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User22438

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Aug 05, 2021
01:52 AM

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I am currently trying to design a Class D power amplifier. however I am unsure of how to interpret the SOA graph.

The Class D PA is a full bridge PA, and has a switching frequency of 300kHz. I intend to drive 100Vdd for the top side MOSFETs.

The modulation factor can be up to 0.95.

I would like to know how to use the SOA graph to find the maximum current I can draw for a continuous PWM signal.

Thanks

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Abhilash_P

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Aug 05, 2021
06:47 AM

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The safe operating area is the voltage and current conditions over which a MOSFET operates without permanent damage or degradation. The MOSFET must not be exposed to conditions outside the safe operating area even for an instant. Conventionally, MOSFETs were known for the absence of secondary breakdown, which was a failure mode specific to bipolar transistors.

The safe operating area of a MOSFET was bound only by the maximum drain-source voltage, the maximum drain current, and a thermal limit between them. However, due to device geometry scaling, recent MOSFETs exhibit secondary breakdown. It is therefore necessary to determine whether the operating locus of the MOSFET is within the safe operating area.

The graph showing the SOA curve with different boundary limits is as shown below,

We have the following limitations,

1) Thermal limit

2) Secondary breakdown limit

3) Current limit

4) Drain-source voltage limit

5) On state resistance limit.

From your application, the Ton duration is 3.508us and Vdd voltage is 100V. These points has to be plotted on the MOSFET SOA curve and the current limit can be determined on the y-axis.

The following image shows the current limit for your MOSFET and the working parameters,

The area between 10ms and 100us has to be considered and the corresponding point on the current axis has to be taken to get the current limit.

Please let me know if this explanation has helped you understand the SOA

Regards,

Abhilash P

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Abhilash_P

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Aug 05, 2021
06:47 AM

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The safe operating area is the voltage and current conditions over which a MOSFET operates without permanent damage or degradation. The MOSFET must not be exposed to conditions outside the safe operating area even for an instant. Conventionally, MOSFETs were known for the absence of secondary breakdown, which was a failure mode specific to bipolar transistors.

The safe operating area of a MOSFET was bound only by the maximum drain-source voltage, the maximum drain current, and a thermal limit between them. However, due to device geometry scaling, recent MOSFETs exhibit secondary breakdown. It is therefore necessary to determine whether the operating locus of the MOSFET is within the safe operating area.

The graph showing the SOA curve with different boundary limits is as shown below,

We have the following limitations,

1) Thermal limit

2) Secondary breakdown limit

3) Current limit

4) Drain-source voltage limit

5) On state resistance limit.

From your application, the Ton duration is 3.508us and Vdd voltage is 100V. These points has to be plotted on the MOSFET SOA curve and the current limit can be determined on the y-axis.

The following image shows the current limit for your MOSFET and the working parameters,

The area between 10ms and 100us has to be considered and the corresponding point on the current axis has to be taken to get the current limit.

Please let me know if this explanation has helped you understand the SOA

Regards,

Abhilash P

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User22438

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Aug 05, 2021
08:47 PM

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Thanks for the explanation. I think my concern is more with the single pulse SOA. It is noted as non-repetitive single pulse, hence i was not sure how the SOA can be interpreted when the pulse is repeated continuously.

Thanks

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Abhilash_P

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Aug 05, 2021
10:15 PM

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The SOA curve gives the pulse drain current limitations at different turn ON durations. The value of this limitation for a particular duty cycle can be seen in the MOSFET's datasheet.

But for continuous operation, please refer the continuous drain current rating at 100C. For continuous operation, the maximum operating condition at 100C has to be considered to be on a safer margin.

Regards,

Abhilash P

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User22438

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Aug 05, 2021
11:08 PM

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Do you mean to use the DC line at 100V for safer margin? Is there an alternative to properly calculate the SOA limit for my operation?

Thanks

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Abhilash_P

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Aug 08, 2021
10:20 PM

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Hi,

The DC limit at 100 degrees has to be considered.

Regards,

Abhilash P

The DC limit at 100 degrees has to be considered.

Regards,

Abhilash P