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.
INPUT VOLTAGE FILTER CAPACITORS
It is recommended that about 10 µF of capacitance (tantalum electrolytic) for bypassing the + and -15V power supplies be placed
as close to the module pins as practical. Adding ceramic bypass capacitors of about 0.1 µF to 1 µF will aid in suppressing noise
transients.
GENERAL LAYOUT
Good PC layout techniques are a must. Ground plane for the analog circuitry must be used and should be tied back to the SIG
GND. Ground plane for the power circuitry should be tied back to the GND pin, pin 16. Pin 16 should be connected to pin 11
external to the hybrid by a single thick trace. This will connect the two ground planes together.
LOW POWER STARTUP
When starting up a system utilizing the MSK 4310 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 dissipation will be
limited. The ±15 volt power should be applied prior to applying the V+ voltage.
5
Rev. B
2/04
MSK [ M.S. KENNEDY CORPORATION ]
