DC/DC Converter Questions and Answers
Time：2022.09.30
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What is a DC/DC converter?
What is DC (Direct Current)? It is a DC power source, such as a dry cell or car battery. A 100V power supply for home use is an alternating current (AC) supply. If a converter can convert a DC voltage (3.0V) to another DC voltage (1.5V or 5.0V), we call this converter a DC/DC converter, or a switching power supply or switching regulator.
A: DC/DC converters generally consist of a control chip, an inductor coil, a diode, a triode, and a capacitor. When discussing the performance of a DC/DC converter, it is not possible to judge the merits of the controller chip alone. The characteristics of the peripheral circuit components and the wiring method of the substrate can change the performance of the power supply circuit, therefore, a comprehensive judgment should be made.
B: Modulation method
1: PFM (Pulse Frequency Modulation)
The switching pulse width is certain, and the output voltage is stabilized by changing the timing of the pulse output.
2: PWM (Pulse Width Modulation)
The frequency of the switching pulse is certain, and the output voltage is stabilized by changing the pulse output width.
C: In general, the performance differences of DC/DC converters with two different modulation methods, PFM and PWM, are as follows.
** The frequency of PWM, the method of selecting duty cycle of PFM.
* PWM modulation method
In the case of selecting higher frequency (e.g., 500KHz) In the case of selecting lower frequency (e.g., 100KHz)
(1) Very low efficiency at small load. (1) High efficiency at small load.
(2)The ripple of the output voltage is small. (2)The ripple of output voltage is large.
Therefore, in the case of small load or long standby time, the frequency of 100KHz is chosen, the efficiency of the conversion circuit is higher, but if the ripple of the output voltage is considered, if the frequency of 500KHz is chosen, the ripple voltage will be smaller. Comparison of ripple voltage at different frequencies with the same reference data inductor coil. For efficiency comparison, please refer to DC/DC converter questions and answers (1-2).
* PFM modulation method
The case of larger duty cycle (e.g. Duty=75%) The case of smaller duty cycle (e.g. Duty=58%)
(1) When small load,the efficiency is low. (1) When small load, the efficiency is higher.
(2) Larger current can be output. (2) The output current is smaller.
Therefore, a duty cycle of 75% is chosen to obtain a larger output current. If the load is not very large, the efficiency will be higher if a duty cycle of 58% is selected. Reference data Comparison of output voltage and efficiency with the same inductor coil and different duty cycle.
* The effect of replacing BJT transistors with MOSFETs as switching components in peripheral circuits on efficiency.
The efficiency will increase accordingly. Because the BJT tube needs to provide drive current to its base, which increases the current consumption of the circuit, while the MOSFET is voltage-driven and does not need to provide current to its gate, which does not increase the current consumption of the circuit.
However, for practical applications, please consider whether the MOSFET has an impact on the surrounding components.
** Spike noise of DC/DC converters (switching regulators)
DC/DC converters (switching regulators) generate spike noise and electromagnetic interference when they are stepped up or down by switching action, especially when transistors or field effect tubes are switched on and off rapidly. Please refer to Technical Information Paper No.0006(Ver.001) for the method of measuring and reducing spike noise.
