Implementation of a DC/DC converter that provides stable power supply,
Depends on checking and adjusting the stability and responsiveness of the output voltage.
You need to know about electrical characteristics as well as frequency characteristics.


Most of the devices on the PCB operate only on DC (direct current).

Each device has its own operating voltage range and voltage accuracy requirements. If an unstable voltage is supplied, problems such as device malfunction or deterioration of characteristics may occur. To prevent this, a DC/DC converter is required.

DC/DC converter is a device that converts DC to DC, and since it is directly related to operation, it affects not only power design but also the overall product design. Power design engineers must be able to evaluate the electrical characteristics of DC/DC converters, such as load regulation, inductor current, output voltage, and power efficiency, as well as frequency characteristics.

In order for a DC/DC converter to stably supply power that satisfies the performance and specifications of the device under actual usage conditions, evaluation of both static and dynamic characteristics is necessary from the design stage. In this process, understanding the frequency characteristics of the DC/DC converter is necessary to check and adjust the stability and responsiveness of the output voltage.

Last February, we asked Na Hee-won, a senior researcher at ROHM Korea Technical Center, who hosted a webinar on the capabilities engineers need to have before evaluating the electrical characteristics of DC/DC converters, about the capabilities engineers need to have to evaluate frequency characteristics.



Q. The response to the last webinar was great. Why do you think so many engineers are looking at DC/DC converters today?

A. All electronic products use DC power, and since the voltage and current used by each device are different, a design that meets the required specifications is required. Currently, the main focus is on improving efficiency to save energy, and it seems that attention is being paid to DC/DC converters (switching regulators) that are more efficient than linear regulators.


Q. What does frequency mean in a DC/DC converter?

A. The DC/DC converter performs negative feedback control, lowering the voltage when the output voltage rises and raising the voltage when the output voltage falls.

Here, a delay time occurs, and as the frequency increases, the phase becomes delayed, which can cause a large output fluctuation due to positive feedback. Therefore, the frequency characteristics must be checked to confirm the output voltage stability and load response speed.



Q. How do you measure frequency response and what?

A. It is measured using a Frequency Response Analyzer (FRA) equipment, and the phase margin, gain margin, and crossover frequency are measured.



Q. Why do we measure phase margin and gain margin, and how much should we secure?

A. Phase margin and gain margin are necessary to verify the output voltage stability.

For phase margin, this is the phase value when the gain is 0dB, and is an item to check how much the phase differs when the amplitude of the input signal and the output signal is 1 times. ROHM generally recommends securing at least 30 degrees, and more than 45 degrees.

For gain margin, this is an item to check how much the output signal is attenuated with respect to the input signal when the phase is 0 degrees, that is, when positive feedback occurs. ROHM generally recommends -10dB or less, and at least -6dB or less.


Q. What is load response and how does it relate to frequency response?

A. Load response is the response speed when the load current changes abruptly.

If the load current increases rapidly, the output voltage will drop momentarily, increasing the possibility of reset false detection. On the other hand, if the load current drops rapidly, the output voltage will rise momentarily, which may exceed the withstand voltage of the load device and cause it to be destroyed.

There is a trade-off between output voltage stability and load response, which can be confirmed by measuring the crossover frequency with FRA equipment. Rohm recommends 1/10 or less of the switching frequency.



Q. What support does ROHM provide to engineers who want to understand the frequency characteristics of DC/DC converters and design peripheral circuits based on them?

A. ROHM provides the 'ROHM Solution Simulator' tool, which enables review of frequency characteristics to check output voltage stability and load response when FRA equipment is not available or when constants are changed.

Additionally, DC/DC converters must place a phase compensation circuit consisting of resistors, capacitors, etc., at the feedback terminal or phase compensation terminal to adjust the frequency characteristics.

ROHM provides information on how to select components for the phase compensation circuit in the data sheets for the DC/DC converter products it designs and manufactures.