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.

REGULATED VOLTAGE FILTER CAPACITORS

It is recommended that about 100 µF of capacitance (tantalum electrolytic) for bypassing the + and -15V regulated outputs 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 suppress-

ing noise transients.

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 4362 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.

6

Rev. G 2/05

APPLICATION NOTES CONTINUED