Critical checkpoint: Output transient response and rising output voltage waveform

We are midway in our discussion of “Critical checkpoint” other than specifications that require attention when evaluating the performance of an isolated flyback converter. In this section, we will explain “Output transient response and rising output voltage waveform”.

• MOSFET VDS and IDS, and rated voltage of output rectifier diode
• Transformer saturation
• V_cc voltage
• Output transient response and rising output voltage waveform
• Measuring temperature and loss
• Electrolytic capacitor

Output Transient Response

Output transient response is one important characteristic of an output voltage. This “transient” means that the output current, which means the load current, fluctuates rapidly, and so a more accurate term would be the output voltage load transient response characteristic. In Japanese, the English term “transient response” is sometimes written phonetically in the katakana alphabet.

This characteristic is important because it relates to stability of the output voltage with respect to the load current. It can be optimized through the components values of an external circuit; in other words, the characteristic can be measured, and can be adjusted so as to result in improvement.

To check the transient response, the voltage waveform is measured for the following combinations of conditions. In addition to an oscilloscope and a load device, a current probe is also necessary to measure the load current waveform.

<Conditions to Check>
・ Input voltage: Minimum, Maximum
・Load current: Minimum → Maximum, Minimum ← Maximum
・Environment: Upper and Lower limits of temperature condition

When the load is switched continuously, a waveform like that seen in the lower-left waveform diagram should be observed. When the load current decreases suddenly, the output voltage rises momentarily, and after a certain time returns essentially to the preset voltage. When the load current rises sharply the opposite happens: the output voltage falls momentarily, then more or less returns to normal. Here, two things should be observed:

1) How much time it takes for the output voltage fluctuations to recover (return to) a stable state

2) Whether there is any disorder in the output voltage fluctuations, such as ringing, overshoot, undershoot, etc.

Regarding 1), it can be said that a characteristic with smaller voltage fluctuations and a faster return to a stable state is desirable; that is, a fast transient response, and rapid convergence of fluctuations, is desirable. Where 2) is concerned, as explained in the waveform diagram, the absence of ringing and the like is preferable. The waveform diagram on the left below is an example of a good waveform.

If a transient response characteristic does not satisfy demands, the feedback loop phase margin and gain margin should be adjusted. More specifically, the component values of the related external circuit, shown in the circuit diagram on the upper right, are adjusted. These components can be adjusted through trial and error, but without some experience in these matters, the engineer will probably not know what to adjust in which way. Early on, the better method is to make quantitative measurements of the phase and gain, check margins, and then make adjustments. In measurements, an FRA (frequency response analyzer) is simple and convenient to use.

In adjustments, there is a general tendency for stability to be reduced when the response is made more rapid, and so adjustments should be made so as to obtain the earliest response while maintaining the phase margin. It should be remembered that output fluctuations resulting from load transients cannot be reduced all the way to zero.

Output Rising Voltage Waveforms

Once again, output voltage waveforms are observed; here, the rising characteristic of the output voltage when the input power supply is turned on is observed. The method and instruments used are the same as those used for response characteristic measurements, and so by conducting a series of measurements as one routine, evaluations can be performed efficiently. One difference is that the output load current need not be continuously switched.

Checks are performed for combinations of the above conditions. For example, with the minimum input voltage, maximum load current, and temperature lower limit as the conditions of one set, observe the output voltage waveform when the input power supply is turned on. This should be performed as a one-shot measurement. Observations should determine whether or not there is ringing, overshoot, or undershoot in the output waveform. If any of these occur, not only is more time required until the output voltage stabilizes, but if fluctuations are too great, there is the possibility that a device being supplied with power will malfunction or be reset.

In fact, this too can be thought of as an output response characteristic. In this sense, there is some overlap with transient response, and so these can be said to be part of the same series of observations. Optimization for ringing, overshoot and undershoot is, as with transient response, achieved by adjusting phase and gain margins. Apart from this, the rising waveform changes with the relationship between soft starts and load capacitance, and so it is also necessary to consider what causes an observed waveform to appear the way it does.

The waveform diagram shown illustrates a satisfactory characteristic.