28 Silicon chip Australia’s electronics magazine siliconchip.com.au
of the board, our Mosfet PCB (shown in Fig.3a) actually has
twelve possible Mosfet mounting locations; six on the top
and six on the bottom, with each pair of Mosfets directly
above and below each other (Q1 and Q1’, Q2 and Q2’ etc).
Fig.3b shows where the Mosfets can be mounted on the
underside of the board.
This gives you the option to mount three or four Mosfets
on one side of the board and the remainder on the other
side, which will help to more evenly distribute what little
heat is generated in the device, and may also make slightly
better use of the copper, reducing losses slightly. But it’s a
minor advantage, and you could just as easily fit them all
one side, which is what we did.
For the control board, install the resistors where shown,
then the 1N4148 diodes, ensuring that in each case, the
cathode stripe faces as indicated.
You can then fit the single zener diode, with its cathode
stripe facing to the left. Next, solder IC1 to the board, ensur-
ing that its pin 1 dot/notch face towards the top as shown.
We don’t recommend that you use a socket as these can
cause failures over time.
Now fit the non-polarised capacitors, which can be either
ceramic or MKT types, followed by the single electrolytic
capacitor, with its longer positive lead through the right-
hand pad (marked with a “+” symbol).
That leaves REG1 and CON1. Gently bend REG1’s leads
to fit the PCB pads, then solder it in place. CON1 is a regu-
lar 5-pin header that’s soldered to the top side of the board.
You can then move on to the Mosfet board.
Building the Mosfet board
This board has eight SMDs (six Mosfets and two TVS di-
odes) plus three through-hole components, not including
the battery connections, which we’ll explain below.
Start by soldering the Mosfets. These are quite large and
are soldered to large, thick copper planes so you will need
a hot iron to solder them.
In each case, start by spreading a thin layer of flux paste
over all the pads, especially the large one for the tab. Then
locate the Mosfet in position and solder its pin 1 (near the
dot). This is the gate connection so should be the easiest
to solder.
Check that all the pins and the tab are lined up correctly.
If not, re-heat that solder joint and nudge the device slightly.
Solder the remaining five small pins next. It doesn’t mat-
ter if you accidentally bridge them to each other, as long as
they don’t bridge to the middle stub pin (which is not con-
nected on this board) or pin 1 (the gate drive).
Finally, flow solder onto the junction of the tab and its
large mounting pad underneath. You will need to apply heat
and feed in solder until the solder flows to form a smooth
fillet between the two. It’s OK to add a little extra solder un-
til it covers the tab. The flux you added earlier should aid
in this process.
Repeat the above for the other five Mosfets. Then solder
the two TVS diodes in place using a similar procedure, ie,
applying flux paste to both pads, tacking the part down on
one side, soldering the other side, then refreshing the first
solder joint to ensure it is reliable.
Next, solder ZD2 and LED1 in place on the top side, with
the orientations shown. It’s a good idea to fit LED1 with some
space between its lens and the PCB, so that it can poke through
a hole in the case. The base of its lens should be a little bit
more than the thickness of one M8 nut above the board.
Having done that, fit 5-pin header socket CON2 on theHere’s the top side of the completed Mosfet PCB. It’s fitted
with six Mosfets as shown in Fig.3a (top). But if you wish,
another six Mosfets can be soldered to the underside of
the PCB for even better current handling (Fig.3b, lower)