not to our dismay that if you place two resistors in parallel,
current should double. In essence, each H-bridge is capable
of handling the 2.8A max before shutting down, allowing
me to reach the 5A mark. I corroborated this fact in two
ways: with a DC motor and with a power resistor.
With a motor, it is very hard to see the maximum
current if the motor is not large enough. Plus, even if you
stall a large motor, once the inductance field is maximized,
the rotor is now seen as a low impedance short. So,
whatever current you get that’s it. What I did to look for a
difference was to switch PHASE very fast. When this is
done, the inrush current is at its worst since the charged
inductor will act as a load opposing the power supply.
To the eyes of the controller, this current inrush may
reach its limit, in which case the H-bridge will be tri-stated.
After some time (defined by tOCP = 1.2 ms as stated on
the A3950 datasheet), the device retries and continues
applying current until the max is found again.
Using a current probe, I was able to count the number
of retries and found this number to be considerably higher
with a single controller than with the parallelized version.
Photo 3 shows what I mean. Note that for the single
controller, the number of pulses is twice that as with the
parallel controller. At the same time, the width of each
pulse is about half for the single controller when compared
52 SERVO 04.2009
to the parallel controller. Also, the maximum peak is higher
on the parallel controller than on the single controller.
The second test consisted of applying a power resistor
as a load instead of a DC motor. Since the high speed
transient is eliminated from the picture, we can better see
the current driven being twice on the parallel version when
compared to the single controller version. To do this, I used
a 5W power resistor. At 24V, this resistor draws a 4.8A
current. If my theory is correct, the single controller will not
be able to drive this load, while the parallel version will.
Photo 4 shows the theory to be correct.
Once again, I used a current probe to measure the
loading. My theory was proven when I saw the square
wave on the single controller but a steady 4.8A DC line
with the parallel version. The scope capture, on the other
hand, shows a current closer to 4.2A, than 4.8A. I take
advantage of this finding to impart a very important fact
about H-bridges and RDS ON.
When you first enable the H-bridge and the FETs are
cold, the RDS ON is as we explained earlier. However, it is a