FN4684 Rev 1.00

Page 14 of 15

March 8, 2005

HIP6021

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PWM 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 equations below). The

conduction losses are the main component of power

dissipation for the lower MOSFETs. Only the upper MOSFET

has significant switching losses, since the lower device turns

on and off into near zero voltage.

The equations below assume linear voltage-current transitions

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

the lower MOSFET’s body diode. The gate-charge losses are

dissipated by the HIP6021 and don't heat the MOSFETs.

which increases the upper MOSFET switching losses. 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.

same device is used for both. This is because the gate drive

applied to the upper MOSFET is different than the lower

MOSFET. Figure 11 shows the gate drive where the upper

MOSFET’s gate-to-source voltage is approximately VCC less

the input supply. For +5V main power and +12VDC for the

bias, the gate-to-source voltage of Q1 is 7V. The lower gate

drive voltage is +12VDC. A logic-level MOSFET is a good

choice for Q1 and a logic-level MOSFET can be used for Q2 if

its absolute gate-to-source voltage rating exceeds the

maximum voltage applied to VCC.

Rectifier CR1 is a clamp that catches the negative inductor

swing during the dead time between the turn off of the lower

MOSFET and the turn on of the upper MOSFET. The diode

must be a Schottky type to prevent the lossy parasitic

MOSFET body diode from conducting. It is acceptable to omit

the diode and let the body diode of the lower MOSFET 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.

Linear Controller Transistor Selection

The main criteria for selection of transistors for the linear

regulators is package selection for efficient removal of heat.

The power dissipated in a linear regulator is:

Select a package and heatsink that maintains the junction

temperature below the rating with a the maximum expected

ambient temperature.

When selecting bipolar NPN transistors for use with the linear

controllers, insure the current gain at the given operating VCE

is sufficiently large to provide the desired output load current

when the base is fed with the minimum driver output current.

2







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







2

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

+

=

2





–





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

=

+12V

PGND

HIP6021

GND

LGATE

UGATE

PHASE

VCC

+5V OR LESS

NOTE:

NOTE:

Q1

Q2

+

-

CR1

–





=