Summary

＜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 V_DS and I_DS 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.