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UC2625: Active Freewheeling

Part Number: UC2625
Other Parts Discussed in Thread: CSD88539ND, , LM5100A

Hi everyone, 

We are using UC2625 for BLDC control along with LM5100A gate drivers and CSD88539ND mosfets. Our problem is that during the LOW side of the PWM generated by the UC2625, all gates are disabled and only diode freewheeling becomes active. Without the active freewheeling, when the PWM duty steps to zero from a non-zero PWM duty, the motor just coasts to zero speed slowly.

With the following speed control loop (from slua106), it can be seen that motor can quickly accelerate to the target speed but cannot decelerate to zero speed as quickly due to this diode freewheeling behaviour.

We could use the brake feature of the UC2625 to quickly stop the motor. However, to quickly decelerate to a non-zero speed or to follow a sharp speed ramp, we can't use this feature.

We have built a speed control loop along with a current control loop that actively slows the motor by applying an opposite current. This way it is possible to slow down the motor very quickly. However, in our application we can't use a closed loop speed control due to other reasons.

If there is a method or circuitry that we can implement to achieve an active freewheeling behaviour with UC2625, we would be glad to know it.

Best Regards.