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.
±15VIN FILTER CAPACITORS
It is recommended that about 10 µF of capacitance (tantalum electrolytic) for bypassing the + and -15V inputs be placed as close
to the module pins as practical. Adding ceramic bypass capacitors of about 0.1 µF or 1 µF will aid in suppressing noise transients.
GENERAL LAYOUT
Good PC layout techniques are important. Ground planes for the analog circuitry must be used and should be tied back to the
small signal grounds, pins 11 and 13. The high power grounds (RTN) pins 19 and 20 get tied back to the small signal ground
internally. DO NOT connect these grounds externally. A ground loop will result.
LOW POWER STARTUP
When starting up a system utilizing the MSK 4364 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. 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 +4 volts positive
peak, or -4 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. For the power up sequence, ±15 volts should be powered at the same time or before the V+ voltage is applied.
5
Rev. A 1/02
MSK [ M.S. KENNEDY CORPORATION ]
