MC34262_11 Datasheet, PDF (9/19 Pages) ON Semiconductor – Power Factor Controllers

MC34262, MC33262
A Quickstart circuit has been incorporated to optimize
converter startup. During initial startup, compensation
capacitor C₁ will be discharged, holding the error amp
output below the Multiplier threshold. This will prevent
Drive Output switching and delay bootstrapping of
capacitor C₄ by diode D₆. If Pin 2 does not reach the
multiplier threshold before C₄ discharges below the lower
UVLO threshold, the converter will “hiccup” and
experience a significant startup delay. The Quickstart
circuit is designed to precharge C₁ to 1.7 V, Figure 8. This
level is slightly below the Pin 2 Multiplier threshold,
allowing immediate Drive Output switching and bootstrap
operation when C₄ crosses the upper UVLO threshold.
Drive Output
The MC34262/MC33262 contain a single totem−pole
output stage specifically designed for direct drive of power
MOSFETs. The Drive Output is capable of up to ±500 mA
peak current with a typical rise and fall time of 50 ns with
a 1.0 nF load. Additional internal circuitry has been added
to keep the Drive Output in a sinking mode whenever the
Undervoltage Lockout is active. This characteristic
eliminates the need for an external gate pulldown resistor.
The totem−pole output has been optimized to minimize
cross−conduction current during high speed operation. The
addition of two 10 W resistors, one in series with the source
output transistor and one in series with the sink output
transistor, helps to reduce the cross−conduction current and
radiated noise by limiting the output rise and fall time. A
16 V clamp has been incorporated into the output stage to
limit the high state V_OH. This prevents rupture of the
MOSFET gate when V_CC exceeds 20 V.
APPLICATIONS INFORMATION
The application circuits shown in Figures 20, 21 and 22
reveal that few external components are required for a
complete power factor preconverter. Each circuit is a peak
detecting current−mode boost converter that operates in
critical conduction mode with a fixed on−time and variable
off−time. A major benefit of critical conduction operation
is that the current loop is inherently stable, thus eliminating
the need for ramp compensation. The application in
Figure 20 operates over an input voltage range of 90 Vac to
138 Vac and provides an output power of 80 W (230 V at
350 mA) with an associated power factor of approximately
0.998 at nominal line. Figures 21 and 22 are universal input
preconverter examples that operate over a continuous input
voltage range of 90 Vac to 268 Vac. Figure 21 provides an
output power of 175 W (400 V at 440 mA) while Figure 22
provides 450 W (400 V at 1.125 A). Both circuits have an
observed worst−case power factor of approximately 0.989.
The input current and voltage waveforms of Figure 21 are
shown in Figure 23 with operation at 115 Vac and 230 Vac.
The data for each of the applications was generated with the
test set−up shown in Figure 25.
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