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I'm designing an application where a power logic level MOSFET is driven using a 5V PWM signal at 20kHz. The drain current will be approx. 10A and the voltage of the load 12V. I looked into the BUK954R8-60E. Which is a good estimation of the power dissipated by the MOSFET? I managed to calculate the conduction power losses (0,49W), but I do not really understand how to evaluate the switching power losses.

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  • \$\begingroup\$ Please add a link to the datasheet. \$\endgroup\$ – jippie Apr 17 '13 at 10:48
  • \$\begingroup\$ nxp.com/documents/data_sheet/BUK954R8-60E.pdf \$\endgroup\$ – Francesco Apr 17 '13 at 11:08
  • \$\begingroup\$ Asking for part shopping information, even adding price range, falls afoul of this site's policies around shopping questions. Please revisit the question and edit, to avoid close-votes. \$\endgroup\$ – Anindo Ghosh Apr 17 '13 at 11:13
  • \$\begingroup\$ switching losses are a function of not only the MOSFET, but the gate driver, since they depend on how fast the switching can happen. \$\endgroup\$ – Phil Frost Apr 17 '13 at 12:28
  • \$\begingroup\$ @Francesco It's best practice to not accept an answer so quickly after posting a question, to stimulate discussion on the site. Upvotes are a better way to support answers until there's a sufficient number to choose from. \$\endgroup\$ – Adam Lawrence Apr 17 '13 at 15:33
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Keep the connections from microcontroller to MOSFET short, both gate and source. The gate-source capacitance is relatively large and wires act as inductors. The combination of long wires, \$C_{GS}\$ and sharp edges will introduce ringing (oscillations). Instead of turning the MOSFET quickly on and off, it will spend a relatively long time in its linear mode of operation, where a lot of heat is dissipated. Low \$C_{GS}\$ and low \$R_{DS,on}\$ are good properties to look for in the datasheet.

To dampen ringing, a small resistor 100~220\$\Omega\$ in series with gate is good practice.

Websites like digikey.com and mouser.com have parametric search, which makes selecting a transistor that answers your criteria quite a bit simpler. Always check local availability of your parts and prices can vary a lot with various shops.

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Which is a good estimation of the power dissipated by the MOSFET?

The power dissipated by the MOSFET is a combination of two parameters:

  • Conduction loss
  • Switching loss

Conduction loss is easy to calculate, and for many applications easily make up 80% of the total losses for the device. Since you're only switching at 20kHz, I would estimate that your circuit is predominantly conduction-loss dominant. The datasheet will give you what you need to estimate \$ R_{DS(on)} \$ vs. \$ V_{GS} \$ for the device, which allows you to easily calculate the power loss.

With 5V \$ V_{GS} \$, your part appears to be somewhere around 3.75 milliohms at 25C, so your conduction estimate isn't too far off (depending on duty cycle, obviously).

Switching loss is another animal altogether. It's a function of not only how quickly the \$ V_{DS} \$ and \$ I_D \$ change, but also the nature of the load - inductive or resistive, generally speaking. Once you know what type of load you're controlling, you can apply an appropriate model to estimate the switching losses.

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