ZXLD1370Q

Document number: DS37117 Rev. 1 - 2

21 of 37

www.diodes.com

August 2015

© Diodes Incorporated

ZXLD1370Q

Resistive Power Losses

The resistive power losses are calculated using the RMS transistor current and the MOSFET on-resistance.

Calculate the current for the different topologies as follows:

Buck Mode

LED

MAX

MAX

MOSFET

I

x

D

I





Boost / Buck-Boost Mode

i

D

1

D

I

LED

MAX

MAX

MAX

MOSFET









The approximate RMS current in the MOSFET will be:

Buck Mode

D

I

I

LED

RMS

MOSFET





Boost / Buck-Boost Mode

LED

RMS

MOSFET

I

x

D

1

D

I







The resistive power dissipation of the MOSFET is:

ON

DS

2

RMS

MOSFET

RESISTIVE

R

x

I

P







Switching Power Losses

Calculating the switching MOSFET's switching loss depends on many factors that influence both turn-on and turn-off. Using a first order rough

approximation, the switching power dissipation of the MOSFET is:

GATE

LOAD

sw

IN

2

RSS

SWITCHING

I

I

x

f

x

V

x

C

P



Where

Matching the MOSFET with the controller is primarily based on the rise and fall time of the gate voltage. The best rise/fall time in the application

is based on many requirements, such as EMI (conducted and radiated), switching losses, lead/circuit inductance, switching frequency, etc. How

fast a MOSFET can be turned on and off is related to how fast the gate capacitance of the MOSFET can be charged and discharged. The

relationship between C (and the relative total gate charge Qg), turn-on/turn-off time and the MOSFET driver current rating can be written as:

I

Qg

I

C

dV

dt







where

dt = turn-on/turn-off time

dV = gate voltage

C = gate capacitance = Qg/V

I = drive current

– constant current source (for the given voltage value)

Here the constant current source” I ” usually is approximated with the peak drive current at a given driver input voltage.

(Example 1)