Datasheet

TCA 3727
Data Sheet 22 Rev. 2.0, 2004-10-01
Calculation of Power Dissipation
The total power dissipation
P
tot
is made up of
• 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
tot
= 2 × P
sat
+ P
q
+ 2 × P
s
(1)
where
•
P
sat
≅ I
N
{V
satl
× d + V
Fu
(1 - d) + V
satuC
× d + V
satuD
(1 - d)}
•
P
q
= I
q
× V
S
+ I
L
× V
L
(2)
•
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 t
p
/T
•
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
P
S
V
S
T
------
i
D
t
DON
×
2
----------------------
i
D
i
R
+ t
ON
×
4
------------------------------
I
N
2
-----
t
DOFF
t
OFF
+++
⎩⎭
⎨⎬
⎧⎫
≅