Tech Tips

Electronic Device is Operating at Direct Current

In general, electronic devices operate at direct current. This is common to home appliances such as smartphones, Computers, TVs, refrigerators, and air conditioners, as well as in-vehicle devices for autos and manufacturing robots operating inside plants. However, not only these electronic devices operate at different voltages, but also within one electronic device, the required voltage varies depending on the circuit. So, it is necessary not only to convert the AC of the outlet into DC but also to convert it to the required voltage and provide it to the circuit.

In addition, the alternating current changes voltage with time. Converting from alternating current to direct current will cause the circuit to become unstable due to voltage fluctuations, so conversion to a stable voltage becomes essential.

Conversion from Alternating Current to Stable Direct Current

Well, we introduce how to get a stable DC voltage. To convert AC power coming from the power company's grid into DC power, convert the voltage with a transformer and then convert AC into DC with a rectifier circuit. However, since the output from the rectifier circuit is in the form of a sine wave and there is voltage fluctuation, it is necessary to further pass a smoothing circuit in order to convert it into a stable DC power supply.

This image explains Conversion from AC power source to stable DC power source

The basic steps to obtain stable DC voltage are as illustrated. However, it is not possible to obtain a completely stable DC voltage. Additional steps are required to extract a stable DC voltage from a commercial power source, and there are two ways. One is a linear power supply, and the other is a switching power supply.

Linear Power Supply

The first is a linear power supply. A resistor is used to remove and stabilize the excess voltage through comparing the unstable DC voltage extracted from the commercial power source with the reference voltage. While it is possible to realize it cheaply and simply by using resistors only, the extra voltage is released as heat, so it is essential to control heat in the circuit. Also, it cannot be used by heat-sensitive circuits.

Circuit Structure of Linear Power Supply as input from commercial power source and battery

Switching Power Supply

The other is a switching power supply. The pulse width is changed using a switching circuit, a high-frequency transformer, a rectifier circuit, a smoothing circuit without a resister, while comparing an unstable DC voltage extracted from a commercial power source with a reference voltage. Although the generation of heat can be suppressed by not using a resistor, there is a generation of noise, so it is necessary to remove this noise. Switching power supplies are characterized by low power consumption compared to linear power supplies. It is a power source originally generated from NASA's space development. Spacecraft cannot waste energy to space where it is difficult to release heat. It was developed as a power source to use energy without waste heat for satellites and spacecraft operated in space.

Circuit Structure of Switching Power Supply

Basic of Linear Power Supply

As introduced in the previous paragraph, the linear power supply is a method of making DC power while removing extra voltage from AC power. So, you can only get a voltage lower than the original one. Linear power supplies are stabilized, bypassing the control circuit after the smoothing circuit. In this part, it is stabilized by releasing the extra current voltage, which could not be equalized in the smoothing circuit as heat. There are two ways in this circuit. One is a shunt regulator, and the other is a series regulator.

The shunt regulator consists of a resistor (R1) and a Zener diode as a voltage regulator diode (ZD) connected in parallel. When the DC voltage of the output changes, the shunt regulator first converts it to the voltage to be output by the resistor in order to stabilize the voltage and break it up into the current to output and the excess current. The excess current is made to flow to the Zener diode, where it is consumed as heat. When the input voltage fluctuates, the current value coming out of the resistor fluctuates. By making the resistance value of the constant voltage diode variable, stabilization is achieved by making the output current value constant.

Circuit Structure of Shunt Regulator

On the other hand, in a series regulator, current flows via a transistor (Tr), which is an energy conversion element. The fluctuated voltage is changed constant voltage in this transistor. It is called a series regulator because the transistor is connected in series to the output side. In this case, a reference voltage is required to make the transistor fluctuate so as to maintain a constant voltage. Therefore, the control circuit is connected in parallel to the transistor, which has the same circuit configuration as the shunt regulator, as you can see from the figure. The difference is that it is just a transistor that stabilizes the voltage by heat generation.

 If the current Iout flowing through the load decreases, the voltage between the transistor EB fluctuates and Iout increases.

Series regulators have the advantage of lower noise and ripple and stability compared to shunt regulators. In any case, the linear power supply has a simple circuit configuration and has the disadvantage of generating heat, but it can produce DC voltage inexpensively.

Basic of Switching Power Supply

The switching power supply was developed to solve the problem that the structure was simple, but there was large heat to compare with the linear power supply. The structure of the switching power supply uses electromagnetic induction by building a transformer (two coils), which converts the voltage to a frequency higher than that of the commercial power supply. This is done by pulsing the current by making and breaking the circuit with the switch (S).

This is Circuit Structure of Switching Power Supply

There are two ways to make this pulse: PWM (Pulse Width Modulation) and PFM (Pulse Frequency Modulation). PWM is a method of controlling by changing the pulse width according to the magnitude of DC voltage while keeping the frequency constant. Although the ripple is smaller than the output voltage, the power consumption increases. It is also characterized by high responsiveness to load.

On the other hand, power consumption may be lower at low frequencies, and PFM may be advantageous, but when the response to load fluctuations is slow, ripples will be larger. These characteristics are generally judged, and PWM is basically used in switching power supplies, but PFM is used when the load is light. Well, there are two types of switching power supplies: non-isolated chopper control and isolated transformer control. The chopper control first converts an unstable DC voltage into an AC voltage (high frequency) of several tens of kHz to several MHz, which is a frequency much high than the commercial AC voltage. Since switching off the power supply, it was named "chopper control."

In the chopper control, both step-up and step-down are supported by using the characteristics of the choke coil (by self-induction), and then a stable DC voltage is obtained by incorporating a control circuit and a smoothing circuit.

On the other hand, in the transformer control, mutual induction by the high-frequency transformer is used to have the same role as the chopper system choke coil.

Reference (Japanese site)