MSK4250_15 Datasheet, PDF (4/6 Pages) M.S. Kennedy Corporation – 75 Volt Motor Supply Voltage

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 30nH 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, pin 17, 18, 23, 24, 33 and 34. The high power grounds (RTN) pins 1,2 and 3 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 4250 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.
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Rev. G 3/11