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UC2625-EP: Safe braking with complementary FETs.

David Steinberg54
Prodigy 20 points
Part Number: UC2625-EP

Complementary FETs have the high and low side drains bonded together, preventing the insertion of the safe-braking steering diode as shown in datasheet Figure 8-C and 8-D. Would there be an issue with inserting the diode between the low-side FET source and Rs? Referring again to figure 8-C, if the FET body diode is used for the low-side diodes shown, their currents will flow through Rs (in the figure they don't). How will this impact the current sense behavior?

  • 0
    TI__Mastermind 28425 points
    Hey David,

    Adding the diode between the low-side FET source and Rs wouldn't cause an issue, but it wouldn't allow for safe breaking as if the bottom side FETs are quickly turned off you wouldn't have a diode protecting the node in-between the high side and low side FETs.

    Thanks,
    Daniel
  • 0 in reply to Daniel Hartung
    Prodigy 20 points

    OK, so looking at this from another angle the datasheet implies that the low-side FETs may turn off as you say because the interaction between the current sensing and the PWM comparator. If one adds circuitry to disable current sensing during braking then the steering diode would be unnecessary, correct?

  • 0 in reply to David Steinberg54
    TI__Mastermind 28425 points

    Hey David,

    As per page 19 of the datasheet:

    "A diode in series with the low-side MOSFETs facilitates PWM current control during braking by insuring that braking current will not flow backwards through low-side MOSFETs."

    While it may not be necessary from a control stand point, you would be giving up negative current prevention.

    Thanks,

    Daniel