Key Mode Power Supply

In this article, we will discuss the Key Mode Power Supply. Linear voltage regulators are often much more efficient and easier to use than equivalent voltage regulator circuits made of a zener diode and discrete components such as a resistance or transistors and even op-amps.

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

Each has its own internal voltage regulation and a wide range of these three-terminal fixed voltage regulators with current limiting circuits. It allows us to create a series of different power supply rails and outputs with single or double feed, suitable for most electronic circuits and applications.

There are even variable voltage linear regulators that provide an ever-changing output voltage from just above zero to a few volts below the maximum voltage output.

Most DC power supplies consist of a filter circuit to eliminate any surge content from the pointed DC to produce a large and heavy downstream mains transformer, full wave or half-wave diode correction, and a properly uniform DC output voltage.

In addition, a linear or switched type of voltage regulator or stabilizer circuit can be used to ensure accurate regulation of the output voltage of power supplies under changing load conditions. Then a typical DC power supply looks like this:

Typical DC Power Supply

Key Mode Power Supply
Typical DC Power Supply

These typical power supply designs include a large mains transformer (which also provides insulation between input and output) and a serial regulator circuit. The regulator circuit can consist of a single zener diode or a three-terminal linear serial regulator to produce the required output voltage. The advantage of the linear regulator is that the power supply circuit only needs an input capacitor, output capacitor and some feedback resistors to adjust the output voltage.

Linear voltage regulators produce an arranged DC output by placing a continuously conductive transistor in series between the input and output, running it in the linear region (hence the name) of the current-voltage (i-v) properties.

Thus, the transistor acts more like a variable resistance that constantly adjusts itself according to the value required to maintain the correct output voltage. Consider this simple serial transit transistor regulator circuit below:

Serial Transistor Regulator Circuit

Key Mode Power Supply
Serial Transistor Regulator Circuit

Here, this simple emitter-monitoring regulator circuit consists of a single NPN transistor and a DC biasing voltage to adjust the required output voltage. Since a emitter tracker circuit has a unity voltage gain, a stabilized output is obtained from the emitter terminal by applying a suitable polarity voltage to the transistor base.

Since a transistor provides current gain, the output load current will be much higher than the basic current if a Darlington transistor arrangement is used.

In addition, the output voltage is controlled by the transistor base voltage, provided that the input voltage is high enough to achieve the desired output voltage. In this example, the load is given as 5.7 volts in about 0.7 volts to produce a 5 volt output. is dropped via the transistor between the base and emiter terminals. Then, depending on the value of the base voltage, any value of the emiter output voltage can be obtained.

Although this simple serial regulator circuit will work, the disadvantage of this is that the serial transistor is constantly biased in the linear region and distributes power in the form of heat. Since the entire load current must pass through the serial transistor, this results in low efficiency, wasted V*I power and continuous heat generation around the transistor.

Also one of the disadvantages that serial voltage regulators have is that maximum continuous output current ratings are limited to only a few amps, so they are often used in applications where low power outputs are required.

When higher output voltage or current power demands are required, normal practice is to use a switching regulator commonly known as a switched power supply to convert the mains voltage into anything that requires higher power output.

Switched Power Supplies, or SMPS, are becoming more and more common. In most cases, it has replaced traditional linear AC-to-DC power supplies as a way to reduce power consumption, reduce heat dissipation, as well as size and weight.

Switched power supplies can now be found on most PCs, power amplifiers, TVs, dc motor drives, etc., and almost anything that requires a highly efficient feed as switched power supplies become an increasingly mature technology.

By definition, a switched power supply (SMPS) is a type of power supply that uses semiconductor switching techniques instead of standard linear methods to ensure the required output voltage. The basic switching converter consists of a power switching stage and a control circuit.

The power switching phase performs the power conversion from circuit input voltage, VIN to output voltage, VOUT, which includes output filtering.

The biggest advantage of the switched power supply is its higher efficiency compared to standard linear regulators, which is achieved by internally changing a transistor (or power MOSFET) between the "ON" state (saturated) and the "OFF" state. Both provide lower power consumption.

This means that when the switching transistor is completely "ON" and transmits current, the voltage drop on it is minimal, and when the transistor is completely "OFF", there is no current flow through it. Thus, the transistor acts as an ideal ON/OFF switch.

Unlike linear regulators that offer voltage reduction regulation only, a switched power supply can reduce, amplify, and negation of input voltage using one or more of the three basic key mode circuit topologies: Buck, Boost, and Buck – Increase. These names refer to how the transistor switch, inductor and softening capacitor are connected within the basic SMPS circuit.

Buck Switch Mode Power Supply

Buck switching regulator is a type of switched power supply circuit designed to efficiently reduce DC voltage from a higher voltage to a lower voltage. That is, it removes the supply voltage or "Bucks", thereby reducing the current voltage at the output. In other words, the buck switching regulator is a downgrade regulator circuit. Therefore, for example, a buck converter can convert +12 volts to +5 volts.

Buck switching regulator is a DC-DC converter and one of the simplest and most popular switching regulators. When used in a switched power supply configuration, the switch switching regulator uses a serial transistor or power MOSFET (ideally an insulated door bipolar transistor or IGBT) as the main switching device, as shown below.

Buck Switching Regulator

Key Mode Power Supply

We can see that the basic circuit configuration for a converter is a series transistor switch, TR1, a related drive circuit that keeps the output voltage as close to the desired level as possible, a diode, D1, an inductor, L1 and a softening. capacitor, C1. This converter has two operating modes, depending on whether the TR1 switching transistor is "ON" or "OFF".

When the transistor is polarized as "ON" (switch off), the D1 diode is inverted and the input voltage, VIN, causes a current to flow from the inductor to the connected load at the output, charging the C1 capacitor.

While a changing current flows from the inductor coil, according to faraday law, the inductor produces a back emf that opposes the flow of the current until it reaches a constant state that forms a magnetic field around L1. This will continue indefinitely as long as TR1 is closed.

When the TR1 transistor is switched to the "OFF" (switch on) position by the control circuit, the input voltage is instantly disconnected from the emiter circuit, which causes the magnetic field around the inductor to collapse and a reverse voltage induction on the inductor.

This reverse voltage causes the diode to be forward-sided, so the energy stored in the magnetic field of the inductors forces the current to flow in the same direction along the load and return from the diode.

The inductor then returns the energy stored in the load, which acts as a source and provides current, until the L1, whichever comes first, the energy of the entire inductor is activated or the transistor switch is turned off again. At the same time, the capacitor discharges the feed current to the load. The combination of inductor and capacitor creates an LC filter that softens any fluctuation generated by the switching motion of the transistor.

Therefore, when the transistor solid state switch is turned off, current is supplied without supply, and when the transistor switch is turned on, the current is provided by the inductor. Note that the current flowing from the inductor is always in the same direction, either directly without feeding or through the diode, but clearly at different times in the switching cycle.

Since the transistor switch is turned on and off continuously, the average output voltage value will therefore be associated with task cycle D, which is defined as the transmission time of the transistor switch during a full switching cycle.

If the supply voltage is VIN and the "ON" and "OFF" periods of the transistor switch are defined as tON and tOFF, the output voltage VOUT is given as follows:

Buck Converter Task Cycle

Key Mode Power Supply

The task cycle of Buck converters can also be defined as follows:

Key Mode Power Supply

Therefore, the larger the task cycle, the higher the average DC output voltage from the switched power supply. From this we can also see that since the task cycle the output voltage will always be lower than the input voltage, D will never reach a (unit), which results in a voltage regulator reducing voltage.

Voltage regulation is achieved by changing the duty cycle and with high switching speeds of up to 200 kHz. Smaller components are available, thereby greatly reducing the size and weight of the switched power supply.

Another advantage of the Buck converter is that the inductor-capacitor (LC) arrangement ensures very good filtering of the inductor current. Ideally, the converter should be operated in a continuous switching mode so that the inductor current never drops to zero. With ideal components with zero voltage drop and "ON" switching losses, the ideal bucket converter can be up to 100% high efficiency.

In addition to the gradual switching regulator for the basic design of the switched power supply, there is another process of the basic switching regulator, which functions as an amplifier voltage regulator called boost converter.

Boost Switch Mode Power Supply

The Boost switching regulator is another switched power supply circuit. It has the same type of components as the previous converter, but this time in different locations. The booster converter is designed to increase the DC voltage from a lower voltage to a higher voltage, that is, it also adds or "Raises" the supply voltage, thereby increasing the usable voltage in the output terminals without changing the polarity. In other words, the boost switching regulator is an amplifier regulator circuit, so for example, a boost converter can convert +5 volts to +12 volts.

We have seen before that the switching regulator uses a serial switching transistor in its basic design. The difference with the design of the Boost switching regulator is that it uses a parallel connected switching transistor to control the output voltage from the switched power supply.

Since the transistor switch is actively connected in parallel with the output, electrical energy switches from the inductor to the load only when the transistor is polarized as "OFF" (switch on), as shown.

Boost Switching Regulator

Key Mode Power Supply

In the Boost Converter circuit, with the transistor switch fully open, the electrical energy from the feed passes through the VIN, inductor and transistor switch and returns to the source. As a result, none of them switch to the output, as the saturated transistor switch effectively shorts out the output.

This increases the flow from the inductor, as it has a shorter internal path to return to the source. Meanwhile, the D1 diode becomes counter-biased, as the anode is connected to the soil through the transistor switch, as the voltage level at the outlet remains quite constant when the capacitor begins to discharge from the load.

When the transistor is completely switched off, the input feed is now connected to the output via the serial connected inductor and diode. As the inductor field decreases, the induced energy stored in the inductor is now pushed to the output by vin via the forward-sided diode.

The result of all this is that the reversal of the voltage induced along the L1 inductor and the addition to the voltage of the input source is VIN + VL, increasing the total output voltage as now.

When the transistor switch is switched off, the current from the C1 softening capacitor used to feed the load is now returned to the capacitor by the input feed via the diode. Then the current supplied to the capacitor is the diode current. The diode will always be "ON" or "OFF" as it is constantly altered between the switching motion of the transistor and its forward and backward state. Next, the softening capacitor should be large enough to produce a smooth and stable output.

Since the induced voltage along the L1 inductor is negative, it is added to the welding voltage, forcing the VIN inductor current to load. The voltage of the riser converters constant state output is given as follows:

Key Mode Power Supply

As with the previous converter, the output voltage from the support converter depends on the input voltage and task cycle. Therefore, output regulation is ensured by checking the task cycle. In addition, this equation is not independent of the value of the inductor, the load current and the output capacitor.

Above, we found that the basic operation of an uninsured switched power supply circuit can use a converter or accelerator converter configuration, depending on whether we need the output voltage. Although Buck converters are more common SMPS switching configurations, booster converters are widely used in capacitive circuit applications such as battery chargers, photo flashes, flash flashes, etc., since the capacitor feeds the entire load current when the switch is off.

But we can combine these two basic switching topologies into a single non-insulating switching regulator circuit called the Buck-Boost Converter, unsurprisingly.

Buck-Boost Switching Regulator

The Buck-Boost switching regulator is a combination of the power converter and the boost converter, which produces an inverted (negative) output voltage, which may be larger or less than the input voltage depending on the task cycle. The booster converter is a variation of the boost converter circuit, in which the converter transmits only the energy stored by the inductor L1 to the load. The basic buck-boost switch mode power supply circuit is given below.

Buck-Boost Switching Regulator

Key Mode Power Supply

When the transistor switch TR1 is fully on (off), the voltage on the inductor is equal to the supply voltage. Thus, the inductor stores the energy from the input source. Since the D1 diode is inverted polar, no current is transmitted to the connected load at the output. When the transistor switch is completely off (on), the diode is polarized forward and the energy previously stored in the inductor is transferred to the load.

In other words, when the switch is "ON", the inductor is energized by the DC source (via the switch) and no energy is supplied to the output, and when the switch is "OFF", the voltage on the inductor is reversed. The inductor now becomes an energy source. Thus, the energy previously stored in the inductor passes to the output (via diode), and none of them come directly from the input DC source. Therefore, when the switching transistor is "OFF", the voltage falling along the load is equal to the inductor voltage.

The result is that the size of the inverted output voltage can be larger or smaller (or equal) than the size of the input voltage, depending on the task cycle. For example, a positive-negative buck-boost converter can convert 5 volts to 12 volts (upgrade) or 12 volts to 5 volts (drop).

Buck-boost switching regulators fixed state output voltage, VOUT is given as follows:

Key Mode Power Supply

The booster regulator takes its name from producing an output voltage that can be higher (such as a power boost stage) or lower (such as a power stage) than the input voltage. However, the output voltage is in contrast to the polarity of the input voltage.