through a fiber washer. Insert the screw with the
washer through the center of the toroid. Put the other
washer on the screw and thread on a nut. Hand-tighten
until snug. DO NOT OVER TIGHTEN. Figure 9 shows
the transformer mounted on the circuit board using the
The transformer is done. Set it aside until needed.
Figure 11 is the schematic of the converter. IC1
(LM3524) drives power MOSFETs Q1
and Q2 which, in turn, drive the
primary of the transformer in a
square wave. Switching frequency is
set by R4 and C3. The stepped-up
voltage at the secondary is rectified
and filtered to raw DC which is
regulated down by IC2 and IC3. R7
prevents C4 and C5 from charging up
to the peak of voltage spikes that can
occur with an unloaded secondary.
R5 and R6 are pull-downs to
speed up the turn-off of Q1 and Q2.
Since the MOSFETs turn on faster
than they turn off, both transistors
would be on for a part of every cycle
if they switched simultaneously,
causing the transistors to get hot
from current spikes. To prevent that,
the LM3524 provides dead time
between turning off one transistor
and turning on the other. Dead time
is set by the voltage at pin 2,
adjusted by trim-pot R3.
carefully inspect the solder joints. Then, mount C1, C2,
C3, and C7. Check polarity on C1 and C7. Next, mount
R1, R2, R4, R5, R6, R7, and trim-pot R3. Then mount Q1
Mount D1, D2, D3, and D4. Next, mount C4, C5, and
C6. Then, mount IC2 and IC3. There are lines near the
holes for IC2 and IC3 showing the positions of their metal
tabs. Mount the two terminal blocks.
Before installing the transformer, connect the board
to nine volts. Use an oscilloscope to look at the gates
of Q1 and Q2. You should see a square pulse with a
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Figure 12 is the board layout.
Components on the primary side
are surface-mount (SMT) devices. (I'm
using SMT since through hole parts
are obsolete). The secondary side has
mostly through-hole parts; I had a lot
of the diodes and regulators, and
wanted to use them.
There's an excellent video on
soldering SMT devices on the
Nuts & Volts Store page
( http://store.nutsvolts.com) under
the High Voltage Power Supply
Circuit kit. I recommend viewing it
before starting this project. Since the
video is so thorough, I won't go into
details about soldering here.
Mount IC1 — a 16 pin SOIC
device — first. With a magnifier,
March 2009 51