DC/DC Converter Questions and Answers
Time：2022.05.13
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What is a DC/DC converter?
What is DC (Direct Current)? It represents a DC power source, such as a dry cell or car battery. The 100V power supply for household use is alternating current (AC)
. If a DC voltage (3.0V) can be converted into other DC voltages (1.5V or 5.0V) through a converter, we call this converter a DC/DC converter,
Or called switching power supply or switching regulator.
A: DC/DC converters are generally composed of control chips, inductor coils, diodes, triodes, and capacitors. When discussing the performance of DC/DC converters,
If it is only for the control chip, it is impossible to judge its pros and cons. The component characteristics of its peripheral circuits and the wiring method of the substrate can change the electrical
The performance of the source circuit, therefore, should be comprehensively judged.
B: Modulation method
1: PFM (Pulse Frequency Modulation)
The switching pulse width is fixed, and the output voltage can be stabilized by changing the pulse output time.
2: PWM (Pulse Width Modulation)
The frequency of the switching pulse is fixed, and the output voltage can be stabilized by changing the pulse output width.
C: In general, the performance differences of DC/DC converters using two different modulation methods, PFM and PWM, are as follows.
** PWM frequency, PFM duty cycle selection method.
* PWM modulation method
In the case of selecting a higher frequency (eg: 500KHz) In the case of selecting a lower frequency (eg: 100KHz)
(1) When the load is small, the efficiency is very low. (1) When the load is small, the efficiency is higher.
(2) The ripple of the output voltage is small. (2) The ripple of the output voltage is large.
Therefore, in the case of a small load or a long standby time, the frequency of 100KHz is selected, and the efficiency of the conversion circuit is high, but if the ripple of the output voltage is considered,
If the frequency of 500KHz is selected, the ripple voltage will be smaller.
The reference data is the comparison of the ripple voltage when the inductance coil is the same and the frequency is different.
See DC/DC Converter Questions and Answers (1-2) for a comparison of efficiency.
* PFM modulation method
When the duty ratio is large (eg: Duty=75%) When the duty ratio is small (eg: Duty=58%)
(1) When the load is small, the efficiency is low. (1) When the load is small, the efficiency is higher.
(2) Larger current can be output. (2) The output current is small.
Therefore, a larger output current can be obtained by choosing a duty cycle of 75%. If the load is not very large, the duty cycle of 58% is selected, and the efficiency will be higher.
Reference data Comparison of output voltage and efficiency with the same inductor and different duty cycle.
* The effect on efficiency of replacing BJT transistors with MOSFETs as switching components of peripheral circuits.
Efficiency will increase accordingly. Because the BJT tube needs to provide driving current to its base, which increases the current consumption of the circuit, and the MOSFET is voltage driven, there is no need for it
The gate supplies the current, which does not increase the current consumption of the circuit.
However, in practical application, please consider whether the MOSFET will affect the peripheral components.
** Spike noise from DC/DC converters (switching regulators).
DC/DC converters (switching regulators) step up or step down by switching, especially when transistors or FETs are fast switching, spikes will occur
noise, and electromagnetic interference.
Please refer to "Technical Information Paper No.0006 (Ver.001) for the evaluation and reduction method of spike noise.
