FEBRUARY 2006 £3.80 - Index of

HANDS-ON RC MODELLING
conduct at the same time, which can
cause a short-circuit and, in general,
destroy the transistors), it is imperative
to prioritize a very fast OFF setting and
a slow ON setting. In this way, the
BAT54C diodes allow for a very fast
pull-up/down to the power MOSFET
gates (to 0 or +Vbattery according to
the type N or P of the MOSFET), while
the ON setting is at 240 Ω (R37, for
28
PWM ON
+VCC
Figure 10. The double-sided board
of the power stage.
PWM OFF
Figure 9. Current during PWM ON and OFF.
+VCC
050157 - 19
example). We can then operate the
motor-control cell of the micro to its
utmost and utilise synchronous rectification.
How about that?
Synchronous rectification
Figure 9 shows what happens in a
bridge branch when the PWM signal is
applied to the P MOSFET. When the
Figure 11. The double-sided board
of the control module.
PWM is ON (closed switch), the current
thus passes through the top MOS-
FET to go into one of the motor phases.
When the PWM is OFF (open switch),
the current (continuous to the coil terminals)
must make a path via the bottom
transistor in the same bridge
branch. If the bottom MOSFET is OFF,
this current shall pass this way via the
internal freewheeling diode (poor
diode quality, in general). A rise in
temperature will result, which is, by
the way, the main reason a MOSFET
stage would overheat in this type of
application. In summary, why not utilise
the bottom MOSFET N when the
top MOSFET P is OFF? That is quite
clearly the definition of ‘synchronous
rectification’.
When the top P MOSFET “is open”, the
bottom N MOSFET closes after an
adjustable time delay called dead time
which is required to avoid any shortcircuit
of the branch; this time delay
has been set at 325 ns in this application.
Then, the current no longer flows
to the freewheeling diode, but rather to
the MOSFET. Transistor heating is
significantly reduced because then
only one resistor, equivalent to a few
milliohms, is visible on the current
path. A fair number of manufacturers
do not take advantage of synchronous
rectification because the microcontrollers
utilised are simply not capable of
generating such signals.
Construction
The boards are both double-sided, and
components must be soldered to each
side (a necessary evil to keep the size of
the circuit within the correct proportions).
Despite the fineness of some
copper traces, it is very well possible
to build these boards with traditional
materials. The author has made numerous
PCBs with a simple laser printer
and a ferric chloride etching solution.
If you do not have access to boards
with through holes, then it is best to
proceed with installing bridges via linking
each side of the circuit (using wire
wrapping, for example) and to cut
them as close as possible to the board.
Next comes installing the integrated
circuits, such as the micro, 5-V regulators,
EEPROM, quartz crystal, as well
as the two rows of six MOSFETs: for
the latter, be careful to align them properly,
avoiding raising some of the
devices with respect to others because
you will have to add a common heatsink.
If not, a fine file can be used to
level the MOSFET modules; however,
elektor electronics - 2/2006