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2017.07.06 AC/DC

Summary

Isolated Flyback Converters: Performance Evaluation and Check Points

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In this series, with the title of "Evaluation Series: Isolated Flyback Converters Performance Evaluation and Check Points" for AC/DC converters, we have explained measurement methods and actual measurement data for evaluation of the performance of isolated flyback AC/DC converter circuits, and have also described important check points for confirming whether a device is operating normally as well as for evaluating performance aspects.

Where performance evaluation is concerned, it is important that the features and characteristics of the power supply IC be fully understood in order to configure a circuit to be evaluated. As we have mentioned a number of times before, in recent power supply designs, generally a power supply IC is used, and consequently design is not possible without an understanding of the power supply IC. Moreover, in essence the datasheet for the power supply IC is used in design, and the characteristics and performance are checked to confirm that they are as expected; the importance of this process was probably well understood. It was also noted that, as a reference for evaluations, use of an evaluation board provided by the power IC manufacturer is extremely effective.

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Where actual evaluations are concerned, we have explained check points and measurement methods to determine whether or not a designed circuit is operating normally. As the procedure to be used, characteristic graphs presented in the datasheet for the power supply IC and evaluation board characteristics are compared with the characteristics of the circuit you have designed yourself; but if operating margins and potential issues are not examined, mass production and shipment are not possible. The check points and judgment criteria are precisely the knowhow needed for power supply design. It is important that latent problems be discovered promptly and without error on the basis of in-house legacy technical information, personal experience and past results, and that reliable design and manufacturing be executed.

Of course, in order to make such knowhow one's own, one must above all repeat the cycle of design and evaluation. In particular, AC/DC converters handle high voltages, and so with respect to measurements, the knowledge and experience to ensure safety are above all important.

The following is a summary of the topics and key points explained in this series. Please have a look at them once again.

<Isolated Flyback Converters Performance Evaluation and Check Points>

  1. Overview and important features of a power supply IC used in example performance evaluation

    Key Points

    ・Evaluating performance involves a verification as to whether given design goals are successfully attained.
    In other words, an evaluation is possible only if clearly defined design goals are established.

    ・An evaluation is possible only if the datasheet for a power supply IC is well understood.

  2. Design goald and circuits in performance evaluation

    Key Points

    ・In the actual design process, evaluate the created PCB to confirm whether the design goals are satisfied.

  3. Critical check point: Measurement method and results

    Key Points

    ・With basic instruments, power parameters can be measured with relative ease.

    ・The measurement process involves the handling of high voltages. Learn the steps necessary to ensure safety, and work in strict adherence to the safety precautions.

  4. Critical check point: MOSFET VDS and IDS, and rated voltage of output rectifier diode

    Key Points

    ・Check whether the MOSFET VDS and IDS values are within the rated values, and whether there are abnormal spikes or ringing.

    ・The reverse voltage Vr applied to a rectifier diode is checked to ensure it is within the rated voltage for the device, and the waveform is also checked.

    ・Comparison with an evaluation board supplied by the manufacturer is a practical and useful way to evaluate one's own circuit.

  5. Critical check point: Transformer saturation

    Key Points

    ・A transformer must not be allowed to reach saturation.

    ・Use an oscilloscope, current probe, etc. to observe the primary-side current waveform.

    ・If a transformer reaches saturation, an excessive current flows, possibly causing destruction of a MOSFET or other device.

  6. Critical check point: Vcc voltage

    Key Points

    ・The Vcc voltage should be checked to confirm an appropriate range given fluctuations in the input and load.

    ・The Vcc voltage should be checked to confirm an appropriate range given fluctuations in the input and load.

    ・Understand the operation of Vcc generation circuits using third windings, diodes and capacitors.

  7. Critical check point: Output transient response and rising output voltage waveform

    Key Points

    ・The load transient response of the output is optimized by adjusting the phase compensation circuit of the feedback circuit.

    ・The same is true of optimization of the output rising waveform.

    ・These evaluations and adjustments are mutually related, and so constitute a series of tasks.

  8. Critical check point: Measuring temperature and loss

    Key Points

    ・Confirming that the junction temperature (Tj) of a device (chip) is within the absolute rated value is essential, for safe operation as well as to ensure reliability.

    ・Temperatures are determined by actual measurements and utilizing the concept of thermal resistance.

    ・Losses are determined by observing switching waveforms and using equations.

    ・Allowed losses are determined by consulting a graph on the datasheet, but ultimately are based on the value of Tj.

  9. Critical check point: Alminum electrolytic capacitors

    Key Points

    ・Aluminum electrolytic capacitors should be selected with particular attention paid to the lifetime.

    ・The lifetime of an aluminum electrolytic capacitor is shortened dramatically at higher temperatures; in general, the "10-degree rule" for halving of the lifetime applies.

    ・Capacitance loss due to degradation is highly likely to cause malfunctions in circuit operation.

With this, we conclude the "AC/DC Evaluation Series: Performance Evaluation and Check Points for Isolated Flyback Converters".

Evaluation and Finding of Isolated Flyback Converter

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