APPLICATION NOTES CONTINUED
BUS VOLTAGE FILTER CAPACITORS
The size and placement of the capacitors for the DC bus has a direct bearing on the amount of noise filtered and also on the
size and duration of the voltage spikes seen by the bridge. What is being created is a series RLC tuned circuit with a resonant
frequency that is seen as a damped ringing every time one of the transistors switches. For the resistance, wire resistance,
power supply impedance and capacitor ESR all add up for the equivalent lumped resistance in the circuit. The inductance can
be figured at about 30 nH per inch from the power supply. Any voltage spikes are on top of the bus voltage and the back EMF
from the motor. All this must be taken into account when designing and laying out the system. If everything has been
minimized, there is another solution. A second capacitance between 5 and 10 times the first capacitor and it should either
have some ESR or a resistor can be added in series with the second capacitor to help damp the voltage spikes.
Be careful of the ripple current in all the capacitors. Excessive ripple current, beyond what the capacitors can handle, will
destroy the capacitors.
GENERAL LAYOUT
Good PC layout techniques are a must. Ground planes for the analog circuitry must be used and should be tied back to the
small pin grounds 9, 14 and 23. Additional ground, pin 26 is an isolated ground that connects internally directly back to the
main DC bus ground pin 27. This can be used as necessary for voltage sensing, etc.
LOW POWER STARTUP
When starting up a system utilizing the MSK 4371 for the first time, there are a few things to keep in mind. First, because of
the small size of the module, short circuiting the output phases either to ground or the DC bus will destroy the bridge. The
current limiting and control only works for current actually flowing through the bridge. The current sense resistor has to see
the current in order for the electronics to control it. If possible, for startup use a lower voltage and lower current power supply
to test out connections and the low current stability. With a limited current supply, even if the controller locks up, the dissipa-
tion will be limited. By observing the E/A OUT pin which is the error amp output, much can be found out about the health and
stability of the system. An even waveform with some rounded triangle wave should be observed. As current goes up, the DC
component of the waveform should move up or down. At full current (with a regular supply) the waveform should not exceed
+8 volts positive peak, or -8 volts negative peak. Some audible noise will be heard which will be the commutation frequency.
If the motor squeals, there is instability and power should be removed immediately unless power dissipation isn't excessive due
to limited supply current. For compensation calculations, refer to the block diagram for all information to determine the
amplifier gain for loop gain calculations.
5
PRELIMINARY Rev. A 6/00
MSK [ M.S. KENNEDY CORPORATION ]
