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參數(shù)資料
| 型號： | MIC2172BN |
| 廠商： | MICREL INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 3.5 A SWITCHING REGULATOR, 115 kHz SWITCHING FREQ-MAX, PDIP8 |
| 封裝： | PLASTIC, DIP-8 |
| 文件頁數(shù)： | 9/20頁 |
| 文件大?。?/td> | 906K |
| 代理商： | MIC2172BN |
Micrel
MIC2172/3172
April 2006
17
M9999-041806
(408) 955-1690
Finally, recalculate the transformer turns ratio to insure
that it is less than the value earlier found in equation (9).
SEC
PRI
L
a
≤
(12)
Then:
5
10
2.54
10
1.8
a
×
≤
a
≤ 0.84 Use 0.8 (same as 1:1.25).
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know
the primary peak current which must be withstood
without saturating the transformer core.
PRI
ON
IN(min)
PEAK(pri)
L
T
V
I
=
So:
18
8
10
5.5
4.0
I
6
PEAK(pri)
×
=
(13)
IPEAK(pri) = 1.22A
Now find the minimum reverse voltage requirement for
the output rectifier. This rectifier must have an average
current rating greater than the maximum output current
of 0.25A.
()
a
BR
OUT
IN(max)
BR
F
V
+
≥
(14)
Where:
VBR = output rectifier maximum peak reverse
voltage rating
a
= transformer turns ratio (0.8)
FBR = reverse voltage safety derating factor (0.8)
Then:
()
15.625V
V
0.8
5.0
6.0
V
BR
≥
×
+
≥
A 1N5817 will safely handle voltage and current
requirements in this example.
Forward Converters
Micrel’s MIC2172/3172 can be used in several circuit
configurations to generate an output voltage which is
less than the input voltage (buck or step-down topology).
Figure 13 shows the MIC3172 in a voltage step-down
application. Because of the internal architecture of these
devices, more external components are required to
implement a step-down regulator than with other devices
offered by Micrel (refer to the LM257x or LM457x family
of buck switchers). However, for step-down conversion
requiring a transformer (forward), the MIC2172/3172 is a
good choice.
A 12V to 5V step-down converter using transformer
isolation (forward) is shown in figure 14. Unlike the
isolated flyback converter which stores energy in the
primary inductance during the controller’s on-time and
releases it to the load during the off-time, the forward
converter transfers energy to the output during the on-
time, using the off-time to reset the transformer core. In
the application shown, the transformer core is reset by
the tertiary winding discharging T1’s peak magnetizing
current through D2.
For most forward converters the duty cycle is limited to
50%, allowing the transformer flux to reset with only two
times the input voltage appearing across the power
switch. Although during normal operation this circuit’s
duty cycle is well below 50%, the MIC2172 (and
MIC3172) has a maximum duty cycle capability of 90%.
If 90% was required during operation (start-up and high
load currents), a complete reset of the transformer
during the off-time would require the voltage across the
power switch to be ten times the input voltage. This
would limit the input voltage to 6V or less for forward
converter applications.
To prevent core saturation, the application given here
uses a duty cycle limiter consisting of Q1, C4 and R3.
Whenever the MIC3172 exceeds a duty cycle of 50%,
T1’s reset winding current turns Q1 on. This action
reduces the duty cycle of the MIC3172 until T1 is able to
reset during each cycle.
Fluorescent Lamp Supply
An extremely useful application of the MIC3172 is
generating an ac voltage for fluorescent lamps used as
liquid crystal display back lighting in portable computers.
Figure 15 shows a complete power supply for lighting a
fluorescent lamp. Transistors Q1 and Q2 together with
capacitor C2 form a Royer oscillator. The Royer
oscillator generates a sine wave whose frequency is
determined by the series L/C circuit comprised of T1 and
C2. Assuming that the MIC3172 and L1 are absent, and
the transistors’ emitters are grounded, circuit operation is
described in “Oscillator Operation.”
Oscillator Operation
Resistor R2 provides initial base current that turns
transistor Q1 on and impresses the input voltage across
one half of T1’s primary winding (Pri 1). T1’s feedback
winding
provides
additional
base
drive
(positive
feedback) to Q1 forcing it well into saturation for a period
determined by the Pri 1/C2 time constant. Once the
voltage across C2 has reached its maximum circuit
value, Q1’s collector current will no longer increase.
Since T1 is in series with Q1, this drop in primary current
causes the flux in T1 to change and because of the
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參數(shù)描述 |
|---|---|
| MIC2172YM | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 1.25A 100kHz Switching Regulator (Lead Free) RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
| MIC2172YM TR | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 1.25A 100kHz Switching Regulator (Lead Free) RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
| MIC2172YMTR | 制造商:Micrel 功能描述:Current Mode PWM Controller 65V 1.25A |
| MIC2172YM-TR | 制造商:Micrel Inc 功能描述: |
| MIC2172YN | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 2.5A 100kHz Switching Regulator (Lead Free) RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
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