Proper PCB layout is extremely important in a
high-current, fast-switching circuit to provide appropriate device operation and
design robustness. The UCC27614-Q1 gate driver incorporates
short propagation delays and powerful output stages capable of delivering large
current peaks with very fast rise and fall times at the gate of power switch to
facilitate voltage transitions very quickly. Very high di/dt can cause unacceptable
ringing if the trace lengths and impedances are not well controlled. The following
circuit layout guidelines are recommended when designing with these high-speed
drivers.
- Place the driver device as
close as possible to power device to minimize the length of high-current
traces between the driver output pins and the gate of the power switch
device.
- Place the bypass capacitors
between VDD pin and the GND pin as close to the driver pins as possible to
minimize trace length for improved noise filtering. TI recommends having two
capacitors; a 100-nF ceramic surface-mount capacitor placed less than 1mm
from the VDD pin of the device and another ceramic surface-mount capacitor
of few microfarads added in parallel. These capacitors support high peak
current being drawn from VDD during turnon of power switch. The use of low
inductance surface-mount components such as chip capacitors is highly
recommended.
- The turnon and turn-off
current loop paths (driver device, power switch and VDD bypass capacitor)
should be minimized as much as possible in order to keep the stray
inductance to a minimum. High di/dt is established in these loops at two
instances – during turnon and turn-off transients, which induces significant
voltage transients on the output pins of the driver device and gate of the
power switch.
- Wherever possible, parallel
the source and return traces of a current loop, taking advantage of flux
cancellation
- Separate power traces and
signal traces, such as output and input signals.
- To minimize switch node
transients and ringing, adding some gate resistance and/or snubbers on the
power devices may be necessary. These measures may also reduce EMI.
- Star-point grounding is a
good way to minimize noise coupling from one current loop to another. The
GND of the driver should be connected to the other circuit nodes such as
source of power switch, ground of PWM controller, and so forth, at a single
point. The connected paths should be as short as possible to reduce
inductance and be as wide as possible to reduce resistance.
- Use a ground plane to provide
noise shielding. Fast rise and fall times at OUT pin may corrupt the input
signals during transitions. The ground plane must not be a conduction path
for any current loop. Instead the ground plane should be connected to the
star-point with one trace to establish the ground potential. In addition to
noise shielding, the ground plane can help in power dissipation as
well.