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| 型號： | AN-19 |
| 英文描述： | TOPSwitch Flyback Power Supply Efficiency |
| 中文描述： | TOPSwitch的反激式電源供應器效率 |
| 文件頁數(shù)： | 15/20頁 |
| 文件大小： | 177K |
| 代理商： | AN-19 |
A
6/96
AN-19
15
APPENDIX A
Peak Primary Currents for Discontinuous-mode
Versus Continuous-mode Flyback
There are two basic modes of operation for a flyback converter.
The first is the discontinuous mode, in which the energy stored
in the flyback transformer is completely discharged each
switching cycle. The second is the continuous mode, where the
transformer begins each switching cycle with a non-zero value
of stored energy. Representative primary current waveforms
are shown for each case in Figure 19. Each mode has its
particular advantages, but the continuous flyback runs at a
lower value of primary and secondary current for the same
operating parameters than a discontinuous flyback. In application
notes AN-16 and AN-17, K
is defined as the ratio of the
primary ripple current to the primary peak current. K
can be
defined in terms of the initial primary current (I
I
), and the peak
current (I
P
) as follows:
K
I
I
I
I
I
RP
P
I
P
R
P
=
=
I
R
denotes the ripple current of the primary current waveform.
K
can be used to describe quantitatively the degree of
continuous operation of a flyback supply. K
can assume
values from zero to unity. K
=1 indicates discontinuous
operation, while K
<1 indicates continuous operation. K
=0
implies the ultimate extreme for continuous operation, with an
infinite primary inductance and a rectangular primary current
waveform. For a given input voltage range, the value of K
can
be selected within the suggested range to optimize either the
TOPSwitch
or the transformer size. A low value of K
indicates
more continuous operation, with a relatively high primary
inductance. This results in low peak and RMS primary currents,
and optimizes the design for the smallest possible
TOPSwitch
,
at the cost of a larger transformer. A larger K
will result in less
continuous operation, lower primary inductance, and higher
primary peak and RMS currents. This optimizes the transformer
size at the expense of a larger
TOPSwitch
. A K
range of
0.4-1.0 is recommended for 100/115 VAC. and universal input
TOPSwitch
supplies, while a K
RP
of 0.6-1.0 is optimum for 230
VAC supplies.
Two design examples are given below showing the differences
in peak and RMS current for a universal input power supply
running in the discontinuous mode at K
= 1, and in the
continuous mode at K
= 0.4. For a more detailed description
of flyback operation, please refer to AN-16 and AN-17.
Discontinuous-mode Operation
Operating Parameters:
K
RP
= 1.0
V
MIN
= Minimum DC bus voltage = 90 VDC
D
MAX
= Duty cycle at V
MIN
= 0.6
P
O
η
= efficiency = 80%
I
can be expressed as a function of I
and K
, and also as a
function of basic supply parameters and I
R
, as shown below:
I
I
K
P
D
V
I
2
P
R
RP
O
MIN
MAX
R
=
=
×
×
+
η
Using the above equation to solve for I
P
, the result is:
I
P
K
V
D
P
O
RP
×
MIN
MAX
=
×
×
2
2
η
For V
= 90VDC, D
MAX
= 0.6, P
O
= 30W, K
RP
= 1.0, and
η
= 0.8, I
P
is:
I
A
P
=
×
×
×
=
30
2
2 1
0 6
.
90
0 8
.
1 39
.
Solving for I
RMS
:
I
I
D
K
K
A
RMS
P
MAX
RP
3
RP
=
×
×
+
=
×
×
+
2
1
1 39
.
0 6
.
1
3
0 62
.
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