The diode-connected transistor is used for synchronous rectifier. I have just drawn bulk diode as below. The fist one is mosfet with its bulk-source, bulk-drain diodes. Second and third pictures are when B-S or B-D are connected together. When B-D is tied together, we have body effect and this increases threshold voltage. However, we get a diode from D to S. I think this diode is good because it helps make the diode-connected transistor more like a real diode.

With the case B-S connected, we don't suffer from body effect but we don't have the help of diode from D to S.

So, my question is which one is better in this case, connect bulk with source or drain?

enter image description here

  • \$\begingroup\$ With BD connected diode becomes conducting with Nch positive drain, so no good \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 12 '16 at 12:21
  • \$\begingroup\$ That is not what we want? We want D and S conducts when D is more positive than S. \$\endgroup\$ – anhnha Oct 12 '16 at 12:29
  • \$\begingroup\$ Do you want Diode to conduct below Vgs threshold? When Vgs is greater than threshold, FET shunts diode. Reverse protection is preferred use. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 12 '16 at 12:34
  • \$\begingroup\$ To make it a real switch, I want it to conduct when Vgs > 0, so no voltage drop across the switch. \$\endgroup\$ – anhnha Oct 12 '16 at 12:43
  • \$\begingroup\$ Then use sub-threshold <1V MOSFET \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 12 '16 at 13:14

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