Power Mosfet Information Mismatch: On Region Characteristics vs Total Charge Graphs

Question

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:

• On Semiconductor NTMFS5C426N:

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.

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.

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:

• On Semiconductor NTD5805N
• ST STL260N4F7

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

• Infineon IPLU300N04S4

Answer 1

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.