ST AN604 APPLICATION NOTE
AN604
Application note
Calculation of conduction losses in a power rectifier
Introduction
This application note explains how to calculate conduction losses in a power diode by taking
into account the forward voltage dependence on temperature and the current waveform.
The ideal current and voltage waveforms of an ultrafast diode in a power supply system
during a switching cycle are shown in Figure 1.
Figure 1. Ideal current and voltage waveforms of a diode in a switch mode power
supply
I
(t)
D
I
Max
I
Min
ID(t)
VD(t)
F Switching frequency
sw
0
V
(t)
D
V
F
0
V
R
·T
δ
sw
T
sw
t
t
T Switching period
sw
Duty cycle
δ
T Duration of diode conduction
δ·
sw
I Maximum forward current
max
I Minimum forward current
min
V Forward voltage
F
V Reverse voltage
R
The conduction losses in a diode appear when the diode is in forward conduction mode due
to the on-state voltage drop (V
). Most of the time the conduction losses are the main
F
contributor to the total diode power losses and the junction temperature rising. This is the
reason why it is important to accurately estimate them.
August 2011 Doc ID 3607 Rev 3 1/12
www.st.com
Contents AN604
Contents
1 Diode forward characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Junction temperature dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Diode forward characteristics modeling: V
), RD(Tj) . . . . . . . . . . . . . . . 3
T0(Tj
2 Conduction losses: basic equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Application parameters: average and rms currents . . . . . . . . . . . . . . . . . . 6
3 An application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Average and rms current calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 V
3.3 Conduction losses expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
(Tj) and RD (Tj) calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
T0
4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2/12 Doc ID 3607 Rev 3
AN604 Diode forward characteristics
1 Diode forward characteristics
1.1 Junction temperature dependence
For two different junction temperatures, the current versus forward voltage curves cross at a
current level point I
the temperature coefficient α
higher, the temperature coefficient becomes positive. This behavior is shown in Figure 2.
For Schottky and bipolar diodes, I
SiC and GaN technologies, I
0, the forward voltage and the conduction losses decrease when the junction temperature
increases.
, depending on the diode technology. When the current is lower than Ic,
c
of the forward voltage is negative. When the current is
VF
is high and the working area corresponds to αVF < 0. For
c
is low and the αVF can be positive or negative. When of αVF <
c
Figure 2. Forward (I
) characteristics of a diode
F,VF
I
F
T
j1
I
c
0
αVF> 0
αVF= 0
αVF< 0
T
j2>Tj1
V
F
1.2 Diode forward characteristics modeling: VT0(Tj), RD(Tj)
Forward characteristics (IF and VF) can be modeled by a straight line defined by a threshold
voltage V
levels (I
Equation 1
Equation 2
, and a dynamic resistance RD. VT0 and RD are calculated for 2 forward current
T0
, IF2) for a given junction temperature as shown in Figure 3. Thus we can write:
F1
I)(TR)(TV)T,(IV ⋅+=
FjDjT0j1FF
1
I)(TR)(TV)T,(IV ⋅+=
FjDjT0j2FF
2
Using Equations 1 and 2, we obtain V
Equation 3
)(TR
=
jD
−
II
−
F
F
2
)T,(IV)T,(IV
j1FFj2FF
1
Equation 4
I)T,(IVI)T,(IV
⋅−⋅
Fj1FF
)(TV
=
jT0
2
II
−
F
F
1
2
Fj2FF
1
Doc ID 3607 Rev 3 3/12
) and RD(Tj) expressions:
T0(Tj
Diode forward characteristics AN604
Figure 3. VT0(Tj) and RD(Tj) parameters
I
F
I
F2
T
jRef2
> T
jRef1
VF(IF,T
jRef1
)
VF(IF,T
jRef2
)
1
TR
jref2D
I
F1
0
and RD are given in each ST diode datasheet. In most cases they are calculated at
V
T0
125 °C with maximum V
VT0(T
) VT0(T
jRef2
VF(I
F1,TjRef2
values for IF1 = I
F
)
V
F(IF2,TjRef2
F(AV)
1
)(
TR
jref1D
)
jRef1
)
V
and IF2 = 2· I
)(
VF(IF2,T
F(IF1,TjRef1
F(AV)
jRef1
)
, where I
)
V
is the
F(AV)
average forward current rating of the diode. For a quick calculation these values can be
used. For more accurate estimation, R
and VT0 must be calculated using the specific
D
application conditions. See the example in Chapter 3.
F
4/12 Doc ID 3607 Rev 3
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