Recent electronic products that pursue high efficiency and miniaturization
Advantages and Efficiency Need to Be Considered When Choosing a Converter
Distinguish between LDO and DC/DC according to output format Most electronic devices that operate by being connected to an outlet require an 'AC/DC converter' that converts AC (alternating current) 220V to DC (direct current). This is because many semiconductor components operate on DC. In addition, each IC on the set board has a different operating voltage range and different voltage requirements.
Therefore, if an unstable voltage is supplied, problems such as malfunction or deterioration of characteristics may occur. Therefore, a 'DC/DC converter' is required to convert or stabilize the required voltage. In this article, let's learn about the basics of DC/DC converter power design and the operating principles of the regulator.
▲ DC/DC converters are used to supply stable voltage to the circuit.
Types of power ICs There are two types of power ICs: the linear type 'LDO' and the switching type 'DC/DC converter'.
In terms of output format, LDO has only one type, △buck, which creates a voltage lower than the input voltage, but DC/DC converters have four types, including △buck, △boost, △buck-boost, and △inverting. There are two types of rectification methods for DC/DC converters: △synchronous rectification and △diode rectification.
LDO vs DC/DC converter LDO has the advantage of low noise and simple circuit. On the other hand, power consumption increases with the difference between input and output voltages, which leads to increased heat generation and lower efficiency. In addition, it can only generate voltages lower than the input.
DC/DC converters can handle large currents with high efficiency even when the input/output voltage difference is large. In addition, they have the advantage of being free in voltage output format. However, the circuit is complex because there are many external components, and the noise that occurs during switching must be considered when designing.
How LDO works LDO consists of an error amplifier, feedback, and output transistor. When the load current changes and fluctuates, the error amplifier controls the output transistor so that the output voltage always remains constant.
This circuit is similar to a spectral amplifier circuit, with an output transistor in the feedback loop. Since the voltage at the non-inverting terminal of the error amplifier is controlled to be equal to V
REF , the current is always kept constant. Therefore, the current is V
REF /R
2 , V
O = R
1 + R
2 /R
2 * V
REF .
Operating Principle of DC/DC Converter The basic circuit of a typical DC/DC converter uses S1 and S2 MOSFET diodes, and the operating principle is as follows.
1. In the comparison circuit, compare the output voltage with the reference voltage to see if it is the set voltage.
2. When the set voltage is lower than the existing voltage, the switch turns on and supplies power from input to output.
3. At this time, current flows and magnetic energy is accumulated in the inductor.
4. When power is supplied and the output voltage rises above the set voltage, the switch turns off.
5. At this time, the current flows through the green path, and the magnetic energy accumulated in the inductor becomes current and is supplied to the load output and returns to the inductor through the diode.
6. When the magnetic energy in the inductor decreases and the set voltage becomes lower than the original voltage, the switch turns on again.
7. Repeat this at the switching frequency.
In this way, the DC/DC converter is highly efficient because it supplies only the necessary power from the input, but ripple voltage occurs in the output voltage.
an style="font-size:11px;">▲ DC/DC converter has a ripple voltage in the output voltage.
Voltage fluctuation occurs [Provided by Rohm]
Comparison by rectification method Asynchronous rectification and synchronous rectification are distinguished based on whether the element used as the lower switch of the equivalent circuit is a diode or a MOSFET.
Discontinuous operation of synchronous rectifier (MOSFET rectifier) type Diodes have the advantage of high efficiency and no penetration at light loads, but low efficiency at medium loads and discontinuous operation at light loads. Synchronous rectification has high efficiency and fast response at medium loads. However, the circuit is somewhat complex and low efficiency at light loads. Therefore, in the case of FET enclosures, measures against penetration current are necessary.
All circuits have losses in the switches, which affect the efficiency because current flows through the switches. Therefore, there is a loss difference in the rectifier method. The loss is the forward voltage V
F for diodes, and the saturation voltage or on-resistance for transistors.
The V
F of a diode changes with the current, but even a Schottky diode with a low V
F has 0.4 to 0.7 V. On the other hand, the R
ON of a MOSFET is very low, from tens of mΩ to hundreds of mΩ. If you expect a voltage drop, it will be much lower than the diode V
F .
Discontinuous operation of asynchronous rectifier (diode rectifier) type DC/DC converters with diode commutation time have continuous and discontinuous operation. While continuous operation refers to the current flowing through an inductor being continuous, discontinuous operation refers to the current flowing through the inductor being intermittent (in the interval where it becomes zero).
The presence of DC/DC converters is increasing due to the demand for high efficiency and miniaturization. Therefore, when selecting a converter, it is necessary to design the analog circuit design by considering each advantage and efficiency. More detailed DC/DC converter power design and operating principles can be found in the e4ds EE webinar 'Low Web Seminar [Introduction to DC/DC Converters in 30 Minutes]'.
