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參數(shù)資料
| 型號(hào)： | KA7526 |
| 廠商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Tantalum Conformal-Coated Capacitor; Capacitance: 1uF; Voltage: 25V; Case Size: 3.2x1.2 mm; Packaging: Tape & Reel |
| 中文描述： | 0.5 A POWER FACTOR CONTROLLER, 20 kHz SWITCHING FREQ-MAX, PDIP8 |
| 封裝： | DIP-8 |
| 文件頁數(shù)： | 11/22頁 |
| 文件大?。?/td> | 250K |
| 代理商： | KA7526 |
KA7526
11
Error Amplifier
An Error Amplifier with access to the inverting input and output is provided. The noninverting input is internally biased at
2.5V and is not pinned out. The output voltage of the power factor converter is typically divided down and monitored by the
inverting input. The error amp output is internally connected to the multiplier and is pinned out for external loop compensa-
tion. Typically, the loopbandwidth is set below 20Hz, so that the amplifer’s output voltage is relatively constant over a given ac
line cycle. In effect, the error amp monitors the average output voltage of the converter over several line cycles. Input bias cur-
rent(0.5uA, max) can cause an output voltage error that is equal to the product of the input bias current and the value of the
upper divider resistor, R1 in Fig. 2.1.
Over Voltage Protection
The low bandwidth (typically below 20Hz) characteristic of Error Amplifier control loop results in output voltage runaway
condition. This condition can occur during initial start-up, sudden load removal, or during output arcing. The over voltage
comparator monitors the output voltage of the error amplifier. When load is removed, error amp output swings lower than
1.8V, comparator is triggered high and output driver is turned off till the error amp inverting input voltage drops below 2.5V.
At this point, the error amp output swings positive, turns the output driver back on. .
Multiplier
A single quadrant, two input multiplier is the critical element that enables this device to get power factor correction. One input
of multiplier(Pin 3) is connected to an external resistor divider which monitors the rectified ac line. The other input is inter-
nally driven by a DC voltage which is the difference of error amplifier output (Pin 2) and reference voltage, Vref. The multi-
plier is designed to have an extremely linear transfer curve over a wide dynamic range, 0V to 3.8V for Pin 3, and 2.5V to 4.5V
for error amplifier output under all line and load conditions.
The multiplier output controls the current sense comparator threshold as the ac voltage traverses sinusoidally from zero to
peak line. This allows the inductor peak current to follow the ac line thus forcing the average input current to be sinusoidal. In
other words, this has the effect of forcing the MOSFET on-time to track the input line voltage, resulting in a fixed drive output
on-time, thus making the preconverter load appear to be resistive to the ac line.
The equation below describes the relationship between multiplier output and inputs.
Vmo = K
×
Vm1
×
(Vm2-Vref)
K : Multiplier gain
Vm1: Voltage at Pin 3
Vm2: Error amp output voltage
Vmo: Multiplier output voltage
-
+
-
+
x
1
2
INV
EA OUT
1.8V
Vref
Over Voltage
Comparator
Vref
To Multiplier
To Drive
Output Shutdown
Ccomp
R1
R2
R3
Band width= 1/(2
π×
R1
×
Ccomp)
+
_
Fig.2.1 Error Amp and Over Voltage Comparator
+
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