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HFC0300 Datasheet(PDF) 13 Page - Monolithic Power Systems

No. de Pieza. HFC0300
Descripción  Variable Off Time Controller
Descarga  21 Pages
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Fabricante  MPS [Monolithic Power Systems]
Página de inicio  http://www.monolithicpower.com
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HFC0300 Datasheet(HTML) 13 Page - Monolithic Power Systems

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HFC0300– VARIABLE OFF TIME CONTROLLER
HFC0300 Rev. 1.0
www.MonolithicPower.com
13
9/23/2011
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
APPLICATION INFORMATION
Design Keys of HFC0300
Current Sense Resistor Section
The peak current level is internally set to 0.5V, so
the current-sense resistor sets the primary-side
peak current, which determines the operation
mode of the converter—such as CCM, BCM or
DCM. If power supply is designed to operate at
BCM at low-line input, it will operate at DCM at
the high line and the same load condition. The
magnetizing inductor current (reflected on the
primary side) and the drain-source voltage (VDS)
of the primary MOSFET is shown in Figure 9.
Inductor
Current (A)
VDS
High line
Low line
Inductor
Current (A)
Tsecond
T
Tsecond
Iprimary
Isecondary/N
0.5V/Rsense
VDS
Iprimary
Isecondary/N
0.5V/Rsense
Figure 9: Inductor Current and Voltage of
Primary MOSFET
The time duration of the secondary current can
be determined by equation (1):
m
peak
sec
o
LI
t
NV
(1)
Where Lm is the primary magnetizing inductance,
Ipeak is the primary peak current, and N is the turn
ratio of the transformer. Ipeak remains the same at
under different inputs and with the same output,
so the time duration of secondary current is the
same.
The switching period can be calculated by:
peak
sec
o
NI
t
t
2I
(2)
From equation (2), the switching period remains
the same at different inputs with the same output
condition. Since the primary-side switch ON time
decreases with the increasing input voltage, then
the higher the input line voltage, the deeper
discontinuous current mode (DCM) it will enter.
Usually, the parameters are designed for the
minimum input condition to guarantee that the
converter can deliver the required maximum
output power.
Since N is pre-determined, if the power supply is
designed to operate at boundary current mode
(BCM) at the low line, the peak current can be
calculated as:
o
peak _ BCM
2I
I
N(1 D)

(3)
Where D is the duty ratio of the switching. Then:
oF
in
o
F
(V
V ) N
D
V(V
V ) N


(4)
If the peak current set by the current-sense
resistor is larger than Ipeak_BCM, the power supply
will enter DCM. On the other hand, if the peak
current set by current sense resistor is less than
Ipeak_BCM, the power supply will enter CCM, as
shown as Figure 10. Here, we define Kdepth as the
depth of CCM.
valley
depth
peak
I
K
I
(5)


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