Power supply ´

The objective of this chapter is to learn how to convert an AC voltage to a DC voltage. `

There are two types of power supply:

• Unregulated: the DC level decreases if the current to the load is increased,

• Regulated: DC voltage is stable with no ripples.

We will also see some integrated circuits regulators. ´

5.1 Introduction to unregulated power supplies

A transformer is required to reduce the nominal 120V mains voltage.

at the desired circuit voltage. It is the first component of the diet. The figure `

5.1 shows the voltage level conversion using the transformer.

The peak-to-peak voltage at the output is 24 ` ×

√

2 = 34V.

The next step is to convert the AC voltage to a pulsating DC voltage. We use

Note: The power supply is floating. There is no specific + or −. If one wants

a positive power supply, terminal 2 must be grounded, which becomes the ` − terminal. Of

likewise, for a negative power supply, terminal 1 must be earthed. `

Obviously, the voltage between terminals 1 and 2 is not a pure DC voltage. We

uses a capacitor to filter the AC components and thus obtain a fixed voltage.

A fairly large capacitor, 500 ´ ÂµF or more, is usually used. The capacitor

is placed after the diode bridge, as in Figure 5.3.

There is no load connected to the output of the circuit in Figure 5.3. The voltage of

output will decrease if we add a load, like the one in Figure 5.4.

The voltage at the load has a lower DC level, 24V in Figure 5.5. Also there are

an AC component, ∆vo

, superimposed on the DC value ( ` ∆vo = 5V in the previous example). ´

We conclude :

1. DC voltage decreases as current increases,

2. AC ripple increases from 0V to a large value under full load. `

The AC ripples of the power supply can be estimated based on the condensing

smoothing factor and the load current, by the following equation

If the ripple is measured with an AC voltmeter, the rms value of the ripples will be

displayed. The rms voltage is related to ∆vo by the following relationship:

∆vo ≈ 3.5Vrms (5.2)

where Vrms is the AC voltmeter reading. `

A last parameter is necessary for the design of an unregulated power supply: ´

it is the minimum instantaneous voltage necessary at full load.

5.2 DC regulation curve ´

We can measure the variation of the DC voltage according to the load for a

unregulated power supply, as in Figure 5.6.

The measured voltage as a function of the measured current gives the curve in Figure 5.7.

Figure 5.7 – Regulation curve: voltage versus current of a power supply ´

not regulated

The point O represents the voltage ´ Vcc = Em and Icc = 0, the voltage without load, and the point

A represents the voltage at full load, ` Vccpc

= 24V and IL = 1A.

We model the behavior of the unregulated power supply by a resistance of

Ro output

The resistance ´ Ro models the internal losses of the transformer, diodes, capacitors

sators and even cabling. It is not necessary to know the contribution of each

element ; only the total effect is important.

Power supply ´

The objective of this chapter is to learn how to convert an AC voltage to a DC voltage. `

There are two types of power supply:

• Unregulated: the DC level decreases if the current to the load is increased,

• Regulated: DC voltage is stable with no ripples.

We will also see some integrated circuits regulators. ´

5.1 Introduction to unregulated power supplies

A transformer is required to reduce the nominal 120V mains voltage.

at the desired circuit voltage. It is the first component of the diet. The figure `

5.1 shows the voltage level conversion using the transformer.

The peak-to-peak voltage at the output is 24 ` ×

√

2 = 34V.

The next step is to convert the AC voltage to a pulsating DC voltage. We use

Note: The power supply is floating. There is no specific + or −. If one wants

a positive power supply, terminal 2 must be grounded, which becomes the ` − terminal. Of

likewise, for a negative power supply, terminal 1 must be earthed. `

Obviously, the voltage between terminals 1 and 2 is not a pure DC voltage. We

uses a capacitor to filter the AC components and thus obtain a fixed voltage.

A fairly large capacitor, 500 ´ ÂµF or more, is usually used. The capacitor

is placed after the diode bridge, as in Figure 5.3.

There is no load connected to the output of the circuit in Figure 5.3. The voltage of

output will decrease if we add a load, like the one in Figure 5.4.

The voltage at the load has a lower DC level, 24V in Figure 5.5. Also there are

an AC component, ∆vo

, superimposed on the DC value ( ` ∆vo = 5V in the previous example). ´

We conclude :

1. DC voltage decreases as current increases,

2. AC ripple increases from 0V to a large value under full load. `

The AC ripples of the power supply can be estimated based on the condensing

smoothing factor and the load current, by the following equation

If the ripple is measured with an AC voltmeter, the rms value of the ripples will be

displayed. The rms voltage is related to ∆vo by the following relationship:

∆vo ≈ 3.5Vrms (5.2)

where Vrms is the AC voltmeter reading. `

A last parameter is necessary for the design of an unregulated power supply: ´

it is the minimum instantaneous voltage necessary at full load.

5.2 DC regulation curve ´

We can measure the variation of the DC voltage according to the load for a

unregulated power supply, as in Figure 5.6.

The measured voltage as a function of the measured current gives the curve in Figure 5.7.

Figure 5.7 – Regulation curve: voltage versus current of a power supply ´

not regulated

The point O represents the voltage ´ Vcc = Em and Icc = 0, the voltage without load, and the point

A represents the voltage at full load, ` Vccpc

= 24V and IL = 1A.

We model the behavior of the unregulated power supply by a resistance of

Ro output

The resistance ´ Ro models the internal losses of the transformer, diodes, capacitors

sators and even cabling. It is not necessary to know the contribution of each

element ; only the total effect is important.

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