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| 型號(hào)： | AN-16 |
| 英文描述： | TOPSwitch Flyback Design Methodology |
| 中文描述： | TOPSwitch的反激式設(shè)計(jì)方法 |
| 文件頁數(shù)： | 29/32頁 |
| 文件大?。?/td> | 247K |
| 代理商： | AN-16 |
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6/96
AN-16
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winding on the power transformer. The capacitor C
is the
sum of C
and C
which are the
TOPSwitch
output capacitance
and the transformer winding capacitance, respectively. These
parasitic circuit elements are present in any real-life flyback
power supply circuit, and greatly affect supply performance.
As previously shown, the discontinuous mode circuit has three
intervals of operation per switching cycle (see Figure 8). The
impact on circuit operation of the parasitic circuit elements in
each of three intervals of operation is discussed below.
In the first interval (1) the
TOPSwitch
turns on, discharging C
and C
. The energy stored by these capacitances at the end of
the previous cycle is dissipated in the
TOPSwitch
at the beginning
of the turn on interval. This dissipated energy is proportional to
the square of the voltage on the parasitic capacitances. Because
of this effect, large values of parasitic capacitance can
dramatically lower the power supply efficiency, especially at
high input voltage. Leakage inductance has little effect during
the turn on interval, since the transformer has no stored energy,
and the initial value of the secondary output current is zero.
In interval (2) of operation, the
TOPSwitch
turns off. The
energy stored in the transformer magnetic field during the
previous interval is now transferred to the secondary circuit. A
problem that arises during this transfer is that leakage inductances
L
KP
and L
are both trying to oppose changes in current flow.
L
is trying to maintain primary current flow, and L
is trying
to block secondary current flow. There is a “crossover region”
during which the primary current ramps down and the secondary
current ramps up. The primary current ramps down to zero with
a slope determined by the value of leakage inductance and
circuit voltage levels. The secondary current ramps up to the
final value with a slope determined by the value of leakage
inductance and circuit voltage levels. The big problem is that
the primary current must continue to flow during this crossover
interval. The decaying primary current ends up flowing into
C
and C
which charge up to a peak voltage V
. This peak
voltage, caused by leakage inductance, will be referred to as the
“l(fā)eakage spike”. In a practical
TOPSwitch
flyback supply, the
leakage spike should be clamped to a value below the
TOPSwitch
breakdown voltage rating.
During interval (3) of operation, the reflected output voltage
goes to zero. The transformer magnetic field has given up all
the energy stored during the first interval. The
TOPSwitch
drain
to source voltage makes a transition from the level equal to the
sum of the reflected output voltage V
and input voltage V
IN
down to a level equal to the input voltage V
alone. This
transition excites the resonant tank circuit formed by the stray
capacitance and the primary inductance to create a decaying
oscillatory waveform, which persists until the
TOPSwitch
turns
on again. This waveform “modulates” the voltage on (and the
amount of energy stored in) C
and C
, determining the
power loss when
TOPSwitch
turns on at the beginning of the
next cycle.
In the continuous mode of operation, the same parasitic elements
are present as in the discontinuous mode. In addition, the non-
ideal aspects of the output rectifier characteristic become
important. An ideal rectifier has no forward voltage drop, and
switches infinitely fast. An actual diode has a finite forward
voltage drop, and takes a finite time to switch off. A PN junction
diode has a finite reverse recovery time ( trr) due to the fact that
the minority charge carriers must be swept from the junction by
the applied reverse voltage before the diode junction can
reverse bias and switch to the off state. In the case of a Schottky
diode, this finite recovery time is caused by junction capacitance.
This recovery time ( trr) is associated with a reverse recovery
current spike that persists until the diode switches off. This
current spike causes reverse power dissipation in the output
rectifier, and loads down the
TOPSwitch
during its turn on
transition. The amplitude and duration of this current spike is
dependent on the speed of the diode. For 100 KHz power
supplies, ultrafast diodes (trr < 50 nsec) are recommended. Use
of slower diodes will cause a loss in efficiency due to excessive
reverse recovery power dissipation, and can result in thermal
runaway of the output rectifier diode.
Non-ideal operating waveforms of a continuous mode flyback
converter are shown in Figure 9. During the interval (1) of
operation,
TOPSwitch
turns on while current is still flowing in
the transformer secondary. This means that the drain voltage at
the instant of turn on is equal to the sum of the input voltage and
the secondary voltage reflected back through the transformer
turns ratio. This results in higher
TOPSwitch
turn-on power
dissipation than in the discontinuous mode, due to the extra
energy stored in the parasitic capacitances of the primary
circuit. In addition, the current in the secondary leakage
inductance must be discharged before the secondary output can
be turned off. This results in a turn on current crossover while
Crossover
Interval
PI-1618-021496
VIN
VDRAIN
Leakage
Spike
Voltage
1
Interval
IPRI
2
VP
ISEC
Slope = di/dt
VIN+VOR
0
D1
Reverse
Recovery
Current
Spike
Figure 9. Non-ideal Flyback Converter Waveforms - Continuous
Mode.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| AN-17 | Flyback Transformer Design for TOPSwitch Power Supplies |
| 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 |
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