NCP1651
http://onsemi.com
14
Ifwelookatasingleapplicationandcomparetheresults.
P
O
= 90 watts
Vin = 85- -265 V
rms
(analyzed at 85 V
rms
input)
Efficiency = 80%
P
in
= 108 W
V
O
= 48 Vdc
Freq = 100 kHz
Transformer turns ration N = 4
Continuous Mode (CCM)
To force the inductor current to be continuous over the
majority of the input voltage range (85- -265 Vac), L
P
needs
to be at least 1 mH. Figure 26 shows the typical current
through the windings of the flyback transformer. During
switch on period, this current flows in the primary and
during the switch off time it flows in the secondary.
TIME
I
PK
I
avg
Figure 26.
The peak current is:
I
PK
= I
avg
+ ((1.414 ?V
in
sin ??t
on
?2)/L
P
)
where I
avg
= 1.414 ?P
in
/V
in
sin ?/DIV>
T
on
= T/((N
S
/N
P
?1.414 ?V
in
sin ?/V
O
) +1)
T
on
= 6.19 ms
I
PK
=(1.414?13)/85sin?(1.414?5?.15ms?)/1 mH
= 3.35 A
Discontinuous Mode (DCM)
In the discontinuous mode of operation, the inductor
current falls to zero prior to the end of the switching period
as shown in Figure 27.
TIME
I
PK
I
avg
Figure 27.
To ensure DCM, LP needs to be reduced to approximately
100 mH.
I
PK
= (V
in
sin ??1.414 ?t
on
)/L
P
I
PK
= 1.414 ?85 sin 90 ?5.18 ms/100 mH = 6.23 A
The results show that the peak current for a flyback
converter operating in the Continuous Conduction Mode is
onehalfthepeakcurrentofaflybackconverteroperatingin
the Discontinuous Conduction Mode.
Continuous Conduction Mode
A second result of running in DCM can be higher input
current distortion, EMI, and a lower Power Factor, in
comparison to CCM. While the higher peak current can be
filtered to produce the same performance result, it will
require a larger filter.
A simple Fast Fourier Transform (FFT) was run in Spice
to provide a comparison between the harmonic current
levelsforCCMandDCM.Theharmoniccurrentlevelswill
affect the size of the input EMI filter which in some
applications are required to meet the levels of C.I.S.P.R. In
theSPICEFFTmodelwedidnotaddanyfrontendfiltering
so the result of the analysis could be compared directly.
Figure 28. Continuous Conduction Mode
300
250
200
150
100
50
0
FREQUENCY (MHz)
0.4
0.2
0.8
0.6
1.2
1.0
1.6
1.4
2.0
1.8
At the 100 kHz switching frequency, the rms value from
the FFT is 260 mA, and the 2nd harmonic (200 kHz) is
55 mA rms.
Figure 29. Discontinuous Conduction Mode
2.8
2.0
1.6
1.2
0.8
0.4
0
FREQUENCY (MHz)
0.4
0.2
0.8
0.6
1.2
1.0
1.6
1.4
2.0
1.8
2.4
At100kHzthermsvaluefromtheFFTare2.8A,andthe
2nd harmonic (200 kHz) is 500 mA rms.
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