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| 型號(hào)： | AN-17 |
| 英文描述： | Flyback Transformer Design for TOPSwitch Power Supplies |
| 中文描述： | 回掃變壓器設(shè)計(jì)的TOPSwitch電源 |
| 文件頁(yè)數(shù)： | 3/12頁(yè) |
| 文件大?。?/td> | 133K |
| 代理商： | AN-17 |
C
6/96
AN-17
3
P
0
10
20
-20 V
-10 V
10 V
0 V
0 V
200 V
VD
0 mA
500 mA
I
0 A
5 A
I
VD
Time (
μ
s)
I
R
I
P
P
0
10
20
Time (
μ
s)
-20 V
-10 V
10 V
0 V
0 V
200 V
VD
0 mA
500 mA
I
0 A
5 A
I
R
I
P
I
VD
500 mA
0 mA
Figure 3. Voltage and Current Waveforms for Transformer
Primary and Secondary in Continuous Mode.
Figure 2. Voltage and Current Waveforms for Transformer
Primary and Secondary in Discontinuous Mode.
primary inductance.
TOPSwitch
turns on to effectively apply
the DC input voltage across the transformer winding with the
“dot” side at lower potential than the “no-dot side”. Primary
current I
increases linearly with a rate of change (di/dt) that
varies directly with DC input voltage and inversely with primary
inductance. Ripple current I
is defined as the incremental
linear current rise (di) over the entire
TOPSwitch
on time (t
).
Peak primary current I
is the final value occurring as
TOPSwitch
turns off. Energy, proportional to the square of peak current I
P
,
is stored by magnetic field in the transformer core as if the
primary winding were a simple inductor. The secondary
winding carries a reflected voltage proportional to primary
voltage by turns ratio with the same “dot” polarity. While
TOPSwitch
is on, output diode D2 and bias diode D3 are reverse
biased which prevents secondary current flow. When
TOPSwitch
turns off, the decreasing magnetic field induces an abrupt
voltage reversal on all transformer windings such that the “dot”
side is now higher potential than the “no-dot” side. Diode D2
and D3 become forward biased and secondary current rises
quickly to a peak value (proportional by the inverse turns ratio
to primary peak current I
). Primary current immediately drops
to zero.
TOPSwitch
drain voltage quickly rises to a voltage
equal to the sum of the DC input voltage and reflected output
voltage. Secondary winding current now linearly decreases at
a rate that varies directly with output voltage and inversely with
secondary inductance. Duty cycle is defined as the ratio of
TOPSwitch
on time t
to switching period T. D can also be
calculated from t
ON
and switching frequency f
S
as shown.
D
t
T
t
f
ON
ON
S
=
=
×
Figure 2 shows
TOPSwitch
and output diode triangular current
waveforms which define “discontinuous” mode of operation
resulting from low primary inductance. The secondary current
linearly decreases to zero before
TOPSwitch
turns on again.
The stored energy is completely delivered to the load.
TOPSwitch
drain voltage V
relaxes and rings back towards the DC bus
voltage when no current is flowing in either primary or secondary.
Figure 3 shows trapezoidal current waveforms which define
“continuous” mode of operation resulting from high primary
inductance. Secondary current is still flowing when
TOPSwitch
turns on at the beginning of the next cycle. The stored energy
is not completely delivered to the load. Energy (due to non-zero
magnetic field) remains in the core when
TOPSwitch
turns on
again which causes the initial step in
TOPSwitch
current. Note
that
TOPSwitch
drain voltage V
stays at a high value equal
to the sum of the DC input voltage and reflected output voltage
until
TOPSwitch
turns on again.
Current never flows in the primary and secondary winding at
the same time. Neither primary or secondary current is actually
continuous. In flyback power supplies, continuous/
discontinuous mode refers to magnetic field continuity in the
transformer core over one complete switching cycle. (The
flyback power supply is an isolated version of the simple buck-
boost converter where continuous and discontinuous modes are
easily defined by inductor current continuity.)
Each primary current waveform has a peak value (I
), a ripple
current value (I
), an average or DC value (I
), and an RMS
value (I
). I
determines the number of primary turns and the
core size necessary to limit peak flux density and must also be
below
TOPSwitch
peak current limit. I
is the average or DC
primary current (as well as the power stage DC input current)
which is proportional to output power. I
causes power losses
due to winding resistance and
TOPSwitch
R
. The ratio
(K
) of ripple current I
to peak current I
defines the continuous
or discontinuous waveform. K
also simplifies subsequent
calculations. Transformers designed for discontinuous operation
have a higher peak current and a ripple current to peak current
ratio K
RP
of one. Practical continuous designs have lower peak
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| AN-18 | TOPSwitch Flyback Transformer Construction Guide |
| AN-19 | TOPSwitch Flyback Power Supply Efficiency |
| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
| AN-22 | OBSOLETE when inventory is depleted. 10% tolerance no l |
| AN-23 | TinySwitch Flyback Design Methodology |
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