18

FN9034.2

December 27, 2004

MOSFET Selection and Considerations

In high-current PWM applications, the MOSFET power

dissipation, package selection and heatsink are the

dominant design factors. The power dissipation includes two

loss components; conduction loss and switching loss. These

losses are distributed between the upper and lower

MOSFETs according to duty factor (see the following

equations). The conduction losses are the main component

of power dissipation for the lower MOSFETs, Q2 and Q4 of

Figure 1. Only the upper MOSFETs, Q1 and Q3 have

significant switching losses, since the lower device turns on

and off into near zero voltage.

The equations assume linear voltage-current transitions and

do not model power loss due to the reverse-recovery of the

lower MOSFETs body diode. The gate-charge losses are

dissipated by the Driver IC and don't heat the MOSFETs.

However, large gate-charge increases the switching time,

Ensure that both MOSFETs are within their maximum

junction temperature at high ambient temperature by

calculating the temperature rise according to package

thermal-resistance specifications. A separate heatsink may

be necessary depending upon MOSFET power, package

type, ambient temperature and air flow.

A diode, anode to ground, may be placed across Q2 and Q4

of Figure 1. These diodes function as a clamp that catches

the negative inductor swing during the dead time between

the turn off of the lower MOSFETs and the turn on of the

upper MOSFETs. The diodes must be a Schottky type to

prevent the lossy parasitic MOSFET body diode from

conducting. It is usually acceptable to omit the diodes and let

the body diodes of the lower MOSFETs clamp the negative

inductor swing, but efficiency could drop one or two percent

as a result. The diode's rated reverse breakdown voltage

must be greater than the maximum input voltage.

P

UPPER

I

O

2

r

DS ON

()

×

V

OUT

×

V

IN

------------------------------------------------------------

I

O

V

IN

×

t

SW

×

F

SW

×

2

----------------------------------------------------------

+

=

P

LOWER

I

O

2

r

DS ON

()

×

V

IN

V

OUT

–

()

×

V

IN

---------------------------------------------------------------------------------

=