AN2623
Application note
Evaluation board
for off-line forward converter based on L5991
Introduction
This application note gives a practical example of a 160 W, isolated, forward converter using
the L5991, high frequency current mode PWM controller. Design procedures for both the
power stage and controller are presented.
Generally for this power level the norm ICE61000-3-2 imposes the use of a PFC pre-
regulator stage, but some countries do not require compliance to this norm. The forward
converter presented here does not have a PFC.
Figure 1.
160 W off-line forward converter, evaluation board
October 2007
Rev 1
1/25
www.st.com
Contents
AN2623
Contents
1
2
3
Basis of forward topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Design circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1
3.2
3.3
3.4
3.5
Primary controller: L5991 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Output diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Power transformer design and MOSFET choice . . . . . . . . . . . . . . . . . . . . 9
Feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4
Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1
4.2
4.3
4.4
4.5
4.6
High frequency ripple of output voltage and load regulation . . . . . . . . . . 16
Dynamic load test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Start-up behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Wake-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Short circuit test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Thermal measurement and global efficiency . . . . . . . . . . . . . . . . . . . . . . 22
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2/25
AN2623
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
160 W off-line forward converter, evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Basic forward converter topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Reset circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Vds and Ids of STW12NK90Z in full load condition at different input voltages . . . . . . . . . 15
High frequency ripple of output voltage in full load condition at different input voltages. . . 16
Output voltage behavior against the load and the V
in
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Behavior of system under dynamic load at different input voltages . . . . . . . . . . . . . . . . . . 18
Behavior of system under dynamic load at different input voltages . . . . . . . . . . . . . . . . . . 19
Wake-up time of the system at different input voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Behavior of the system in short circuit condition at different input voltages . . . . . . . . . . . . 21
Efficiency of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3/25
Basis of forward topology
AN2623
1
Basis of forward topology
A forward converter is typically used in off-line applications in the 100 W - 300 W power
range. A simplified schematic of the forward converter can be seen in
Figure 2.
Figure 2.
Basic forward converter topology
D
1
+
D
2
V
d
Reset
Circuit
L
+
C
V
0
_
_
A natural limitation of the forward converter is the need to completely reset the transformer,
cycle by cycle, before the next MOSFET switches on. Different circuits are used for this
purpose with advantages and drawbacks. The two simplest and most commonly used reset
schemes are: the RCD reset circuit and the reset auxiliary winding both shown in
Figure 3
(a-b).
In the design presented in this document, the reset winding was used. It is
advantageous with respect to efficiency because the energy stored in the magnetizing
inductor goes back to the input and is not lost as using an RCD snubber net. The drawback
of the reset circuit is that, generally, a higher voltage Power Mosfet is needed. In the present
design a 900 V MOSFET was used.
Figure 3.
Reset circuits
C
R
R
R
N
1
N
2
N
R
N
1
N
2
D
R
D
R
(a)
(b)
The primary controller IC used is the L5991. It is based on a standard current mode PWM
controller and includes features such as programmable soft start, adjustable duty cycle
limitation and a standby function that reduces the switching frequency when the converter is
lightly loaded. The standby function, in this case, is not used to prevent the transformer from
saturation. The output voltage regulation is obtained through a voltage reference and an
error amplifier (TL1431) placed at the secondary side. A charge pump connected to an
auxiliary winding guarantees a stable supply at the controller itself.
4/25
AN2623
Main characteristics
2
Main characteristics
The design procedure is presented in this section and we will refer to the electrical
schematic in
Figure 4.
The power supply electrical specifications are shown in
Table 1
below.
Table 1.
Input and output parameters
Input parameters
V
in
f
line
Input voltage
Line frequency
Output parameters
V
out
I
out
P
out
∆V
out
%
∆V
out HF
T
A max
Output voltage
Output current
Output power
Efficiency at full load
Max tolerance on output voltage
Max output voltage ripple at switching frequency
Maximum ambient temperature
35 V
4.5 A max continuous, 0.45 A min
160 W max
80%
3%
350 mV
70 °C
88 ÷ 290 V
RMS
50/60 Hz
5/25
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