Trinamic TMC603-EVAL evaluation Board TMC603-EVAL 데이터 시트

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TMC603-EVAL

TMC603 DATA SHEET (V. 1.05 / 11. Mar. 2009)

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 Q

substantially larger than the gate-source charge Q

In order to guarantee a safe operation of the Q

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 Q

and

the gate-drain charge Q

(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 Q

needs to be charged to the gate in order to complete

the switching event. Wherever Q

is larger than Q

, 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 Q

protection ensures a reliable operation, as long as the slopes

are not set too fast.

Calculating the maximum slope setting for high Q

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 Q

and Q

while maintaining a gate voltage below the threshold voltage.

Therefore

Thus the minimum value required for I

OFFQGD

can be calculated:

Where I

is the gate current set via R

SLP

, and I

OFFQGD

is the Q

protection gate current.

The low side driver has a lower Q

protection current capability than the high side driver, thus we

need to check the low side. With its R

LSOFFQGD

of roughly 15 Ohm, the TMC603 can keep the gate

voltage to a level of:

Now we just need to check U

GOFF

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
U

GOFF

= 133mA * 15Ω = 2V

This is sufficient to keep the MOSFET safely off.