ON Semiconductor NCP1351B, Application Note

Page: 12 / 28

AND8344/D
www.onsemi.com
12
The rectified voltage is filtered by C17, C18, C19, C20, C21
and L6 and is connected to the output of the 12 V terminal.
Transformer and LLC Tank:
For a detailed description on how to design the LLC
transformer and resonant tank, please see application notes
AND8311/D , AND8257/D and AND8255/D .
The transformer used in this application is manufactured by
Jepuls, model no. BCK3501−078. The parameters of this
transformer are as follow:
Primary Winding: 34 Turns
Secondary Winding 24 V: 2
×
4 Turns
Secondary Winding 12 V: 2
×
2 Turns
Aux Winding: 3 Turns
Primary Inductance: L
mag
= 670
m
H
Leakage Inductance: L
leak
=105
m
H
Accuracy of the primary and leakage inductance: 5%
Let’s review this transformer. Necessary gain for our
application is:
G
min
+
2
@
(V
out
)
V
f
)
V
in_max
+
2
@
(24
)
0.6)
425
^
0.116
(eq. 13)
G
nom
+
2
@
(V
out
)
V
f
)
V
in_nom
+
2
@
(24
)
0.6)
385
^
0.128 (eq. 14)
G
max
+
2
@
(V
out
)
V
f
)
V
in_min
+
2
@
(24
)
0.6)
295
^
0.167 (eq. 15)
The resonant frequency that best keeps this LLC in
operation can be calculated as follows:
f
res
+
1
2
@ p @
L
leak
@
C
res
Ǹ
+
(eq. 16)
+
1
2
@ p @
105
@
10
*
6
@
33
@
10
*
9
Ǹ
^
85.5 kHz
The minimum resonant frequency can be calculated as
follows:
f
min
+
1
2
@ p @
(L
leak
)
L
mag
)
@
C
res
Ǹ
+
(eq. 17)
+
1
2
@ p @
(105
@
10
*
6
)
670
@
10
*
6
)
@
33
@
10
*
9
Ǹ
^
^
31.5 kHz
It is very important to keep the operating frequency
always above the peak gain. This peak depends on the output
load as it is seen on Figure 14 and Figure 15. If the LLC
operating frequency were to drop below this peak, Q1 or Q3
is turned on when the opposite transistor’s body diode
conducts. This short, but very high current can damage
them. A simulation tool is used to see how the resonant tank
will operate with this transformer. The results are shown in
Figure 14.
V
min
V
nom
V
max
Gain chart
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
10 100 1000
Frequency [kHz]
Gain
Figure 14. Behavior of the Resonant Tank
for Full Load
50.8 85.5 113
V
min
V
nom
V
max
Gain chart
0
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
10 100 1000
Frequency [kHz]
Gain
Figure 15. Behavior of the Resonant Tank
for 10% Load
56.5 127
For only 10% of the nominal load, see Figure 15.
As one can see, although V
bulk
drops to 295 V (voltage set
by the BO divider), there is still some gain margin necessary
for tolerances of the devices used and so the LLC cannot fall
into ZCS. As is seen in Figure 30 and Figure 31, the
operating frequency measured by the oscilloscope is
84.5 kHz, which is very close to the frequency calculated in
Equation 16. Also notice that ZVS conditions are ensured
for both MOSFETs. Because the operating frequency is
slightly smaller than calculated, the primary current seen in
Figure 30 and Figure 31 is slightly distorted. This frequency
changes over time due the ripple of the PFC front stage.
PCB Design
The PCB layout of the LLC’s primary side is not very
critical because switching of the main MOSFETs happens
only under ZVS conditions and the influence of PCB
parasitic inductances on the operating frequency is
negligible. More critical is the secondary side of the LLC. It
is recommended that both halves of the path from the
secondary winding be made the same length because
a difference in path leakages means a different resonant