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Take you to unlock AC/DC, DC/DC converter basics
First, let's go over the concepts of AC (Alternating Current) and DC (Direct Current).What is AC
Acronym for Alternating Current.
AC is a current whose magnitude and polarity (direction) changes periodically over time.
The number of times the current polarity changes in 1 second is called the frequency and is expressed in Hz.
What is DC
Acronym for Direct Current.
DC is a current whose polarity (direction) does not change with time.
①The current whose polarity (direction) and size do not change with time is usually called DC.
②The flow polarity does not change with time, but the current whose magnitude changes with time is also DC, which is usually called ripple current (Ripple current).
1. AC/DC converter
What is an AC/DC converter?
An AC/DC converter is an element that converts AC (alternating voltage) into DC (direct current voltage).
Why do you need an AC/DC converter?
That's because family homes and buildings receive 100V or 200V AC voltage. However, most of the electrical appliances you use operate on a DC voltage of 5V or 3.3V. That is, the appliance cannot work without converting the AC voltage to a DC voltage.
Among them, there are also products that can be driven by AC voltage, such as motors and light bulbs, but the motors are connected to the control circuit of the microcontroller, and the light bulbs also become energy-saving LEDs, so it is necessary to perform ACDC conversion.
Why is AC voltage transmitted?
Some people may think "Since the appliance uses DC, why not transmit DC in the first place?"
As we all know, electricity comes from hydroelectric power plants, thermal power plants, nuclear power plants, etc. These power stations are located in areas such as mountains or coastal areas, and from these areas to urban areas, AC voltage is more advantageous.
In short, by transmitting AC voltage with high voltage and low current, transmission loss (energy loss) can be reduced. However, in an actual home, since high voltage cannot be used directly, it needs to be transformed (stepped down) through several substations in stages, and finally converted into 100V or 200V and then entered into the home. These conversions are also simpler because AC, so the AC voltage is transmitted.
Full-wave rectification and half-wave rectification (AC/DC conversion)
There are full-wave rectification and half-wave rectification for converting AC (alternating voltage) into DC (direct voltage). In both cases, the forward current flow characteristics of the diode are used for rectification. Diodes are sold in Weisamp Mall, with various models, and the original factory authorized genuine spot.
Full-wave rectification is to convert the negative voltage component of the input voltage into a positive voltage through a diode bridge circuit structure and then rectify it into a DC voltage (pulse voltage). The half-wave rectification uses a diode to eliminate the input negative voltage component and rectify it into a DC voltage (pulse voltage). Afterwards, the waveform is smoothed by the charging and discharging function of the capacitor, which is converted into a pure DC voltage. Therefore, it can be said that full-wave rectification is a more efficient rectification method than half-wave rectification that does not utilize input negative voltage components. Also, the smoothed ripple voltage varies depending on the capacitor capacity and load (LOAD).
Full-wave rectification and half-wave rectification under the same capacitor capacity and load conditions, the ripple voltage of full-wave rectification is smaller. The smaller the ripple voltage, the higher the stability and the better the performance.
AC/DC conversion method
AC/DC conversion has a transformer method and a switching method.
Transformer mode
This is the transformer circuit structure of a common AC/DC converter.
【Example of circuit configuration of transformer method】
The following figure shows the change of voltage waveform in transformer mode.
The transformer method first requires stepping down the AC voltage through a transformer to the proper AC voltage (eg, from AC100V to AC10V, etc.). This is an AC/AC conversion, and the step-down value is set by the winding ratio of the transformer.
Next, the AC voltage stepped down by the transformer is full-wave rectified by a diode bridge rectifier and converted into a pulse voltage. Finally, the capacitor smoothes and outputs a DC voltage with a small ripple, which is the most traditional AC/DC conversion method.
【Transformation of the waveform of the transformer method】
Switch style
This is the switching circuit structure of an ordinary AC/DC converter.
【Example of circuit configuration of switching method】
The figure below shows the change in the switching mode voltage waveform.
The transformer method is to first step down the AC/AC through a transformer, and the switch method is to directly rectify the AC voltage with a diode bridge rectifier. Since the power consumption of ordinary households is AC100V or AC200V, the diode bridge rectifier must have specifications that can withstand high voltages.
Next, smooth the DC voltage (pulse voltage) with a capacitor. Capacitors also require high-voltage-resistant capacitors. Then, the DC voltage is chopped (cut) by the ON/OFF of the switching element, and the voltage is stepped down by a high-frequency transformer and sent to the secondary side. At this point, the chopping waveform becomes a square wave.
The switching element uses a higher frequency (eg 100kHz) compared to the household frequency (50/60Hz). Due to the high frequency operation, miniaturization and weight reduction of the transformer can be realized.
【Waveform transition of switching method】
The square wave is half-wave rectified by a rectifier diode on the secondary side, and then smoothed by a capacitor, and a DC voltage is output.
The switching method is to use the control circuit to control the switching element to obtain a stable and expected DC output (for example, DC12V).
Compared with the transformer method, the switch method consists of switching elements and control circuits, and the circuit structure is more complicated, but due to the high-frequency control, a small transformer can be used, So it helps to miniaturize the device, which is one of its great advantages.
Contradictions
What is feedback control?
The switching AC/DC converter confirms the actual output DC voltage value and controls the switching element according to the voltage information, thereby ensuring stable implementation of the regulations the DC output. This mechanism of confirming the output voltage value to control the switching element is called feedback control (FB control).
【Example of switching circuit structure】
Schematic of feedback control span>
The switching AC/DC converter rectifies the AC voltage through a diode bridge, and then smoothes it through a capacitor to convert the AC voltage to a DC voltage. Then, the DC voltage is chopped (ON/OFF) by a switching element, then stepped down by a high-frequency transformer and transmitted to the secondary side, and then smoothed by a capacitor to output a predetermined DC voltage (VDC).
FB control circuit detects whether the actual output voltage value reaches the specified target voltage value.
【Schematic diagram of smoothed output voltage】
When the actual output voltage value is lower than the target voltage value, the switching element will be controlled to make the ON time longer. In this way, the output voltage value will rise. Conversely, when the voltage is higher than the target voltage value, the control ON time becomes shorter.
In this way, the feedback control circuit always confirms the actual output voltage value, and adjusts the ON/OFF time of the switching element according to the value to ensure the target output voltage value stability.
Light load mode
What is light load mode?
The technique to improve efficiency when using less output current is called light load mode. In DC/DC converters, etc., it is also called burst mode.
Switching AC/DC and DC/DC converters perform voltage chopping and capacitor smoothing through ON/OFF conversion to stably provide the target output voltage value . However, this switching generates momentary leakage current (through current) during ON/OFF. That is to say, the more ON/OFF times per unit time, the greater the loss caused by leakage current and the lower the efficiency.
When the period is constant (PWM control), even if the ON/OFF time ratio changes, the number of times is constant per unit time. As a result, the amount of self-dissipated power is also constant, and this loss of switching leakage current at light loads reduces efficiency. Therefore, in the case of using less current, frequency modulation (PFM control) is used to lengthen and slow down the cycle, thereby reducing the number of ON/OFF transitions per unit time and reducing losses. This technique is called light load mode.
【PWM mode and PFM mode】
Using PWM and PFM can further improve efficiency according to the situation, such as using constant period PWM control at high load (using current), and light load (not using current) using cycle-changing PFM control.
■PWM (Pulse Width Modulation): The frequency is constant, and the control method of calling the output part from the input voltage through the ON switch.
■PFM (Pulse Frequency Modulation): The method of calling the output part by fixing the ON time and changing the frequency (changing the OFF time). Conversely, there are methods of fixing the OFF time and changing the ON time.
The PFM method changes the frequency according to the amount of output current, which is more efficient, but there will be occasional noise during switching. This kind of noise whose frequency cannot be determined is difficult to eliminate. To solve the noise, it is easier to use a PWM method with a constant frequency.
In this way, PWM with low noise and PFM with high efficiency can complement each other. PWM is used for high load (more noise) driven by high frequency. PFM is used for low loads with less current usage, and the optimal use can improve the efficiency as much as possible.
Second, DC/DC converter
What is a DC/DC converter?
DC/DC converter is a component that converts DC (direct current) to DC (direct current), specifically refers to the use of DC (direct current) to convert voltage element. Electronic components such as ICs have different operating voltage ranges, so they need to be converted to corresponding voltages.
Converters that produce a voltage lower than the initial voltage are called "buck converters"; converters that produce a voltage higher than the initial voltage are called " Boost Converter".
Name description
DC/DC converter is the name of a device that converts direct current to direct current.
It is often called linear regulator or switching regulator, etc., named after the conversion method.
Why do I need a DC/DC converter?
Electrical products that are plugged into an outlet to work require an "AC/DC converter" that converts AC (alternating current) 100V to DC (direct current). This is because most semiconductor components can only operate at DC. The ICs mounted on the circuit board of the whole machine have their own inherent operating voltage range, and the voltage accuracy requirements are also different. Supplying power from a power supply with an unstable voltage may cause malfunctions or deterioration of characteristics. Therefore, a "DC/DC converter" is used to convert to the desired voltage and stabilize it.
A device that achieves voltage stabilization through a DC/DC converter is called a voltage regulator.
Power IC type
Power ICs are roughly divided into two types: linear regulators and switching regulators.
As their respective output forms, linear regulators can only step-down output voltages lower than the input voltage.
Switching regulators have degrees of freedom, and the output forms include the following 4 types:
・Step-down output voltage lower than input voltage
・Boost output voltage higher than input voltage
・The buck-boost output constant voltage has nothing to do with the input voltage
・Invert output from positive voltage to negative voltage
In addition, the rectification methods of switching regulators include synchronous rectification and non-synchronous rectification (diode rectification).
[Type of power supply IC]
Linear and switching regulators
A device that achieves voltage stabilization through a DC/DC converter is called a voltage regulator.
According to the conversion method, voltage regulators are divided into two types: linear regulators and switching regulators.
Linear regulator
Because the relationship between input and output is linear during operation, it is called "linear regulator".
It is sometimes called a "series regulator" due to the series connection between the input and output.
The voltage is stepped down through the control element, so the greater the voltage difference between the input and the output (the degree of step-down), the greater the loss and the lower the efficiency. Therefore, it is suitable for low-power power supplies.
Advantages:Circuit Simpleless external partsless noise
Disadvantages: Low efficiencyGreat heatBuck Converters Only
Switching Regulators
Turn on the switching element (MOSFET) to supply power from the input to the output until the output voltage reaches the desired voltage.
When the output voltage reaches the specified value, the switching element is turned off and no longer consumes input power.
By repeating this action at high speed, the output voltage is adjusted to the specified value.
Advantages:High efficiencySmall heat generationCan realize boost/buck/negative voltage conversion
Disadvantages: Many external componentsDifficult to designNoisy
How Linear Regulators Work
General Pin Configuration
A linear regulator basically consists of three pins: VIN (input), VO (output), and GND (ground).
Added FB (feedback pin) for feedback output voltage on linear regulator with variable output.
In simple terms, a voltage-fixed type is a voltage regulator with a built-in voltage-variable external resistor.
The outline of the internal circuit of the linear regulator is shown in the figure below.
Its working principle is the same as that of the inverting amplifier circuit. The non-inverting pin (FB) voltage of the error amplifier is the same as the reference voltage (VREF), so the output voltage value (VO) is determined by the ratio of the resistance values of the two resistors (R1 and R2). Decide.
VO=[ (R1+R2) / R2 ] x VREF
The output transistor in the picture below is a MOSFET, but there are also products using bipolar transistors.
Classification of Linear Regulators
By function
When linear regulators are classified by function, they can be divided into two types: positive voltage and negative voltage.
Depending on the circuit used, there are also products that do not require positive power but negative power.
If there is only a positive side power supply, it cannot handle the voltage below ground potential, and the voltage of the output pin of the transistor cannot be assigned to the negative level. Load the control transistor into the negative output line, which produces a negative voltage.
[By function]
Secondly, there are two types of voltage fixed type and voltage variable type. The fixed type consists of three pins, input, output, and GND, and has a built-in resistor for setting the output voltage.
When the variable type is GND reference type, adding feedback pins becomes four pins. The variable type also has a floating operation type without a GND pin, but in this case it has three pins.
The voltage fixed type and the voltage variable type can also be divided into two types: standard type and LDO.
LDO is the abbreviation of Low Dropout, which is a linear regulator that reduces the potential difference between input and output. The minimum potential difference between the input and output of the standard type is about 2V, while the LDO can be controlled below 1V.
What is LDO?
LDO is the abbreviation of Low Dropout, which is a linear regulator that can work even with a low potential difference between input and output.
Sometimes called low loss linear regulators or low saturation linear regulators.
There is no numerical definition of the potential difference between the input and output of the LDO. Generally, it means that the minimum potential difference can be controlled below 1V when the regulator is working stably. regulator.
For example, for an IC that requires a 3.3V power supply, since the standard type cannot make a 5V to 3.3V power supply, it requires a low potential difference between the input and output LDOs.
This allows the LDO to set a lower input voltage while outputting the same voltage as a standard regulator.
Working with a low potential difference can reduce energy loss, and can be designed to suppress heat dissipation.
Pressure drop
In the interior of the linear regulator, a transistor is added from VIN to VO, and the minimum potential difference between the input and output required for the stable operation of this transistor is called for the pressure drop.
When the voltage difference between input and output is lower than the voltage drop, it is difficult for the transistor to maintain stable operation, and the output voltage will decrease.
In this way, in order to make the linear regulator containing LDO work, the required minimum input voltage value is set, and at this time (VO + voltage drop) is the minimum operating voltage value of the regulator.
When the input voltage value (VIN) is lower than the minimum operating voltage, the output voltage cannot work stably.
Source: techclass.rohm
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