TLE 4726

Data Sheet

19

1999-09-15

Calculation of Power Dissipation

The total power dissipation

P

saturation losses

P

sat

(transistor saturation voltage and diode forward voltages),

quiescent losses

P

q

(quiescent current times supply voltage) and

switching losses

P

s

(turn-ON / turn-OFF operations).

The following equations give the power dissipation for chopper operation without phase

reversal. This is the worst case, because full current flows for the entire time and

switching losses occur in addition.

P

where

P

P

I

N

= nominal current (mean value)

I

q

= quiescent current

i

D

= reverse current during turn-on delay

i

R

= peak reverse current

t

p

= conducting time of chopper transistor

t

ON

= turn-ON time

t

OFF

= turn-OFF time

t

DON

= turn-ON delay

t

DOFF

= turn-OFF delay

T

= cycle duration

d

= duty cycle

V

satl

= saturation voltage of sink transistor (T3, T4)

V

satuC

= saturation voltage of source transistor (T1, T2) during charge cycle

V

satuD

= saturation voltage of source transistor (T1, T2) during discharge cycle

V

Fu

= forward voltage of free-wheeling diode (D1, D2)

V

S

= supply voltage

V

L

= logic supply voltage

I

L

= current from logic supply

T

------

i

D

t

DON

×

2

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

i

D

i

R

+

t

ON

×

4

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

2

DOFF

t

OFF

+

++

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