Switching Mode Power Supply/ Automatic Cut off Power Supply

Switching Mode Power Supply

Power electronics world

The 2nd switching power supply that brought about a power revolution

Electronic devices equipped with ICs and microcomputers require stabilized direct current with little voltage fluctuation.

There are two types of stabilized power supplies:

Linear power supply and switching power supply.

Switching power supplies breakthrough the limits of conventional linear power supplies and achieve breakthrough miniaturization, weight reduction, and high efficiency. The technical essence of power solutions is concentrated in switching power supplies.

Why a linear AC adapter is heavy and bulky?

A good place to know the basic technology of power supply is an AC adapter that converts commercial AC into DC. AC adapters used to be heavy and heavy, but they are now replaced by lighter and more compact ones, such as mobile phone chargers. This is because the switching method has become the mainstream since around 2000, replacing the conventional linear method.

The differences between the linear system and the switching system will be described later, but first we will explain the conventional simple AC adapter (a simple linear power supply without the stabilization circuit). This type of AC adapter is characterized by its simple circuit and low cost, and it is used in speakers for cordless phones, desktop PCs, power tools, etc. Although not visible from the outside, most of its weight and volume come from the power transformer with the coil wound around the iron core. This power transformer transforms 220V AC voltage into AC low voltage, then forward current flows and rectifies AC by using the property of diode that does not flow reverse current.

However, even though it has been rectified, since it is a pulsating current that is far from DC, it is smoothed by a smoothing circuit using a capacitor. Storing charge is one of the basic properties of capacitors. 

The rectifier circuit shown in the figure is an example that uses a bridge-type diode (full-wave rectification method).

Even if the direction of the alternating current is switched, the current always flows in the same direction in the capacitor and accumulates electric charge. . The pulsating current has large fluctuations in voltage due to repeated increases and decreases in current, so the capacitor releases the accumulated charge and suppresses fluctuations in voltage. This is the role of the smoothing circuit. Since a large-capacity smoothing capacitor is required, aluminum electrolytic capacitors are generally used. In the smoothing circuit, a choke coil may be used in series with the capacitor together with the capacitor. It utilizes the property of the coil that tries to prevent the current change, and can achieve smoothing more.

"DC regulated power supply" is important for digitized electronic devices

Obtaining DC from commercial AC is the basis of AC-DC power supplies, including AC adapters. However, even though it is a direct current, its quality is from pin to sharp. Even if the pulsating flow is smoothed with a simple circuit like the AC adapter mentioned above, ripples like ripples still remain. In addition, the output DC voltage also changes due to fluctuations in the commercial AC voltage. Although such a fluctuation does not cause many problems in charging the battery, it may cause malfunction in ICs that operate at low voltage, so a flatter and more stable DC is required. A power supply with a stabilizing circuit (regulator) for that purpose is called a stabilized power supply.

Stabilized power supplies are roughly classified into linear power supplies and switching power supplies depending on the method. Linear power supply is a method that has been used since the age of vacuum tubes. The principle is quite simple, and the output voltage is adjusted by incorporating a variable resistor in the circuit. Zener diodes and three-terminal ICs (three-terminal regulators) are used as the regulator.

Zener diodes are also called constant voltage diodes. A general diode is used as a rectifying element that allows current to flow in a certain direction and does not flow in the opposite direction. However, if the voltage applied in the reverse direction to the diode is increased, it will not be able to withstand it and current will suddenly flow. This phenomenon is utilized by the Zener diode, which functions as a constant-voltage diode through which a current flows at a certain voltage, which allows the output voltage to be stabilized.

A three-terminal IC is an element that detects the error between the constant voltage (reference voltage) and the output voltage due to the Zener diode, and amplifies and corrects this with a transistor to stabilize the voltage. It is called a 3-terminal IC because the entire circuit is integrated into one chip and has three terminals, IN, OUT, and GND (ground).

3-terminal ICs have been widely used in electronic devices because they are small and easy to use. However, since the heat loss is large, a heat sink (heat sink) for heat dissipation is required. For this reason, it is not suitable for high-output power supplies, but it has the advantages of a simple circuit and low noise, so it is used in measuring instruments, medical equipment, high-end audio equipment, etc. Switching power supply that promotes miniaturization, weight reduction, and high efficiency of the power supply

Now, let's move on to the explanation of switching power supplies. The most familiar switching power supply is the AC adapter for mobile phones. The circuit is much more complicated than the simple AC adapter mentioned above, but it is extremely compact due to the IC of the stabilizing circuit. In addition, the fact that it does not have a large and heavy power transformer like a simple AC adapter greatly contributes to the reduction in size and weight.

The technical essence of the past power electronics is condensed in the switching power supply. From around 1960, vacuum tubes had replaced semiconductor elements (diodes, transistors, etc.), but the miniaturization and efficiency of power supplies did not progress much. This is the fate of linear power supplies. A heat sink was required to dissipate the heat from the transistors, and the power transformer was still heavy and bulky.

The switching power supply has solved this problem by a completely different method from the linear power supply (developed by NASA's Apollo project). The first difference between the two methods is that in a linear power supply, commercial AC is voltage-converted by a transformer and then rectified, whereas in a switching power supply, commercial AC is first rectified to DC and then voltage-converted. However, if it is rectified, it cannot be converted by a transformer.

Therefore, in the switching power supply, the rectified current is converted into pulse wave AC by high-speed switching of semiconductor elements (transistor or MOS FET), and this is sent to a high frequency transformer. Naturally, the circuit becomes complicated and the number of parts increases. Then, why doing such a complicated thing is the key point of the switching power supply.

There are various control methods for switching power supplies, but the most typical one is PWM (pulse width modulation). This is a method for stabilizing the voltage by adjusting the pulse wave width (ON time of switching ON / OFF cycle) and making each pulse area the same. In terms of efficiency, the linear power supply always cuts off part of the electric power as heat to stabilize it, so it is inevitably low in efficiency, but the switching power supply outputs the electric power as if cutting and pasting the power without waste. It is extremely efficient.

Also, the size of the transformer is inversely proportional to the frequency. Since the AC frequency is as low as 50Hz / 60Hz, the transformer of the linear power supply is inevitably heavy and large, but the pulse frequency of the switching power supply is a high frequency of several tens of kHz to several hundreds of kHz, so the transformer can be small and lightweight. However, at high frequencies, the iron core cannot be used as a transformer core due to the large loss. Therefore, the ferrite core is indispensable. Even if the efficiency of the power supply is increased by only 1%, a great amount of energy saving effect can be obtained for the whole society. However, switching power supplies also have their weaknesses. It is the generation of noise due to high speed switching. Power supplies have been said to fight heat, but since the advent of switching power supplies, the fight against noise has been added.