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I was looking at the schematic on page 10 of this application note: http://www.ti.com/lit/an/snva557/snva557.pdf

I was looking to order parts for it but I am having trouble finding parts for Q1 and Q2. The schematic symbol is for a N-Channel Depletion Mode Single Gate MOSFET and they are labled as VN01A. I tried searching for this component, but nothing came up.

Does VN01A refer to a family of MOSFETs or is it an obsolete part? Can anyone recommend a component that works with this schematic?

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  • \$\begingroup\$ NDMOSFETs are incredibly hard to find as of late. Even PDMOSFETs are rare. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Nov 18, 2016 at 23:42
  • \$\begingroup\$ @IgnacioVazquez-Abrams Am I right in thinking that ND and PD MOSFETS are part of the LDMOSFET family? \$\endgroup\$
    – Polynomial
    Commented Nov 18, 2016 at 23:45
  • \$\begingroup\$ @Polynomial: No, "LD" stands for "Laterally-Diffused" whereas the "D" stands for "Depletion-mode". They are different things. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Nov 18, 2016 at 23:46
  • \$\begingroup\$ @IgnacioVazquez-Abrams Cool. Wasn't sure, as I'd come across LDMOSFET but not the PD or ND prefixes before. \$\endgroup\$
    – Polynomial
    Commented Nov 18, 2016 at 23:48
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    \$\begingroup\$ @IgnacioVazquez-Abrams -- actually, I know of several NDMOSFET parts but no PDMOSFETs whatsoever \$\endgroup\$
    – ThreePhaseEel
    Commented Nov 19, 2016 at 0:59

1 Answer 1

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First off -- the schematic is using the wrong symbol. The VN01A appears to be an enhancement mode part from its role in the circuit and the part number -- all Supertex depletion mode parts have a D in their part number such as LND01 or DN3765. (Microchip still makes them, BTW.)

As to the part number itself, the VN01A is an older Supertex N-channel part that appears to be obsolete. Given the noncritical small-signal switching application the circuit from the linked application note uses them in, and the note about the schematic symbol listed above, I'd substitute any jellybean N-channel FET in TO-92 or SOT-23.

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    \$\begingroup\$ Those FETs are only used to switch R2 and R11 in and out of the circuit when needed. As the answer states many devices would suit - BSS138 is one of my favourites. \$\endgroup\$
    – Kevin White
    Commented Nov 19, 2016 at 2:14
  • \$\begingroup\$ Ok, so the transistors are just working as on/off switches to control the type of current protection being used, meaning that the component choice is non-critical. That makes sense. \$\endgroup\$
    – Emmett P
    Commented Nov 19, 2016 at 19:03
  • \$\begingroup\$ Also because of their odd choice to use NDMOSFETs that means that when the logic levels are low, both resistors with be in the circuit, which would then lead to the lowest trickle current level. Is that correct? \$\endgroup\$
    – Emmett P
    Commented Nov 19, 2016 at 19:07
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    \$\begingroup\$ I am planning to use this circuit with Ni-Mh batteries, which need the lowest level of trickle charge, so I could just leave the transistors out entirely and have both resistors always in the circuit. \$\endgroup\$
    – Emmett P
    Commented Nov 19, 2016 at 19:09
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    \$\begingroup\$ @ThreePhaseEel Okay, so these resistors being in circuit actually increases the current going to the battery. So to have a trickle charge current of c/40 when the overcharge protection is activated, which is my goal, I could just leave them out of the circuit entirely. \$\endgroup\$
    – Emmett P
    Commented Nov 19, 2016 at 19:21

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