Asking about the use condition of STD4NK100Z

[SSONY]

Hello.

We are considering to apply your MOSFET (STD4NK100Z).

Particularily, we are going to apply the MOSFET with the source follower application.

What I want to know is whether the use condition that we are planing is alright.

First, Vgs will be fixed at 5V with a resistor at source side with 4.7kohm

Second, Vds will be placed up to 800V ~ 1000V (Current is restrained by MOSFET Output characteristic)

Third, This operation is constant whenever electrical power is applied. (2.3mA is always flowing through the MOSFET and resistor)

Fourth, Vgs can be changed according to input voltage fluctuation up to 5~6V

With these condtions, I would like you to let me know whether you can guarantee the MOSFET can be used in safety operation area.

Please reply to me with expert's opinion.

Thank you in advance.

[Peter BENSCH]

Welcome, @SSONY​, to the community!

A few mathematical calculations to answer your questions:

1. If you set Vgs to 5V, i.e. not the voltage between gate and GND, but actually 5V between gate and source, then according to fig. 5 of the data sheet about 125mA could flow. With a 4.7K resistor at the source, this would result in a voltage of 587.5V and a power loss of 73.4W. If you then also apply 800V drain voltage, 212.5V drop out at the MOSFET, which would correspond to a power loss of 26.6W at 125mA, and even 51.6W at 1000V (see point 2 below).
2. 1000V is the absolute maximum, i.e. if this value is exceeded even slightly, the function of the MOSFET is no longer guaranteed or it can be destroyed immediately.
3. The indicated 2.3mA are not clearly predictable, because the threshold voltage has a relatively wide range: 3...4.5V, but this is also only valid for 50µA. To obtain 2.3mA drain current, a voltage of 4.7k*2.3 mA + 3...4.5V = 13.81...15.31V would have to be applied between gate and GND.
4. I cannot understand this point.

It is probably useful to examine and discuss the use case in detail. With a source follower aka common drain amplifier, the MOSFET, or more precisely an enhancement MOSFET, operates in linear mode, which can be set up with a MOSFET in planar technology.

[edit] even if the following statement is correct, it does not apply here because it is not a Super Junction MOSFET but a planar one:[/edit]

However, today's high voltage MOSFETs are usually build with super-junction (trench) technology in order to achieve inter alia the lowest possible values for Rdson, which can lead to local heat hotspots in linear operation. This linear operation of super-junction MOSFETs is therefore not recommended.

Can you please explain why you want to use a source follower at such a high drain voltage?

Regards

/Peter

[SSONY]

Hello.

Thank you for your sincere reply.

Let me reply to you about your question and more information,

1. The reason for application of a source follower is to flow current continously through the MOSFET and source resistor.
2. I want to know SOA diagram for DC (continous Id not pulse). Cause we are planning to use this circuit as the linear operation in constant DC at Vgs = 5V, Vds = 800V, Id = 2.3mA

Conclusively, Do you think or recommend this circuit we are planning dipicted on above?

I'm not sure this plan is in SOA area and recommendable or not. Cause you mentioned the linear operation with High voltage MOSFET is not recommended.

Thank you for in advance.

I'm wating for you reply again.

[Peter BENSCH]

[edit] even if the following statement is partly correct, it does not apply here because it is not a Super Junction MOSFET:[/edit]

The Some high-voltage MOSFETs are manufactured using super-junction MDmesh technology, which, as already mentioned, is not suitable for linear operation, which is why no DC mode is indicated in the SOA diagram. The MDmesh MOSFETs are optimised for switching operation due to their structure, as found in LLC converters, PFC stages, etc. In this respect, you do not need to pursue this further for the application.

What you could consider, however, is the use of a biolar transistor (e.g. the BUL741), which is still manufactured with planar technology. Bipolar technology has the disadvantage that you can't control powerlessly, but the advantage of more predictable base-emitter voltage.

If the problem is resolved, please mark this thread as answered by selecting Select as best under your preferred answer. This will help other users find that answer faster.

Regards

/Peter

[SSONY]

I feel really thanksful and I'm gonna reflect your advice to my plan.

Finally, Let me ask you only one thing about it.

If we apply this MOSFET for the condition that I mentioned, what problems are expected to happen? and

What surrounding condition could make it worse?

[Peter BENSCH]

As this would be operation outside the guaranteed specifications, no reliable statement can be made about the possible consequences. In the worst case, a thermal hotspot forms in the gate structure, which can expand exponentially at such high operating voltages, which then would lead to the sudden destruction of the MOSFET.

But also think of the maximum power losses that can occur! You had only given insufficient information, so that a more precise assessment is not possible.

Regards

/Peter

[Peter BENSCH]

I am very sorry for the confusion, but my statement was not correct. After consultation, I can inform: SuperMESH is planar, and since STD4NK100Z is made with SuperMESH, it follows that STD4NK100Z is suitable for linear operation.

I have already crossed out my incorrect statements above.

Regards

/Peter

[SSONY]

Ok. Thank you for your correction.

If so, one thing that should be sure.

This operation condition is always ON meaning DC mode. But the SOA in the datasheet doesn't have DC line. In the past, you mentioned it can't be drawn due to not guaranteed condition. I would like you to present me with DC line on the SOA for this MOSFET officially. Otherwise, I couldn't help thinking that this MOSFET is not appropriate for DC condition.

[SSONY]

In addition to above question, I found regarding specific reference applicable to this Linear Operation Mode with DC for general POWER MOSFET (not specified in super junction but all MOSFET)

According to this reference, the linear operation mode with DC is in thermal instability boundary.

This is in accordance with the pointing out that you did. This application is thermally instable.

So, I assume that STD4NK100Z's datasheet doesn't have DC line on the SOA diagram.

and It means this MOSFET was made only for switching application not always ON conditon.

Thus, I really want you to confirm that STD4NK100Z can be used safely for the linear operation mode with DC and DC line can be drawn on the SOA diagram.

I attached original reference about this and indicated the raised operation condition on the SOA in the article

[SSONY]

One more thing I found regarding this operating condition (Linear Mode Operation with DC)

This application is using the lowest Vgs 5V to regulat Id using the principle of source follower.

But, the linear mode operation with low Vgs is under conditon below ZTC (Zero Temperatur Coefficient)

Using low Vgs for long term below ZTC is known as really dangerous condition for MOSFET due to thermal runaway.

I would like you to confirm whether this MOSFET(STD4NK100Z) is all right to be used under this condition.