Critical checkpoint: MOSFET VDS and IDS, and rated voltage of output rectifier diode

In the previous section, we began from the desktop design of isolated flyback converters, discussing prototypes and evaluation, and considered design targets, that is, evaluation of whether or not the specifications as a power supply such as output voltage and efficiency are being satisfied. Here, we explain important points other than the specifications that need to be confirmed. Planned topics are listed below.

We begin by explaining differences between ordinary specification evaluation and the points to be checked that are explained below. The following table describes design goals for the circuit being used as an example; this table has appeared a number of times before.

The internal MOSFET has the drain connected to the DRAIN pin of the IC and the source connected to the SOURCE pin. This MOSFET serves to switch the primary side of a transformer to transmit energy to the secondary side, and performs the vital function of generating an output for use as a power supply.

Of course, if this switching operation and waveform are abnormal, a normal output cannot be obtained.

Below, the V_DS and I_DS for this MOSFET (IC) are displayed.

On the left is the waveform at the time of startup, and the waveform on the right is for steady-state operation. During startup, switching operation begins when the input voltage reaches the operating voltage; V_DS basically indicates the switching waveform of V_IN+VOR and the GND potential. I_DS is linked to this. The waveform is a fairly good example. This is because there is no disturbance after startup and a smooth transition to steady-state operation, and although spikes are caused in relation to switching, there are no excessive spikes, and all are within the MOSFET rated voltage (650 V).

The time axis for the steady-state waveform on the right is expanded, and the relationship between V_DS and I_DS accompanying switching is evident. Here, we can see that it is easy to anticipate the various waveforms from the circuit configuration, and that abnormal spikes, ringing, and in addition oscillation, do not occur. (Details on the circuit operation and waveforms, refer to “Isolated Flyback Converter basics”.

In essence, it should be confirmed that V_DS and I_DS, including spikes and ringing, do not exceed the rated values. As conditions, a measurement matrix of upper and lower limits is prepared for the input voltage, load current, and temperature. Using this, trends in fluctuation of characteristics for various conditions can be observed, and when this is supported by the circuit operation and component characteristics, the results become knowhow for future use.

Actually observed waveforms, voltages and currents will not conform exactly to calculations, and will differ from the theoretical values appearing in textbooks. Waveforms in particular are affected by such external factors as parasitic capacitances and inductances, and are also influenced by the measurement method used. It requires some experience to be able to judge whether or not an observed waveform is satisfactory or not. In making such judgments, an evaluation board supplied by the manufacturer is useful. Even when circuits differ somewhat, the operating state of the evaluation board is essentially close to the ideal state, and so comparing the operation of this board with one’s own device is an effective and practical method of evaluation.