MSK4370HU Ver la hoja de datos (PDF) - M.S. Kennedy Corporation
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MSK4370HU Datasheet PDF : 8 Pages
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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.
REGULATED VOLTAGE FILTER CAPACITORS
It is recommended that about 10 µF of capacitance (tantalum) 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 suppressing 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, 28. This can be used as necessary for voltage sensing, etc.
LOW POWER STARTUP
When starting up a system utilizing the MSK4370 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.
COMPENSATION AND ANTI-WINDUP
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.
Because this high voltage torque amplifier contains only high-side bootstrap supplies, it must continuously PWM the bridge to refresh
the high-side supply capacitors. Additionally, this type of amplifier controls the loop through PWM, so it must PWM all of the time in order
to maintain control.
When the bridge commutates the motor through each phasing state, the current path switches windings. The current must be ramped
back up after each transition. With an integrating compensation scheme, it is very possible for the error amplifier to exceed the PWM
maximum and minimum voltages. When this happens, the loop stops PWM control and the error amplifier will continue to integrate and
ramp up to the voltage rail. This is called integrator windup.
Recovery from integrator windup can take a significant amount of time, long enough for the bootstrap supply capacitors to be depleted
and shut off the gate the gate drive. This must be prevented from happening. By placing zener diode clamps across the error amplifier
output to the inverting input, it will clamp the amplifier from running past the PWM maximum and minimum.
For a free-running clock with no synchronization, the zeners used should be 7.5V. In each direction, the voltage will be 7.5V plus one
diode drop, or 8.2V to 8.5V. If the synchronization pin is being used for clock sync, then that voltage may have to be lower, as the sync
scheme effectively shortens up the PWM ramp to increase frequency and sync the clock. A 100 ohm series current limiting resistor is
necessary to prevent the error amplifier from driving too much current back through the feedback when the diodes are conducting
5
8548-24 Rev. L 1/15
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