MC34262_11 Datasheet, PDF (15/19 Pages) ON Semiconductor – Power Factor Controllers

MC34262, MC33262
10mA
Error
Amp
+
R₂
1
R₁
6
2
C₁
Figure 26. Error Amp Compensation
The Error Amp output is a high impedance node and is susceptible to noise pickup. To minimize pickup, compensation
capacitor C₁ must be connected as close to Pin 2 as possible with a short, heavy ground returning directly to Pin 6. When
operating at high ac line, the voltage at Pin 2 may approach the lower threshold of the Multiplier, ≈ 2.0 V. If there is
excessive ripple on Pin 2, the Multiplier will be driven into cut−off causing circuit instability, high distortion and poor power
factor. This problem can be eliminated by increasing the value of C₁.
7
7
Current
Sense
Comparator
22k
4
R
10pF
C
R₇
Figure 27. Current Waveform Spike Suppression
A narrow turn−on spike is usually present on the leading edge of
the current waveform and can cause circuit instability. The
MC34262 provides an internal RC filter with a time constant of
220 ns. An additional external RC filter may be required in
universal input applications that are above 200 W. It is
suggested that the external filter be placed directly at the Current
Sense Input and have a time constant that approximates the
spike duration.
Current
Sense
Comparator
22k
4
10pF
D₁
R₇
Figure 28. Negative Current Waveform
Spike Suppression
A negative turn−off spike can be observed on the trailing edge of
the current waveform. This spike is due to the parasitic
inductance of resistor R₇, and if it is excessive, it can cause
circuit instability. The addition of Schottky diode D₁ can
effectively clamp the negative spike. The addition of the external
RC filter shown in Figure 27 may provide sufficient spike
attenuation.
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