MP6528 – 5V TO 60V, H-BRIDGE GATE DRIVER

MP6528 Rev. 1.03

www.MonolithicPower.com

15

6/20/2017

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Gate Drive Considerations

The

gate

characteristics

of

the

selected

MOSFETs will affect how fast they will be

switched and off. The gate drive outputs of the

device can be connected directly to the gates of

the power MOSFETs, which results in the fastest

possible turn-on and turn-off times. However, it

may

be

advantageous

to

add

external

components (resistors and/or diodes) to modify

the MOSFET turn-on and turn-off characteristics.

Adding external series resistance

– typically

between 10 and 100 ohms

– will limit the current

that charges and discharges the gate of the

MOSFET, which will slow down the turn-on and

turn-off times.

Sometimes this is desirable to

control EMI and noise.

Slowing the transition

down too much, however, results in large power

dissipation in the MOSFET during switching.

In some cases, it is desirable to have a slow turn-

on, but a fast turn-off. This can be implemented

by using a series resistor in parallel with a diode.

At turn-on, the resistor limits the current flow into

the gate; at turn-off, the gate is discharged

quickly through the diode.

1N4148

1N4148

This waveform below shows the gates of the LS

and HS MOSFETs, and the phase node (output)

with no series resistance. You can see that the

gates transition quickly. The resulting rise time

on the phase node is quite fast. Note the scale

of 100nS/div.

This waveform shows the effect of adding a

100

Ω series resistor between the GLA and GHA

pins and the MOSFET gates. Rise time on the

phase node has been slowed significantly. The

scale here is 200nS/div.

This waveform shows the effect of adding a

1N4148 diode in parallel with the 100Ω resistors

(with the cathode connected to the IC). You can

see that the fall time of the LS gate is quite fast

compared to the HS gate rise time. The phase

node moves even slower, because of a longer

period of time between when the LS FET is

turned off, and the HS FET is turned on.

PCB Layout

Proper PCB layout is critical to the performance

of MOSFET gate drivers.

In particular, the

connection between the HS source and LS drain

needs to be as direct as possible, to avoid

negative undershoot on the phase node due to

parasitic inductances. The pre-driver is designed

to accommodate negative undershoot, but if it is

excessive, unpredictable operation or damage to

the IC can result.

An example PCB layout (showing the MP6530,

which is a 3-channel part, but otherwise similar to

the MP6528) is shown below.

It uses surface

mount N-channel MOSFETs, which allows very