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.

LOW POWER STARTUP

When starting up a system utilizing the MSK4411 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 limit sensor has to see the current in order for the electronics to control it.

If possible, for startup use a 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.

5

APPLICATION NOTES CONTINUED

8548-148 Rev. B 1/15