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2017.05.11 AC/DC

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

Isolated Flyback Converters: Performance Evaluation and Check Points

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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.

Parameter Min Typ. Max Units Conditions
Input voltage 90 - 264 VAC -
No load input power - - 50 mW Input: 100VAC/230VAC
Output voltage 11.4 12 12.6 V -
Output current 1.5 - - A -
Output ripple voltage - - 100 mV Bandwidth: 20MHz
Efficiency 80 - - % Output: 12V/1.5A

These parameters and numerical values are typical as power supply specifications, and methods of their measurement and the like have already been explained. For example, conditions are set for the output voltage, and a voltmeter is first used to measure the voltage; if the measured value is within the upper and lower limits set in advance, then we can conclude that the specifications have been satisfied.

However, in actuality it is extremely important that parameters be examined a little more precisely, or evaluated from a different point of view, checking for latent problems and confirming operating margins in advance, to ensure a smooth move to mass production. For this level of evaluations, experience and knowhow, knowing "what to look for where", are vital.

MOSFET Drain Voltage and Current
This example uses the BM2P014 power supply IC with an internal MOSFET which is used as the design example. The diagram on the right shows the internal block of the output stage of the BM2P014, and a portion of the external circuit used when configured as a flyback converter.

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 VDS and IDS 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; VDS basically indicates the switching waveform of VIN+VOR and the GND potential. IDS 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 VDS and IDS 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 VDS and IDS, 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.

<MOSFET Checkpoints>

  • Is startup smooth?
  • Are there abnormal spikes or ringing?
  • Are waveforms as expected from circuit operation?
  • Are VDS and IDS as expected, and are ratings exceeded?
<Conditions to Set during Checks>

  • Input: From minimum to maximum values
  • Load current: From minimum to maximum
  • Temperature: From minimum to maximum
  • Prepare measurement matrix, observe trends

Rated Voltage of Output Rectifier Diode
The reverse voltage Vr applied to the output rectifier diode on the secondary side of the circuit diagram shown above is checked for problems. A reverse voltage, the maximum value of which is VIN × winding ratio + VOUT, is applied to this diode. The voltage actually applied is observed to ensure it does not exceed the Vr rating and is not an abnormal waveform. (Details on the circuit operation and waveforms, refer to “Isolated Flyback Converter Basics”.

The waveform on the right is one example of a Vr waveform for an output rectifying diode.

<Output Rectifier Diode Checkpoints>

  • Are there abnormal spikes or ringing?
  • Are waveforms as expected from circuit operation?
  • Is Vr as expected, and is the rating exceeded?
<Conditions to Set during Checks>

  • Input: From minimum to maximum values
  • Load current: From minimum to maximum
  • Temperature: From minimum to maximum
  • Prepare measurement matrix, observe trends

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.

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.

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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