TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
16
Note:
Do not add gate series resistors to your MOSFETs! This would eliminate the effect of the QGD
protection. Gate series resistors of a few Ohms only may make sense, when paralleling
multiple MOSFETs in order to avoid parasitic oscillations due to interconnection inductivities.
5.2.7 Effects of the MOSFET bulk diode
Whenever inductive loads are driven, the inductivity will try to sustain current when current becomes
switched off. During bridge switching events, it is important to ensure break-before-make operation,
e.g. one MOSFET becomes switches off, before the opposite MOSFET is switched on. Depending on
the actual direction of the current, this results in a short moment of a few 100 nanoseconds, where the
current flowing through the inductive load forces the bridge output below the lower supply rail or above
the upper supply rail. The respective MOSFET bulk diode in this case takes over the current. The
diode saturates at about -1.2V. But the bulk diode is not an optimum device. It typically has reverse
recovery time of a few ten to several 100ns and a reverse recovery charge in the range of some
100nC or more. Assuming, that the bulk diode of the switching off MOSFET takes over the current, the
complementary MOSFET sees the sum of the coil current and the instantaneous current needed to
recover the bulk diode when trying to switch on. The reverse recovery current may even be higher
than the coil current itself! As a result, a number of very quick oscillations on the output appear,
whenever the bulk diode leaves the reverse recovery area, because up to the half load current
becomes switched off in a short moment. The effect becomes visible as an oscillation due to the
parasitic inductivities of the PCB traces and interconnections. While this is normal, it adds high current
spikes, some amount of dynamic power dissipation and high frequency electromagnetic emission. Due
to its high frequency, the ringing of this current can also be seen on the gate drives and thus can be
easily mistaken as a gate driving problem.
VVM
HS takes over
output current
UBMX
0V
-1.2V
IOUT
IHS
0A
0A
ILSBULK
-IOUT
HS curr.
rise up to
IOUT
LS bulk
reverse
recovery
Phase of switching
event
LS bulk diode
conducting IOUT
switching
complete
overshoot +
ringing
normal slope
HS starts
conducting
figure 10: effect of bulk diode recovery
A further conclusion from this discussion: Do not set the bridge slope time higher than or near to the
reverse recovery time of the MOSFETs, as the parasitic current spikes will multiply the instantaneous
current across the bridge. A plausible time is a factor of three or more for the slope time. If this cannot
be tolerated please see the discussion on adding Schottky diodes.
5.2.8 Adding Schottky diodes across the MOSFET bulk diodes
In order to avoid effects of bulk diode reverse recovery, choose a fast recovery switching MOSFET.
The MOSFET transistors can also be bridged by a Schottky diode, which has a substantially faster
reverse recovery time. This Schottky diode needs to be chosen in a way that it can take over the full
bridge current for a short moment of time only. During this time, the forward voltage needs to be lower
than the MOSFETs forward voltage. A small 5A diode like the SK56 can take over a current of 20A at
a forward voltage of roughly 0.8V.
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
