IRLR/U7833

8

www.irf.com

Control FET

Special attention has been given to the power losses

in the switching elements of the circuit - Q1 and Q2.

Power losses in the high side switch Q1, also called

the Control FET, are impacted by the R

MOSFET, but these conduction losses are only about

one half of the total losses.

Power losses in the control switch Q1 are given

by;

P

This can be expanded and approximated by;

P

I

rms

2

× R

ds(on )

()

+ I ×

Q

gd

i

g

× V











 + I ×

Q

gs 2

i

g

× V













+ Q

()

+

Q

oss

2

×V









This simplified loss equation includes the terms Q

gs2

and Q

Q

charge that is included in all MOSFET data sheets.

The importance of splitting this gate-source charge

into two sub elements, Q

Fig 16.

Q

the gate driver between the time that the threshold

voltage has been reached and the time the drain cur-

rent rises to I

gins to change. Minimizing Q

reducing switching losses in Q1.

Q

put capacitance of the MOSFET during every switch-

ing cycle. Figure A shows how Q

parallel combination of the voltage dependant (non-

linear) capacitance’s C

the power supply input buss voltage.

Synchronous FET

The power loss equation for Q2 is approximated

by;

P

*

P

I

rms

ds(on)

()

+ Q

()

+

Q

oss

2

× V











+ Q

(

)

*dissipated primarily in Q1.

For the synchronous MOSFET Q2, R

portant characteristic; however, once again the im-

portance of gate charge must not be overlooked since

it impacts three critical areas. Under light load the

MOSFET must still be turned on and off by the con-

trol IC so the gate drive losses become much more

significant. Secondly, the output charge Q

verse recovery charge Q

are transfered to Q1 and increase the dissipation in

that device. Thirdly, gate charge will impact the

MOSFETs’ susceptibility to Cdv/dt turn on.

The drain of Q2 is connected to the switching node

of the converter and therefore sees transitions be-

tween ground and V

a rate of change of drain voltage dV/dt which is ca-

pacitively coupled to the gate of Q2 and can induce

a voltage spike on the gate that is sufficient to turn

the MOSFET on, resulting in shoot-through current .

The ratio of Q

potential for Cdv/dt turn on.

Power MOSFET Selection for Non-Isolated DC/DC Converters

Figure A: Q