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one of the 3 HalfbridgesFor a BLDC driver I`m using this Mosfet: http://www.farnell.com/datasheets/526719.pdf?_ga=1.38505579.956125949.1445440291 on page 6 there are some diode data. Is this diode enough for my application?

Some data:

  • pwm frequency 20Khz

  • Supply Voltage 50V

  • Max current 100A

  • Each bridge has 3 mosfets parallel

  • Gate driver ir2110

how do I choose the right freewheeling diode?

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    \$\begingroup\$ Generally the internal diode is sufficiently matched to the device rating so no further diode is necessary. \$\endgroup\$ – Trevor_G Mar 9 '17 at 18:55
  • \$\begingroup\$ Thanks! How do I know that no further diodes were needed? \$\endgroup\$ – rusky Mar 9 '17 at 19:02
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    \$\begingroup\$ well it really depends on the drive cycle and motor so you can never be really sure till you build it. For some reason the spec-sheets never give nice data on the internal diode. I might design it to include some additional diodes and populate if required. A shotky diode able to handle the full motor current + tolerance is best. \$\endgroup\$ – Trevor_G Mar 9 '17 at 19:08
  • \$\begingroup\$ The voltage drop and current determines the power dissipation. Make sure your device does not over-heat. \$\endgroup\$ – skvery Mar 9 '17 at 20:01
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The "body diode" of a MOSFET is the P-N junction between the drain terminal and the substrate. It passes current whenever the source is more positive than the drain (in an N-channel MOSFET) because there is a direct connection between the source terminal and the substrate as well.

The body diode is an ordinary silicon diode, with a nominal forward voltage drop of 0.7 V or so. The voltage drop increases relatively rapidly with increasing current, because the substrate is not generally heavily doped, and has a higher intrinsic resistance as a result. The body diode is also relatively slow, because of the large capacitance associated with its electrode structure.

If you use external Schottky freewheeling diodes in parallel with your MOSFETs, the overall power dissipation during freewheeling will be roughly halved, improving overall efficiency. More importantly, the freewheeling dissipation will be occurring in a different device from the switching MOSFETs themselves (which have other sources of dissipation such as the losses during the switching transitions and the I2R losses while conducting), relieving them of a significant amount of stress, which should improve reliability.

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