7

IXDD415SI

APPLICATIONS INFORMATION

High Frequency Gate Drive Circuit

The circuit diagram in figure 14 is a circuit diagram for a

very high switching speed, high frequency gate driver

circuit using the IXDD415SI. This is the circuit used in

the EVDD415 Evaluation Board,and is capable of driving

a MOSFET at up to the maximum operating limits of the

IXDD415. The circuit's very high switching speed and

high frequency operation dictates the close attention to

several important issues with respect to circuit design.

The three key elements are circuit loop inductance, Vcc

bypassing and grounding.

Circuit Loop Inductance

Referring to Figure 14, the Vcc to Vcc ground current

path defines the loop which will generate the inductive

term. This loop must be kept as short as possible. The

output leads (pins 24, 23, 22, 21, 20, and 19) must be

no further than 0.375 inches (9.5mm) from the gate of

the MOSFET. Furthermore the output ground leads (pins

25, 26, 27 and 28 on one end of the IC and pins 15, 16,

17, and 18 on the other end of the IC) must provide a

balanced symmetric coplanar ground return for optimum

operation.

Vcc Bypassing

In order for the circuit to turn the MOSFET on properly,

the IXDD415 must be able to draw up to 15A of current

per output channel from the Vcc power supply in 2-6ns

(depending upon the input capacitance of the MOSFET

being driven). This means that there must be very low

impedance between the driver and the power supply.

The most common method of achieving this low

impedance is to bypass the power supply at the driver

with a capacitance value that is at least two orders of

magnitude larger than the load capacitance. Usually,

this is achieved by placing two or three different types of

bypassing capacitors, with complementary impedance

curves, very close to the driver itself. (These capacitors

should be carefully selected, low inductance, low

resistance, high-pulse current-service capacitors). Care

should be taken to keep the lengths of the leads

between these bypass capacitors and the IXDD415 to an

absolute minimum.

The bypassing should be comprised of several values of

chip capacitors symmetrically placed on ether side of

the IC. Recommended values are .01uF, .47uF chips

and at least two 4.7uF tantalums.

Grounding

In order for the design to turn the load off properly, the

IXDD415 must be able to drain this 15A of current into an

adequate grounding system. There are three paths for

returning current that need to be considered: Path #1 is

between the IXDD415 and its load. Path #2 is between the

IXDD415 and its power supply. Path #3 is between the

IXDD415 and whatever logic is driving it. All three of these

paths should be as low in resistance and inductance as

possible, and thus as short as practical.

Output Lead Inductance

Of equal importance to supply bypassing and grounding are

issues related to the output lead inductance. Every effort

should be made to keep the leads between the driver and

its load as short and wide as possible, and treated as

coplanar transmission lines.

In configurations where the optimum configuration of circuit

layout and bypassing cannot be used, a series resistance

of a few Ohms in the gate lead may be necessary to prevent

ringing.

Heat Sinking

For high power operation, the bottom side metalized heat

sink pad should be epoxied to the circuit board ground

plane, or attached to an appropriate heat sink, using

thermally conductive epoxy. The heat sink tab is connected

to ground.

Figure 15: IXDD415SI Bottom Side

Heat Sinking Metalization