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I am a second year BS Electrical Engineering student, and I am working on making an autonomous robot in my summer vacations, nowadays.

Right now I am designing an H-Bridge using discrete components for my robots' drive motors.

So, here's what I have done already.

After selecting the package of the MOSFET, and then finding the amount of power it can dissipate, given the operating temperatures, I was able to calculate the maximum allowed Rds(on) of the MOSFET to be selected. I then went on to select the right MOSFET i.e. IRF1010Z.

Now, when I went to look the output characteristics of this MOSFET in order to find the right gate voltage to apply to, so that Vds is low and that the MOSFET is able to conduct a maximum of 5A. However, the output characteristics of the device are given on a log-log scale that I don't seem to understand. The graph doesn't exist for low current values of (0-5A), which is the range I want to work on. So please help me understand the output characteristics of this MOSFET device. Here are the characteristics, enter image description here

Maybe, a Vgs of 5-6 V will be fine, but what confuses me is the nonexistence of the graph for low current values.

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  • \$\begingroup\$ There are a lot of combinations for which the graphs do not exist, this isn't mean as an exhaustive chart. Try interpolating, or play it safe. \$\endgroup\$ – PlasmaHH Jun 16 '16 at 10:53
  • \$\begingroup\$ That's because that device isn't usually used for currents that piddly. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 16 '16 at 10:53
  • \$\begingroup\$ @PlasmaHH do you mean that I should use a Vgs of 10V or so? \$\endgroup\$ – Muhammad Faique Shakeel Jun 16 '16 at 11:14
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You can project the slopes back to get a reasonable approximation. Or you can assume current is ten times higher at 50 A. For instance with a gate voltage of 6V you'll get this: -

enter image description here

Volt drop will be 0.43 V at 50A so just assume a straight line and at 5 A you will see a volt drop of about 43 mV = 215 mW dissipation.

MOSFETs like this have a Rds(on) value dictated by Vgs in the "linear region" and it's a fairly good approximation to assume Rds is also linear in this region.

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