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2016.08.18 DC/DC

Evaluating a Switching Regulator: Output Voltage

Characteristics and Evaluation Method of Switching Regulators

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From this section, we begin a new chapter on [Evaluation of Switching Regulators]. The two chapters [Switching Regulator Basics] and [How to Read Power Supply IC Datasheets] constituted the basic knowledge necessary for actually evaluating switching regulators. From this section on, we turn to a discussion of "what to look at, and how to look at it" with respect to actual circuit operation and characteristics.

The following is an overview that includes future planned material. Existing sections already include quite a lot of information relating to "evaluation of switching regulators", and so should be referenced as necessary.

We begin this chapter on [Evaluation of Switching Regulators] with the first section, an explanation of [Output Voltage].

Output Voltage

The job of a switching regulator is to generate a regulated output voltage, for use as the power supply of a load (another device). Hence, at least for the time being, the main order of business is evaluation of the output characteristics. Major output characteristic parameters include voltage, current, transient response, and noise; these are related to each other. In this section, we begin with an explanation of the output voltage.

In general, important points in evaluating output voltage are as follows.

14D_graf

・Items for evaluation 
- Voltage accuracy: Minimum and maximum values
  including ripples
- Magnitude of ripple voltage: Voltage amplitude,
  waveform
- Harmonic noise, spikes etc.

・Evaluation methods
- Observe the output voltage with an oscilloscope
- (Spectrum analyzer)

・Condition settings
- Output load current: A variable load device is required.
- Input voltage: A variable DC power supply is required.
- Temperature: Even simple spot heating/cooling is adequate.

More specifically, the output voltage is observed with an oscilloscope while varying the conditions. A spectrum analyzer may be effective depending on the circumstances, but an oscilloscope alone can be used for nearly all such observations. Here, it is important that an oscilloscope be used in observations. Originally an oscilloscope was not used for accurate voltage measurements. But the output voltage of a switching regulator contains various components such as ripples and noise, which cannot be observed by using a voltmeter or other instrument that averages the measured values of all the components as the displayed result.

Voltage accuracy is used to verify that the minimum and maximum values of the ripple voltage are within the accuracy required by the load device. FPGAs and other recent high-performance devices have extremely rigorous power supply voltage accuracy requirements, such as 2% or lower. Many power supply voltages are as low as 1 V or so, and actual allowed voltage fluctuations are very small. The output voltage requirement must be satisfied not only as an averaged value, but also including the peak amplitude of the ripple voltage.

The ripple voltage is checked to determine whether it is the voltage set by the following formula at design time, and to see whether there are any anomalies in the ripple waveform.

14D_formulas01

Of course, even if the ripple voltage is as intended in the design, if the voltage does not ultimately satisfy the accuracy requirement of the load device, the design must be reconsidered. Similarly, the output is observed to determine whether it contains abnormal spikes or harmonics.

These characteristics change state depending on the load and the temperature, and so observations should always be performed with varying factors included.

In evaluations, an example of an ideal output voltage and waveform is necessary. In many cases, typical and ideal waveforms are described on the data sheet for the power supply IC (see the figures below). These were also mentioned in [How to Read a Power Supply IC Datasheet].

14D_graf02

Further, when the IC manufacturer provides an evaluation board, it is possible to compare performance with that board. Evaluations can be performed using the same instruments and environment, making this a satisfactory and effective method.

Where oscilloscope observations are concerned, some caution is required relating to handling of fast waveforms. The following photos and graphs indicate an example in which the correct waveform cannot be observed depending on how the probe is used, even with the same board.

13D_graf03

The photo and graph on the left are for a case in which the oscilloscope probe is brought into contact with the test terminal and a clip is used for grounding; numerous high-frequency spikes are seen in the output waveform. The photo and graph on the right are for a case in which a specialized connector is used to connect the probe; in this case, no spikes are seen. The waveform on the right is the original output voltage; the waveform on the left is an example in which spikes that did not originally exist are caused by the probe grounding wire or the like.

Thus when evaluating a switching power supply, skills for handling and measuring high frequencies are necessary.

Key Points:

・In order to evaluate the characteristics of a switching regulator, the measurements required must be known, and measurement skills are necessary.

・When evaluating an output voltage, an oscilloscope is used to observe ripples and noise while changing various conditions.

・The graphs of data sheets and evaluation boards are used as a reference for comparison.

Power Supply Design Technical Materials Free Download

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