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I'm dealing with Power N-Chanel Mosfets and, while looking at the data sheet of several units, I noticed that the information about the On-Region (\$ I_{D}\$ vs \$ V_{DS}\$) and the Gate Charge (\$ V_{GS}\$ vs \$ Q_{G}\$) does not match. Before going any further I think it would be useful to mention the assumptions I'm making while interpreting this data:

  • The Gate Charge graph is obtained by supplying constant current to the DUT while switching (see section 2 in this app note from IR or figure 13 in this data sheet from ST).
  • The constant current switching operation is done in 4 steps. First, \$ V_{GS}\$ rises until the threshold voltage is reached. Then \$ I_{D}\$ starts rising until the current set by the constant current source is reached (the Plateau begins). Thirdly, \$ V_{DS}\$ decreases until the device enters the Ohmic region (Plateau ends). Finally, \$ V_{GS}\$ keeps rising until the final value is reached.
  • All the graphs in the data sheets are given for the same Junction Temperature values.

The point of conflict is that, in the Gate Charge graph, the Plateau voltage for the given test conditions (i.e. \$ V_{DS}\$ and \$ I_{D}\$) is frequently much higher than the one you would deduce using the On-Region characteristic. Here are a couple of examples:

The Gate Charge graph shows that, for an \$ I_{D}\$ of 50A the Plateau \$ V_{GS}\$ is 4.74V. The \$ V_{DS}\$ at this point should be very close to 20V.

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Checking the On-Region graph shows that for, the same \$ V_{DS}\$ and \$ V_{GS}\$ values, the \$ I_{D}\$ should be clearly higher than 160A.

enter image description here

It is my understanding that these figures should match. I would highly appreciate if you could help me understand why this is not the case. I have tried contacting the manufacturers but so far none has replied.

This behavior can also be seen on further Mosfets. I won't put the images in here but here are the links to their data sheets if you are interested in checking them out:

This is not always the case, there are some devices where the values match:

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  • \$\begingroup\$ I'd also like to know how they keep Tj at 25 when pumping \$V_{DS}*I_D\$ = 1000W into it (or where my assumption fails). \$\endgroup\$ – Huisman Sep 17 '19 at 20:54
  • \$\begingroup\$ Really tiny pulses with a super big sink \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 17 '19 at 22:50
  • \$\begingroup\$ @SunnyskyguyEE75 is right. The losses generated for during a short pulse (turn on - conduction - turn off) are very low, which allows the device's temperature to remain fairly constant. \$\endgroup\$ – Cristián Ladisla Sep 18 '19 at 10:03
  • \$\begingroup\$ I think I found out the reason for this discrepancy. I will post an answer soon but I need some time to make sure my analysis is correct. \$\endgroup\$ – Cristián Ladisla Sep 24 '19 at 6:28
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I believe the Fig 8 Vds , Ids values only represent 1 point on the conduction curve which is not constant during the plateau charge region.

This device which adds a dual Y axis Vds curve for fig 8 at the fixed 50A limit gives more information and may anwser your query.

It assumes you understand the Id current is limited to the value stated in the graph and also by Vds/Ron controlled by Vgs during the total dynamic charge range. Generally Q increases as RdsOn decreases.

Since Q=CV also consider Ic=C *dV/dt + V * dC/dt how this affects conduction noise during switching.

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  • \$\begingroup\$ Thank you for your answer. During a theoretical the turn on process the \$V_{DS}\$ and \$I_{D}\$ values given in the Gate Charge graph will only be simultaneously present at one instant, that is correct. First the current rises, then the plateau is reached and then the voltage starts to drop. This, however, does not explain why the information does not match between both figures. For example, in the data sheet you provided, figure 1 shows that for a plateau voltage of 2.8V the current should be about 110A and not 50A as stated in figure 8. What am I missing? \$\endgroup\$ – Cristián Ladisla Sep 18 '19 at 6:12
  • \$\begingroup\$ Tolerances? Nominal? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 18 '19 at 8:41
  • \$\begingroup\$ Do you mean that the differences in current could be due to the tolerances of the parameters and graphs? That could be an explanation for this, but having such large tolerances (>100%) seems odd. \$\endgroup\$ – Cristián Ladisla Sep 18 '19 at 8:54

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