ST AN1453 APPLICATION NOTE

AN1453
®
NEW FAMILY OF 150V POWER SCHOTTKY
INTRODUCTION
Nowadays, the Switch Mode Power Supply (SMPS) is becoming more widespread as a result of computer, telecom and consumer applications.
The constant increase in services (more peripherals) and performance, which offers us these applications, tends to move conversion systems towards higher output power.
APPLICATION NOTE
By F. GAUTIER
In the following examples, the conduction losses between a 150V Schottky and a 200V bipolar diode in a Flybackand aForward converter willbe compared.
The conduction losses in the diode are calculated from the classical formula:
P=VI+RI
cond T0 F(AV) d IF(RMS)
V :threshold voltage with V = V +R .I
t0 F(@IF) T0 d F
R : dynamic resistance with R = V / I
ddFF
where V
and Rdare calculated from the current
T0
⋅⋅
range of current view by the diode (Fig. 1), for better accuracy.
2
and
I
IF(RMS)
:
Fig. 1: Typical current through a rectification diode
2
∆∆
F(AV)
Consequently, this application note will underline the advantages of a 150V Schottky technology compared to a 200V ultra fast diode.
In order to do this, the example of a Flyback converter will be used, and thestatic and dynamic parameters of the 150V Schottky will be detailed, as well as their influence in this converter.
1.CONDUCTIONLOSSES&EFFICIENCY GAIN
) versus forward current (IF),
F
and obviously the best gain in efficiency will be obtained with the lowest V
July 2001
.
F
I
D
I
ma x
I
min
0
α
I=(I I)
F(AV)
2
I(III
F(RMS)
R=
d
ID
+
max min
2
α
2
ID
=++
VV
max min2m
3
F(@Imax) F(@Imin)
II
max min
αI.T
D
I
)
ax min
VV
=
T0 F
NB:
-In the datasheet, the V values given for I
and2IFat 125°C.
F
-In discontinuous mode I
and Rdare maximum
T0
=0.
min
T
(@imax) d max
RI−⋅
1/9
t
APPLICATION NOTE
1.1. Example 1: FLYBACK
The first example is a 24V/48W Flyback converter working in continuous mode (Vmains=90V) with the following conditions:
0.4, I 6.66A, I 3.33A, I 2A
== = =
α
ID max min out
ID ID
Fig. 2: Rectification diode in a Flyback converter
I
V
I
in
o u t
D
Calculations per diode give:
I = 1A and I = 1.6A
F(AV)per diode F(RMS)
per diode
We can now calculate the efficiency gain (∆η(%)=
- η) for this Flyback converter which has a
η
ref
Fig. 3: Example of efficiency gain in Flyback converter
V
P V
out out
=48W =24V
T0
typ(V)
1.5A, 3A, 125°C
R
m
d
P
cond
(W)
P
(W)
η=85
%
∆η%
STPR102CT 2x5A / 200V
0.58 46.5 1.4 0 (ref) 0 (ref)
PN diode
1.2. Example 2: FORWARD
Fig. 4: Rectification diode in a Forward converter
I
I
o u
L
V
D1
in
D2
α
0.3, I 9A, I 7A, I 8A====
D1 Lmax Lmin out
Calculations per diode give:
==
I 2.4A, I 4.39A
F(AV)D1 F(RMS)D1
=
I 5.6A, I
F(AV)D2 F(R
MS)D2
The difference of efficiency between a STPR1620CT (2x8A, 200V Ultrafast) and a STPS16150CT (2x8A, 150V Schottky) for a 12V output, are given in table Fig. 5:
Fig. 5: Example of efficiency gain in Flyback converter
V
R
T0
d
m
P V
out out
=96W =12V
typ(V)
7A, 9A,
125°C
STPR1620CT
0.8 20 6.48 Ref Ref
P
(W)
cond
6.71A=
η=85
P
%
(W)
∆η%
STPR162CT 2x8A / 200V PN diode
STPS10150CT 2x5A / 150V Schottky diode
STPS16150CT 2x8A / 150V Schottky diode
2/9
0.54 46.5 1.32 -0.08 +0.12
0.50 43 1.22 -0.18 +0.27
0.47 40 1.14 -0.26 +0.39
STPS16150CT
0.68 20 5.60 -0.95 +0.72
APPLICATION NOTE
2. REVERSE LOSSES AND T
JMAX
2.1. Reverse losses: Prev
The reverse losses can be determined by:
=⋅⋅−α)
PVI(1
rev R R
with:
): duty cycle when the reverse voltage (VR)is
(1-
applied IR: leakage current versus VRand operating
junction temperature (T
V
: reapplied voltage accross the diode
R
)
j
Fig. 6 shows an example of reverse losses in a Flyback converter with the following conditions:
()1−= = = °α0.4, V 80V, T 125 C
Rj
Fig. 6: Example of reverse losses in a Flyback converter
STPS10150CT
per diode
I
Rtyp
100V, 125°C
130µA 4.2mW
P
rev
per diode
Thus, the reverse losses are very low due to the low value of the leakage current.
The following paragraph will show that due to these low values of reverse current, the thermal runaway limit is only reached for high junction temperature.
2.2. T
before thermal instability is reached
jmax
Remembering that the stability criterion is given by:
dP
rev
<
dT1R
j th(j a)
with:
=−α)
P V .I .(1
rev R R(VR,Tjmax)
The above formulae give the critical value of the leakage currentbefore the thermal runaway limit is reached:
I
R(VR,Tjmax)
=
Vc.R
R th(j a)
The evolution of the leakagecurrent versus T
is given by:
V
R
IIexp
=
R(V ,Tj) R(V ,125)
RR
1
⋅⋅
1()α
c(Tj 125)
and
j
From these physical laws, it can be deduced that:
Example: Flyback converter with 2 diodes in parallel
()1−= = =α0.4, c 0.069, V 80V
R 2.4 C / W, R 7.6 C / W
th(j c)total th(c a)−−
Fig. 7: Example of T
For a dual diode
STPS10150CT
with STPS10150CT
jmax
I
Rmax
(80V,125°C)
1.3mA 45.28mA 176.5°C
R
I
R(VR,Tjmax)Tjmax
This example shows that in a typicalapplication, a 150V Schottky can be used up to 175°C. STMicroelectronics specifies in the datasheet
at 175°C.
T
jmax
3. SWITCHING BEHAVIOUR
3.1. Turn-on behaviour
The behaviour at turn-on is characterized bya low value of peak forward voltage (V reverse recovery time (t
) (Fig. 8).
fr
) and forward
FP
Fig. 8: VFPand tfrfor STPS16150CT
=16A
I
F
/dt=100A/µs
dI
F
=25°C
T
j
t
fr
(ns)
V
(V)
FP
Per diode
STPS16150CT
These values depends mainly on the dI
100 2.2
/dt. The
F
switching losses at turn-on are always negligible.
3.2. Turn-off behaviour
The turn-off behaviour isa transitoryphenomenon (ns), but repetitive depending on the switching frequency. It is a source of spike voltage, noise and for high switching frequency, ofnon-negligible switching losses.
Inorder to illustrate thisphenomenon, the example of a Flyback converter will be used once again.
The difference in behaviour between a 150V Schottky and 200V bipolar diode will be compared for the three following points: spike voltage, EMC and switching losses.
T 125
=+
jmax
1
In
c
I
R (V ,125 C)datashee
max R
I
R(V ,Tjmax)
R
° t
3/9
APPLICATION NOTE
3.2.1. Difference of spike voltage between a 150V Schottky and 200V PN diode
In a Flyback converter, the reverse voltage (V used in §2) across the diode will be maximum, for the maximum mains voltage (V
n
VV
=⋅+
R INmax
s
n
p
V
out
In addition to this nominal reverse voltage (V
):
INmax
(cf Fig. 9)
R
generallyan overvoltage spike at the turn-off ofthe diode is observed (Fig. 9). It can be shown that with a conventional bipolar diode, this spike is more important for a Flyback converter working in continuous mode than in a discontinuous mode.
In the case of a high spike voltage, the Maximum Repetive Reverse Voltage (V to be oversized, compared with the real need (V
) of the diode has
RRM
R
defined in Fig. 9. To limit this peak and to preserve a "guard band"
with the V
(in order to avoid reaching the
RRM
breakdown voltage), the designer places a snubber circuit (R
) in parallel with the diode.
S,CS
Generally, the "guard band" is such that the maximum voltage reapplied to the diode does not exceed 80% of the V
RRM
.
3.2.1.1 Turn-off behaviour for a PN diode
In the datasheet are specified the main turn-off parameters (Q
R
rr,IRM,trr
…). These parameters are
represented in Fig. 10:
Fig. 10: Key parameters at turn-off for a bipolar diode without snubber
Q = Q + Q t = t + t
I R
Q
rrb
rrab
S =
t
V
R
V
Rma x
),
I
V
)
d /dt
I F
Q
rra
I
RM
t
atb
d /dt
The following oscillogram shows the turn-off behaviour for a bipolar diode (STPR1620CT) with snubber and without snubber, in a 24V/45W Flyback working in continuous mode.
rr rra rrb
t
b
t
a
Fig.9: Spike voltageacross the rectificationdiode
R
C
s
s
Turn-off diode
V
V
npn
in
V
P
D
s
I
D
V
s
V=V
RIN+Vout
I
D
V
o u t
V
D
n
s
n
p
V
Rmax
I
RM
V
RRM
Thisspike voltage isdue to theleakage inductance of the transformer (L
) and to the nature of the
f
recovery charge of the diode,which itselfdepends on the diode technology: bipolar diode or Schottky diode.
To observe the phenomenon correctly, it is necessary to compensate the delay time between the voltage and the current, (by temporal shift) due to the measuring equipment Fig. 11.
Fig. 11: Switching behaviour of a 200V bipolar diode
dI /dt=130A/µs
F
I =4A
RM
delay time
t0
t0
Tj=100°C
dI /dt=600A/µs
R
Compensative curve
V =250V
Rmax
dV/dt
V =90V
Rmax
I
V =42V
R
I
R =22ohms
s
V
V
20V/div 2A/div 50ns/div
V
C =2.2nF
s
V
I
I
4/9
APPLICATION NOTE
Without a snubber, in this example the diode is repeatedly in conduction because the oscillationis very strong. Furthermore, the voltage is close to the breakdown voltage. This means that the systemis no longer reliableand a snubbercircuit is necessary.
Onthese 2 oscillograms, we cansee that the value of the maximum reverse current (I
) is defined
RM
when the reverse voltage rises (typical behaviour of a bipolar diode). At this time the voltage is not fixed by the diode.
The curve Q
versus dIF/dt and Tjis given in
rr,IRR
the datasheet. For example in Fig. 12, the evolution of I
versus dIF/dt for a STPR1620CT
RM
can be observed.
Fig. 12: Peak reverse recovery current versus
/dt (per diode)
dI
F
IRM(A)
20
IF=IF(av) 90% confidence
10
Tj=125°C
STPR1620CG/CT
Fig.13: Equivalent modelat t0fora bipolar diode
R
C
s
s
L
f
I=I
V
n
p
L
P
D
n
s
L
s
V
s
V
o u t
V=V+V
R S o u t
L f RM
C
s
V
R
C
Qrrb
s
C
j
Where:
n
V
: secondary voltage
s
:leakageinductance of the transformer
L
f
C
: junction capacitance
j
C
: equivalent capacitance modeling the
Qrrb
V
=⋅
S
s
V
IN
n
p
reversecharge, necessary forthe establishment of the potential barrier, which supports the reverse voltage.
: output voltage
V
out
D
1
10 20 50 100 200 500
It can be also noticed, that the parameter I
dIF/dt(A/µs)
RM
significantly increases with the temperature. In continuous mode the dI
/dt (few hundred A/µs)
F
is fixed by theleakage inductanceand thereverse voltage (V
):
R
dIdtV
FR
with V
==+
L
f
n
R
n
s
VV
IN out
p
It is many time higher than in discontinuous mode (lower than 1A/µs):
dI
F out
=
dtVLL
: Secondary inductance)
(L
S
Sf
withL L
+
〉〉
Sf
Thus, with this curve we can see that, in continuous mode (high dI
/dt), the bipolar diode
F
must evacuate a non-negligible charge, which means a higher I
. This is verified onoscillogram
RM
Fig. 11.
With this value of I
, an equivalent model at t
RM
with a snubber circuit can be established:
With the following initial conditions at t=t
I I and V 0
=≈
LRM D
f bipolar
The equivalent schematic can be used to define
=
VV
DR
max
NB:
1) Without snubber, there is a L
(C=C
+C
j
) which lead to a second order
Qrrb
differential equation:
2
dV
dt
2
C
+⋅+⋅= = ⋅ωω ω
0
2
2
V V 0 and 1/ L C
CR f
0
with initial conditions at t=t0:
===
I I and V V 0
LRM C D
f0
In this equation, an approximation is made with C constant, because in reality C
and C
j
the voltage applied. The solution of the differential equation gives us:
VVVVI
==+ + ⋅
RDRR
max
Therefore we can see that the V leakage inductance (L
). Thus,
0
(I
RM
temperature.
V
R
max
is very dependent on the
2
RM
) and on recovery charge
f
Rmax
:
0
, C circuit
f
2 0
vary with
Qrrb
2
L
f
C
depends the
5/9
APPLICATION NOTE
If C is a low value of capacitance, the expression leads to a first order differential equation:
==+⋅
VVVLdI/dt
RDRfR
max
with:
dI /dt I /t and t Q / I /2 dI / dt I / 2 Q
==
R RM b b rrb RM
2
=⋅
RRM
()
b
rr
()
If the diode is of a snap-off type, we have t0,Q 0
→→
b rrb
consequently to reach the breakdown voltage of the diode. (It is
and dI is considerable. There is a risk
V
R
max
/dt is very high,
R
not guaranteed by the manufacturer)
2) With a snubber, if it is supposed that
>> +
CCC
S j Qrrb
diode), we have a simple R
(more true for an ultrafast PN
s,Lf,CS
circuit to define
by the second order differential equation:
2
dV
C
SS
++=ωωω
2
dt
2m
dV
0
C
dt
V.V
02R0
2
R
where:
R2C
m: the absorption coefficient
:natural frequency
0
ω
0
=
LC
m
=
1
fS
SS
L
f
There are 3 possible cases:
-m>1 behaviour withoutoscillation (over damping)
Fig. 14: Turn-off for a perfect Schottky diode without snubber
I
D
V
Rma x
t
0
V
D
In this perfect case at t=t
I I 0 and V 0
==
LRM D
f
, we have:
0
t
V
R
Without a snubber circuit, the new solution of the differential equation gives:
VV 2V
==
DR R
max
Unfortunately, it is difficult to realize a perfect high voltage Schottky. The reason is the presence of a parasitic bipolardiode in parallel with the Schottky. If it is polarized, the recovery charge is added at the turn-off.The phenomenon begins to appear for a 100V technology.
In the same conditions as before, the STPS16150CT is used:
- m<1 behaviour with oscillation damped (under
damping), where the frequency oscillation can be determined by:
ωω
r
0
2
1m=−
-m=1 limit of behaviour without oscillation (critical
damping)
Like this, in this case,it ispossible tosuppress the oscillation across the diode with m>1.
3.2.1.2 Turn-off behaviour for a Schottky diode
For an ideal Schottky diode, there is no recovery charge (Q
=0). Therefore, it is said that the diode
rr
has a capacitive type recovery Fig. 14.
6/4
Fig. 15: Switching behaviour of a STPS16150CT
dI /dt=130A/µs
F
I =1.6A
RM
V =58V
Rmax
t0
t0
Tcase=100°C
dI /dt=250A/µs
R
V =120V
Rmax
dV/dt
V =42V
R
I
I
R =22ohms
s
V
V
C =2.2nF
s
20V/div 2A/div 50ns/div
I
V
V
I
APPLICATION NOTE
It can be observed that this is not an ideal Schottky. In fact, when the voltage rises at , we have a value of I
. The charge Q
RM
is not easily
rrB
identifiable because it is embedded in the capacitive current.
However, the slope dI
/dt can be observed.
R
Unlike a PN diode, we can see that with the 150V Schottky the maximum reverse voltage (V and the maximum reverse current (I
RM
Rmax
) are
distinctly lower.
The equivalent model at t
for a STPR1620CT and
0
a STPS16150CT is the same, with thelower initial conditions at t
In Fig. 16, we can see the curve C
:
0
I I and V 0
=≈
LRM D
f schottky
versus VRfor
j
the 200V bipolar and 150V Schottky diode. Whatever the reverse voltage, the junction capacitance of the 150V Schottkyis alwayshigher than for a PN diode. This justifies, the lower observed with the Schottky diode.
Fig. 16: Cjversus V
F=1MHz,Tj=125°C
300.0
250.0
R
STPR1620CT STPS16150CT
Thus, a different efficiency of the snubber circuit with a bipolar diode and a Schottky diode is observed.In most cases, we cansay that the 150V Schottky behaves better at turn-off, due to its larger capacity and its softness recovery.
Model showed in Fig. 13can beused todefine the snubber circuit.
NB:
)
In the case of a Forward converter with multiple outputs (12V, 5V, 3.3V…) and cross regulation with coupled inductor, the poor behaviour at the turn-off with a bipolar diode on 12V output, will be reflected on the other coupled outputs (that means an overvoltage on rectification diode of 5Voutput). A150V Schottky willdecrease the coupledeffects.
3.2.2. EMC Comparison between a 150V Schottky and a 200V bipolar diode
The better the behaviour at turn-off of the 150V Schottky in comparisonwith a 200V bipolar diode, the better performance in the EMC.
Fig. 16 shows the comparison of electromagnetic disturbance conducted in a 45W/24V Flyback converter (in continuous mode) between a STPR1620CT and a STPS16150CT with a snubber circuit.
Fig. 17: Electromagnetic disturbances conducted between a STPR1620CT and a STPS16150CT
200.0
F)
p
150.0
C(
100.0
50.0
0.0 1 10 100 1000
VR(V)
In summary, when we compare the different parameters with those of the bipolar diode, we have:
II
>
RM RM
bipolar schottky
CC
<
jj
bipolar schottky
dI / dt
()
R
dV / dt dV / dt
()()
V
Rma
polar
bi
bipolar schottky
x Rmax
bipolar schottky
dI / dt
>
()
R
>
V>
schottky
STPR1620CT
STPS16150CT
We can see near30MHz, thatthere is a difference of -10dB. This difference is partially explained by the higher dV/dt with a bipolar diode at turn-off than with a 150V Schottky. In fact, the lower capacitance junction of the PN diode favors the high dV/dt at t mode. (
, and therefore thecommon current
0
i C dV / dt, C
=⋅
CM
equivalent capacitance
junction-heatsink) The other high dV/dt, which could take place due
to the strong oscillation, are suppressed bya good choice of the snubber circuit.
7/9
APPLICATION NOTE
In the case of anEMC problem,the firstsolution is to reduce the current slope (dI gate resistance (R
and dIR/dt decrease as well as the
I
RM
about 10 ohms). In this way,
G
/dt) by adding a
F
.
V
R
max
3.2.3. Switching losses
We have evaluated the consequences of poor behaviour at the turn-off: spike reverse voltage, possible oscillations and EMC problem. For these reasons, the designer may wish to use a soft recovery PN diode, but which, in return, will increase the switching time and particularly the switching losses at turn-off.
Switching losses at turn-off due to the diode are the sum of losses inside the diode and the energy dissipated in the other elements of the circuit. In fact, during this time the I
current, due to the
RM
recovery charge, flows through the transformer, the power MOS transistor and the primary bulk capacitor. Thus, there are additional losses. The distribution of power losses at turn-off can be detailed:
- Losses inside the diode: P
turn off b RM R
1
tI VF
=⋅⋅ ⋅⋅
diode
2
4. RESULT OF EXPERIMENTS
Experimental measurements (Fig. 18) were carried out in a 45W/24V Flyback converter working in the following conditions:
V
= 90V P
IN
F
= 100kHz Tc= 100°C
s
= 45W V
out
out
= 24V
These experiments confirm the interest in a 150V Schottky in comparison with a 200V bipolar diode.
Fig. 18: Experiments of efficiency in a Flyback converter
η% P(W) STPR1620CT (2x8A) STPS16150CT (2x8A) STPS20150CT (2x10A)
84.04 0.41
84.4 0.18
84.69 Ref
CONCLUSION
We have been able tohighlight that whenwe have the choice between 150V Schottky and 200V PN diode, the 150V Schottky is the best choicefor the safety of the component and the environment, the limitation of parasitic effects and for the efficiency of the converter.
- Losses due to the energy store in the leakage inductance:
1 2
2
LI
RM
W
=⋅⋅
Lf f
which is mainly dissipated in the snubber resistor
).
(R
S
- Losses due to the eddy current in the transformer (view AN1262)
- Losses dueto the all additional resistor of circuit, defined by:
2
PI R
=⋅
turn off
R
RMS
(IRM)
As described before (in §1), in a typical converter working with a switch frequency lower than 100kHz, these different losses can be considered negligible compared to the conduction losses.
However in applications such as the DC/DC converter (12V-48V) working with a switching frequency around 300kHz, these losses can be predominant, and a 150V Schottky can be very interesting to reduce the switching losses.
In fact, in addition to the low V
, the 150V Schottky
F
has a better switching behaviour, due to its essentially capacitive recovery (less sensibility to the temperature). We have the advantage ofa soft recovery diode in terms of EMC and the Schottky is preferable to a fast recovery diode in terms of losses. The 150V Schottky diode is the better choice versus the 200V bipolar as for EMC and losses at turn-off are concerned. Experimental measurements confirm this.
Moreover, future advancements will mean that this product will be developed.
In fact, with the arrival of theEN6100-3-2 standard andthe introduction of thePFC, whatever the input voltageis, there will be acontinuous voltage on the primary. This will lead to a reduction of the transformation ratio, and in the same time, the reverse voltage of the diode.
Consequently, a lower breakdown voltage diode will be needed in the future to replace a 200V PN diode used today.
Also, the tendency is for the output power of adaptors to increase. This involves an increase in the output voltages. The voltage requirements of the diode in thiscase will be higher than 100Vand a 150V diode is likely to be the appropriate component.
8/9
APPLICATION NOTE
Informationfurnishedis believed to be accurate and reliable. However, STMicroelectronics assumes noresponsibilityfor the consequences of useof such information nor for any infringement of patentsor other rights of third parties which mayresultfrom its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written ap­proval of STMicroelectronics.
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