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MC34261D 데이터시트(PDF) 7 Page - ON Semiconductor

부품명 MC34261D
상세설명  Power Factor Controllers
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제조업체  ONSEMI [ON Semiconductor]
홈페이지  http://www.onsemi.com
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MC34261D 데이터시트(HTML) 7 Page - ON Semiconductor

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MC34261, MC33261
http://onsemi.com
7
With the component values shown in Figure 16, the
Current Sense Comparator threshold, at the peak of the
haversine varies from 1.1 V at 90 Vac to 100 mV at 268 Vac.
The Current Sense Input to Drive Output propagation delay
is typically 200 ns.
Timer
A watchdog timer function was added to the IC to
eliminate the need for an external oscillator when used in
stand alone applications. The Timer provides a means to
automatically start or restart the preconverter if the Drive
Output has been off for more than 400 μs after the inductor
current reaches zero.
Undervoltage Lockout
An Undervoltage Lockout comparator guarantees that the
IC is fully functional before enabling the output stage. The
positive power supply terminal (VCC) is monitored by the
UVLO comparator with the upper threshold set at 10 V and
the lower threshold at 8.0 V (Figure 14). In the standby
mode, with VCC at 7.0 V, the required supply current is less
than 0.5 mA (Figure 13). This hysteresis and low startup
current allow the implementation of efficient bootstrap
startup techniques, making these devices ideally suited for
wide input range off line preconverter applications. An
internal 36 V clamp has been added from VCC to ground to
protect the IC and capacitor C5 from an overvoltage
condition. This feature is desirable if external circuitry is
used to delay the startup of the preconverter.
Output
The MC34261/MC33261 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 pull−down resistor.
The totem pole output has been optimized to minimize cross
conduction current during high speed operation. The
addition of two 10 Ω resistors, one in series with the source
output transistor and one in series with the sink output
transistor, reduces the cross conduction current, as shown in
Figure 12. A 16 V clamp has been incorporated into the
output stage to limit the high state VOH. This prevents
rupture of the MOSFET gate when VCC exceeds 20 V.
Table 1. Design Equations
Notes
Calculation
Formula
Calculate the maximum required output power.
Required Converter Output Power
PO = VO IO
Calculated at the minimum required ac line for regula-
tion. Let the efficiency n = 0.95.
Peak Inductor Current
IL(pk) =
22 PO
ηVac(LL)
Let the switching cycle t = 20 μs.
Inductance
L =
2t
2
VO Vac(LL) IL(pk)
VO
− Vac Vac2
In theory the on−time ton is constant. In practice ton
tends to increase at the ac line zero crossings due to
the charge on capacitor C6.
Switch On−Time
η Vac2
ton =
2 PO L
The off−time toff is greatest at peak ac line
and approaches zero at the ac line zero
crossings. Theta (θ) represents the angle of
the ac line voltage.
Switch Off−Time
VO
− 1
toff =
2 Vac ⎪Sin θ⎜
ton
The minimum switching frequency occurs at peak ac
line and increases as toff decreases.
Switching Frequency
f =
ton + toff
1
Set the current sense threshold VCS to 1.0 V for
universal input (85 Vac to 265 Vac) operation
and to 0.5 V for fixed input (92 Vac to 138 Vac,
or 184 to 276 Vac) operation.
Peak Switch Current
R9 =
IL(pk)
VCS
Set the multiplier input voltage VM to 3.0 V at high
line. Empirically adjust VM for the lowest distortion
over the ac line range while guaranteeing startup
at minimum line.
Multiplier Input Voltage
+ 1
Vac
VM =
R7
2
R3
The IIB R1 error term can be minimized with a divider
current in excess of 100 μA.
Converter Output Voltage
− IIB R2
VO = Vref
R2
+ 1
R1
The bandwidth is typically set to 20 Hz for minimum
output ripple over the ac line haversine.
Error Amplifier Bandwidth
BW =
R1 R2
1
R1 + R2
2 π
C1
The following converter characteristics must be chosen:
VO
IO
Vac
Vac(LL)
− Desired output voltage
− AC RMS line voltage
− Desired output current
− AC RMS low line voltage


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