Basic design of a modern power supply

Basic design of a modern power supply

The linear voltage stabilizers with IC were the basis of the supply projects for many years now the time has come to modernize and replace them with analogous to much more efficient switching and equally easy to manage.

The linear voltage regulators are generally much more efficient and easier to use than equivalent circuits of voltage regulators realized with discrete components such as a zener diode and a resistor, or even transistors and operational amplifiers.

The most popular types of voltage regulators in a linear output are fixed and by far the series for positive output voltages and the 78XX series for negative output voltages 79XX These two types of complementary voltage regulators produce a precise voltage output and stable from about 5 volts until about 24 volts for the use in many electronic circuits.

A wide range of these voltage regulators adjustable fixing to three terminals, each with its own voltage regulation and current limitation of the embedded circuits. This allows us to create a whole series of different power outputs, both single or dual power supply, suitable for most of the circuits and electronic applications.

The majority of DC power supplies includes a large and heavy step-down mains transformer, the rectification diode, the wave interaction semionda, a filtering circuit to remove any content of ripple from the DC rectified to produce a DC voltage sufficiently regular, and a stabilizer circuit, to ensure the correct adjustment of the output voltage of the power supply under varying load conditions. So a typical DC power would look like this:

Typical DC power supply

These typical power supplies contain a large mains transformer (which also provides isolation between the input and the output) and a regulator circuit series dissipative. The regulator circuit may be constituted by a single zener diode or a linear series regulator to three terminals to produce the output voltage required. The advantage of a linear regulator is that the power supply circuit requires only one input capacitor, an output capacitor and some feedback resistors to set the output voltage.

The linear voltage regulators produce a regulated output DC using a continuous conduction transistor in series between the input and the output, operandolo in its linear region.

Therefore, the transistor acts more as a variable resistance that continuously adapts to any value necessary to maintain the correct output voltage.

Circuit regulator transistor series

Here this simple emitter-follower regulator circuit consists of a single NPN transistor, and a DC bias voltage to set the output voltage required. Since an emitter follower circuit has a unitary voltage gain, applying a proper bias voltage to the base of the transistors, you get a stabilized output from the emitter terminal.

Given that a transistor provides current gain, the output load current will be much higher than the base current, even higher if you use a Darlington configuration.

The only condition is that the input voltage is sufficiently high to obtain the desired output voltage, the output voltage is controlled by the base voltage of the transistor.

In this example it comes 5,7 volts to produce an output of 5 volts to the load, additional 0.7V serve to compensate for the voltage drop between the terminals of the base and emitter. Then, based on the value of the base voltage, it is possible to obtain any value of the emitter output voltage.

The downside of this is that the series regulator transistor is continuously biased in its linear region, dissipates power in the form of heat as a result of its product VxI, since the entire load current must pass through the transistor, resulting in poor efficiency, wasted power and heat generation continues.

Furthermore, one of the disadvantages that the series voltage regulators have is that their nominal current of maximum continuous output is limited to a few amperes. So they are generally used in applications where low output power are required.

When you are prompted multiple output voltages or high currents, the normal practice is to use a switching regulator commonly known as power supply switching to convert the mains voltage to the power required in any other.

power supplies switching, O SMPS, They are becoming commonplace and replaced in most cases the traditional linear power as a way to reduce energy consumption, reduce the heat dissipation, as well as the size and weight.

Switching power supplies are now available in most PCs, power amplifiers, TV, DC motors, etc. Pretty much anything that requires highly efficient power because switching power supplies are becoming a mature technology.

For definition, a switching mode power supply (SMPS) It is a type of power source that uses semiconductor switching techniques, rather than standard linear methods to provide the output voltage required. The basic scheme consists in a power switching stage and a control circuit. The power switching stage performs the circuits from the input voltage of the power conversion, Wine to its output voltage, Vout which includes the output filter.

The main advantage switching power supply is its greater efficiency, compared to standard linear regulators, and this is achieved by exploiting a transistor (or power MOSFET) including their status “ON” (full) and their status “OFF” ( cut-off), states that produce a lower power dissipation.

This means that when the switching transistor is completely “ON”, the voltage drop on it is at its minimum value and when the transistor is fully “OFF” there is no power dissipation. Then, the transistor behaves as an ideal switch.

Consequently, unlike linear regulators that provide only step-down voltage regulation, a switching power supply can provide step-down, step-up and negation of the input voltage using at least one of the three basic switch circuit topologies:Buck, Boost e Buck-Boost. The three types differ in the way in which the transistor, the inductor and the smoothing capacitor are connected inside the basic circuit.

Feeder Buck

The Buck regulator It is designed to effectively reduce the DC voltage from a higher voltage to a lower without changing the polarity. In other words, the buck switching regulator is a step-down regulator circuit, thus for example a buck converter can convert +12 It was the +5 volt.

The buck switching regulator is a DC to DC converter and one of the easiest and most popular. The buck regulator switch uses a transistor in series or a power MOSFET or an IGBT as a main switching device as shown below, conventionally in the following schemes I will use a MOSFET as a switching element only because around the vast majority uses those but, the speech is equally true for BJT or IGBT.

The buck switching regulator

We can see that the basic configuration of the circuit for a buck converter is a MOSFET switch in series,T1 with an associated control circuit which keeps the output voltage as close as possible to the desired level, a diode,Dl, an inductor,L1e a filter capacitor,C1. The buck converter has two operating modes, depending on whether the transistor is on commutazioneT1 “ON” or “OFF”.

When the T1 is “ON” (closed switch), the diode D1 is reverse biased and the input voltage Vin causes a current to flow through the inductor to the load connected to the output, loading at the same time the capacitor C1. When a variable current through the inductor, It produces an opposite induced voltage that opposes the flow of current, according to the Faraday's law, when it reaches a steady state creates a magnetic field around the inductor, L1. This situation continues indefinitely until T1 is conductive.

When the transistor turns off and goes into “OFF” (open switch) commanded by the control circuit, the input voltage is instantaneously interrupted causing the collapse of the magnetic field around the inductor inductor inducing a reverse voltage to the principle of conservation of energy. This reverse voltage causes the forward bias of the diode, Therefore, the energy stored in the magnetic field strength of the inductor current to continue to flow through the load in the same direction.

Therefore, the inductor L1 returns the stored energy to the load by acting as a source and providing power until all the energy stored is not returned to the circuit or until the switch (MOSFET) It closes again. At the same time discharging the capacitor contributes to the load. The combination of the inductor and the capacitor form an LC filter that attenuates any ripple created by the action of the switching transistor.

Therefore, when the solid-state switch is closed, power is supplied from power supply and when the switch is open, power is supplied from the inductor. Note that the current flowing through the inductor is always in the same direction, either directly or through it from the diode, but obviously at different times within the switching cycle.

Since the transistor switch is continuously closed and opened, the average value of the output voltage will then be dependent on the duty cycle D which is defined as the conduction time of the switch transistor during a whole switching cycle. If there is the supply voltage and timing “ON” e “OFF” for the switch are defined as: Ton e Toff, therefore the output voltage Vout is given as:

The duty cycle of the buck converters can also be defined as:

from here if we apply the previous formula we

So the greater the duty cycle D, the greater the power supply output voltage in switch mode. From this we can also see that the output voltage will always be lower than the input voltage because the work cycle, D can never achieve unity. The voltage regulation is obtained by varying the duty cycle and high switching speed, up to 200kHz, it is possible to use smaller components greatly reducing the size and weight in switch mode power supply, recently this limit has been largely overcome reaching Mhz for SMD assembly in which the coil must have a similar size to the other components.

With ideal components, or if the switching losses in the state “ON” were zero, the ideal buck converter may have efficiencies up to 100%.

In addition to the controller step-down buck for the basic design of a switching power supply, There is another configuration, namely step-up O Boost Converter.

Power boost

The Boost switching regulator is another type of power supply circuit in switch mode. It has the same components as the previous buck converter, but this time in different positions. The boost converter is designed to increase the DC voltage from a lower to a higher voltage, or increases the supply voltage, thereby increasing the available voltage across the output terminals without changing the polarity. In other words, the boost switching regulator is a step-up regulator circuit, thus for example a boost converter can convert the voltage to be +5 It was the +12 volt.

We have seen earlier that the buck switching regulator using a switching transistor in series within its basic design. The difference with the design of boost regulator It is that it uses a switching transistor connected in parallel to control the output voltage from the sheet feeder in switch mode. Since the switch is effectively connected in parallel with the output, the energy passes through the inductor to the load only when the transistor is in “OFF” (open switch) as shown.

The regulator switching boost

In the circuit Boost Converter , when the switch is in the "ON", the energy supply, Wine passes through the inductor and the transistor and feeding back. Consequently nothing passes the exit because the transistor switch creates a short circuit to the output. This increases the current flowing through the inductor as it has a shorter path to return to power. Meantime, the diode D1 inversely polarized because its anode is connected to ground through the switch while the cathode is at the voltage level at the output while the capacitor begins to discharge through the load.

When the transistor is off, the input power is now connected to the output through the inductor and diode connected in series. When the inductor field decreases, the energy stored in the inductor is induced thrust output from Vin, through the diode directly biased. The result of all this is that the reverse voltage induced by the inductor L1 is added to the supply voltage by increasing the total output voltage that becomes Vout = Vin + VL.

The current from the smoothing capacitor, C1 that was used to power the load when the switch was closed, is now provided to the input it from the condenser through the diode. So the current supplied to the capacitor is the diode current, that will always be ON or OFF since the diode is continuously switched between the state direct and the inverse value by the transistor switching actions. Then, the smoothing capacitor must be large enough to produce an output adjust and filter these peaks.

Since the induced voltage across the inductor L1 is negative, it is added to the source voltage, Wine, forcing the inductor current in the load. The output voltage of the boost converters is given by:

As with the previous buck converter, the output voltage from the boost converter depends on the input voltage and the duty cycle. Therefore, controlling the work cycle, You are obtained by the output regulation. Furthermore, this equation does not depend from the inductor value, ment from the load current or the output capacitor.

We have seen above that the basic operation of a switching power supply circuit may use a non-isolated buck configuration, or a boost configuration depending on whether you require a step-down output voltage (buck) o step-up (boost).

But we can also combine these two basic switching topologies in a single switching regulator circuit does not isolanto called, (coincidentally), with a huge stretch of the imagination, Buck-Boost Converter.

buck-boost power supply

The Buck-Boost switching regulator It is a combination of the buck converter and the boost converter which produces an inverted output voltage (negative) which may be greater or less than the input voltage according to the work cycle. The buck-boost converter is a boost converter circuit variant in which the inverter converter delivers only to load only the energy stored by the inductor L1.

Below is the power supply circuit in buck-boost mode.

The regulator switching buck boost

When the switch T1 is turned on (closed), the voltage across the inductor is equal to the supply voltage so that the inductor store energy input it from. No current is supplied to the load connected at its output because the diode, D1 is reverse biased. When the transistor is off (open), the diode becomes forward biased and the energy previously stored in the inductor is transferred to the load.

In other words, when the switch is on “ON”, the energy in the inductor is delivered from the supply and nothing is output.

When the switch is “OFF”, the inductor voltage is reversed and then the inductor itself becomes a source of energy, then the energy stored previously in the inductor is switched output (through the diode), nothing comes directly from the input source. Therefore, the voltage supplied to the load when the switching transistor is “OFF” It is equal to the induced voltage of the inductor.

The result is that the amplitude of the output voltage inverted can be greater than or less than or equal to the input voltage according to the work cycle. Eg, a buck-boost converter can convert from 5 It was the 12 volt (step-up) Oh yes 12 It was the 5 volt (step-down).

In the buck-boost switching regulators the Vout is given as:

So the buck-boost regulator has this name because the output voltage that may be higher (as one of boost power stage) or less (as a buck power stage) of the input voltage. However, the output voltage has opposite polarity to the input voltage.

Switch Mode Riepilogo

The modern switching power supply, o SMPS, It uses solid state switches to convert a DC input voltage is not regulated in a DC regulated output voltage at different voltage levels. The power supply of DC voltage can be a real input from a battery or a solar panel, or a rectified DC voltage from an AC power supply using a diode bridge together with some additional capacitive filters.

The main advantage of this is that the energy efficiency of the controller can be quite high because the transistor is fully turned on and conductor (full) or completely off (break).

There are different types of converter DC to DC available, the most commonly used three switching power supply topologies are known as Buck, Boost e Buck-Boost. All three of these topologies are not isolated, ie their input and output voltages share a common ground line.

The adjustment of the output voltage is obtained by controlling the percentage of time in which the switching transistor is in the state “ON” out of the total ON time / OFF. This ratio is called the duty cycle and varying the duty cycle, D can be controlled by the amplitude of the output voltage Vout.

The use of a single inductor and diode and solid state switches with fast switching able to operate at switching frequencies in the order of kilohertz can greatly reduce the size and weight feeder. However, if it is required the isolation between the input and output terminals, it is necessary to include a first converter transformer.

The buck converter is designed to convert electrical energy from a starting voltage to a lower. The buck converter operates with a switching transistor connected in series. Since the duty cycle, D <1 , the buck output voltage is always lower than the input voltage.

The boost converter is designed to convert electrical energy from a starting voltage to a higher. The boost converter operates with a switching transistor connected in parallel which determines a direct current path between Vin and Vout through the inductor L1 and the diode D1. This means that there is no protection against output short-circuit.

By varying the duty cycle, (D) a boost converter, the output voltage can be controlled and with D <1, the DC output from boost converter is greater than the input voltage Vin as a result of self-induced voltage of the inductors .

Furthermore, it is assumed that the output smoothing capacitors in power supplies to commutation They are old enough, such as not to be discharged in the absence periods of energy supplied by the source with the switching, which translates into a constant output voltage.

With this I end this discussion on the types of switching power supplies with the hope of being able to persuade people to come to this type in the design of their works be it hobby or semi-professional. Basically once we understand how we can stop watching them with suspicion and fully exploit them for our needs.

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