DC / DC
■ FIGURE 1. The motor controller block
outlines the major board sections.
From the diagram, four or eight N
channel FETs (only four are shown)
can be populated depending upon
the total drive requirements.
The FETs we are using are rated for
162A each, however this is not really
the truth in a real world application.
Practically speaking, the maximum
power that can be switched is dictated
by what the package (in this case, a
TO220AB) can safely dissipate. The
PWM frequency, duty cycle, motor
resistance, and input power level all
have a bearing on how the part will
perform. In addition, the leads on the
package and the lead frame (the
internal part that holds the chip) will
not pass this current without glowing
cherry red ... there is a big difference
between pulse power and continuous
power operation, something most data
sheets forget to mention.
For larger currents, these problems can be mitigated by paralleling
FETs together or using a package that
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will dissipate more heat. The OSMC
design uses up to four FETs per leg,
however we are only interested in 50A
max with adequate cooling, so this
will be more than enough.
The driver, HIP4081A contains
circuitry to generate the necessary
gate drive voltage and logic to ensure
that two FETs are never on at the same
time causing a condition known as
“shoot through” ... smoke to us ... the
time delay associated with this switching action is controlled by two external resistors — one per drive section.
The logic section ensures the
direction input toggles the correct
phases and routes the PWM signal
accordingly. The logic is also set up
to ensure the driver comes up in a
“safe” mode with the motor off if your
controller is not powered up.
The isolation section consists of a
quad opto-isolator and a single higher
speed opto-isolator. The digital signals
are essentially at DC levels, so a slow
opto-coupler is all that is required. In
addition, this part was chosen to present
a light load to the controller. Most LEDs
require at least 10 mA of current, but
this part can be driven by 5 mA or less.
The high-speed coupler is
required to correctly pass the PWM
signal with a minimum of edge distortion. I left some flexibility here to
experiment with higher PWM speeds
and different opto-isolators. Running
the controller at different PWM