222

must be exercised with regard to the capacitor surge current

rating. These capacitors must be capable of handling the

surge-current at power-up. The TPS series available from

AVX, and the 593D series from Sprague are both surge

current tested.

MOSFET Selection/Considerations

The HIP6017 requires 3 N-Channel power MOSFETs. Two

MOSFETs are used in the synchronous-rectified buck

topology of the PWM converter. The linear controller drives a

MOSFET as a pass transistor. These should be selected

management requirements.

PWM1 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 loss is the only component of power

dissipation for the lower MOSFET. Only the upper MOSFET

has switching losses, since the lower device turns on 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 proportional to the switching frequency

contributing to the MOSFETs’ temperature rise. However,

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.

if the type device is used for both. This is because the gate

drive applied to the upper MOSFET is different than the

lower MOSFET. Figure 14 shows the gate drive where 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

Rectifier CR1 is a clamp that catches the negative inductor

voltage 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 might drop one or two percent as a result. The

diode's rated reverse breakdown voltage must be greater

than twice the maximum input voltage.

Linear Controller MOSFET Selection

The main criteria for selection of MOSFET for the linear

regulator 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 maximum rating while operating at

the highest expected ambient temperature.

P

UPPER

I

O

2

r

DS ON

()

×

V

OU T

×

V

IN

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

I

O

V

IN

×

t

SW

×

F

S

×

2

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

+

=

P

LOWER

I

O

2

r

DS ON

()

×

V

IN

V

OU T

–

()

×

V

IN

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

=

+12V

PGND

HIP6017

GND

LGATE

UGATE

PHASE

+5V OR LESS

NOTE:

NOTE:

Q1

Q2

+

-

FIGURE 14. OUTPUT GATE DRIVERS

CR1

P

LIN EAR

I

O

V

IN

V

OUT

–

()

×

=

HIP6017