(tr)uSDX BS170 woes

(tr)uSDX users seem beset by a number of common problems, and one of them is failure of the BS170 PA FETs.

Further, lots of users have bought replacements that appear as a dead short circuit when wired into the board. This seems to be that pins 1 and 3 are transposed on some product bought online.

These FETs have an integral body diode, and if Source and Drain are swapped, the body diode will conduct, appearing to be a near short circuit on the supply rail in the (tr)uSDX.

So a possible explanation is that Chinese cheats may have labelled 2N7000 as BS170, the specs are similar in many respects except:

  1. 2N7000 has a lower current rating;
  2. the BS170 saturates at lower Vgs; and
  3. Drain and Source pins are transposed.

Now if the original (tr)uSDX was made with 2N7000 (wired appropriately) or 2N7000 labelled as BS170, it has lower current rating FETs and may be easier to damage. This is one reason why people are buying more FETs.

So, ‘fixing' the pinout problem by rewiring the FETs may not be a fix at all. If in fact the dodgy FETs are mislabeled 2N7000, you have degraded the (tr)uSDX.

Substitution of 2N7000 might also explain low efficiencies reported by many users, the gate characteristics are different to the BS170 (2. above) and that is highly relevant to the necessary drive level for Class E.

If it were me, for this critical application, I would buy BS170 from a reputable source.