = 150°C and P = 15W. Usually 0cs + 0sa = 5, and TA = 50°C, so we have:



For the power supply of Figure 10, P = 10,4W. Substituting this value we have: 0hs = 4,6°C/W. Here we use a cooler with 0hs = 4°C/W.


Calculation of Filter Capacitor

Usually the value of the filter capacitor should not be less than 2000uF for each ampere of output current, i.e. a power supply with a maximum output current 5A, the filter capacitor should have at least a value of 5 x 2000uF = 10000uF. In the case of power supply at Figure 10, the value of C must be at least 1 x 2000uF = 2000uF. The value of the filter capacitor C can be calculated using the relationship: C = I / (V x f).

Where I is the maximum current provided by the power supply, f = 100 for a full rectification, and V is the voltage ripple we want to have at the input of the IC, usually is about V = 1 - 2V.