TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
15
5.2.6 Considerations for QGD protection
This chapter gives the background understanding to ensure a safe operation for MOSFETs with a
gate-drain (Miller) charge QGD substantially larger than the gate-source charge QGS.
In order to guarantee a safe operation of the QGD protection, it is important to spend a few thoughts on
the slope control setting. Please check your transistors’ data sheet for the gate-source charge QGS and
the gate-drain charge QGD (Miller charge). In order to turn on the MOSFET, first the gate-source
charge needs to be charged to the transistor’s gate. Now, the transistor conducts and switching starts.
During the switching event, the additional QGD needs to be charged to the gate in order to complete
the switching event. Wherever QGD is larger than QGS, a switching event of the complementary
MOSFET may force the gate of the switched off MOSFET to a voltage above the gate threshold
voltage. For these MOSFETs the QGD protection ensures a reliable operation, as long as the slopes
are not set too fast.
Calculating the maximum slope setting for high QGD MOSFETs:
Taking into account effects of the MOSFET bulk diode (compare chapter 5.2.7), the maximum slope of
a MOSFET bridge will be around the double slope as calculated from the Miller charge and the gate
current. Based on this, we can estimate the current required to hold the MOSFET safely switched off:
During the bridge switching period, the driver must be able to discharge the difference of QGD and QGS
while maintaining a gate voltage below the threshold voltage.
Therefore
Thus the minimum value required for IOFFQGD can be calculated:
Where ION is the gate current set via RSLP, and IOFFQGD is the QGD protection gate current.
The low side driver has a lower QGD protection current capability than the high side driver, thus we
need to check the low side. With its RLSOFFQGD of roughly 15 Ohm, the TMC603 can keep the gate
voltage to a level of:
Now we just need to check UGOFF against the MOSFETs output characteristics, to make sure, that no
significant amount of drain current can flow.
Example:
A MOSFET, where QGD is 3 times larger than QGS is driven with 100mA gate current.
The TMC603 thus can keep the gate voltage level to a maximum voltage of
UGOFF = 133mA * 15Ω = 2V
This is sufficient to keep the MOSFET safely off.
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