mp6001dn MPS, mp6001dn Datasheet - Page 8

• Manufacturer Part Number: mp6001dn
• Description: Monolithic Flyback/forward Dc-dc Converter
• Manufacturer: MPS

APPLICATION INFORMATION
Switching Frequency
The frequency (f
selection of the transformer (Tr), the output cap,
(C2), and the input cap, (C1). The higher the
frequency, the smaller the sizes for Tr, C2, and
C1. However, a higher frequency also leads to
higher AC power losses in the power switch,
control circuitry, transformer, and in the external
interconnection. The general rule states that
lower the output power, higher the optimum
switching frequency. For low current (<10A)
applications, f
synchronous rectifiers are used and 300KHz to
500KHz if Schottky rectifiers are used.
Fundamental Equations
The transformer turns ratio N is defined as:
Where N
primary
respectively.
The output voltage V
Where D is the duty cycle.
The steady-state drain to source voltage of the
primary power switch when it is off is estimated
as:
The
Schottky diode D2 is estimated as:
The output current is calculated as:
Where I
Schottky diode when it is conducting.
The input current is calculated as:
Where I
primary power switch when it is conducting.
MP6001 Rev. 0.91
9/21/2007
steady-state
S
P
D
and N
and
is the average current through the
is the average current through
TM
S
V
is usually 200KHz to 300KHz if
V
S
DS
V
I
secondary
O
O
are the number of turns of the
D
S
I
2
IN
) has big effects on the
=
=
=
N =
O
reverse
=
I
1
V
=
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D
is estimated to be:
V
IN
−
D
I
×
O
S
N
N
D
+
1 (
P
×
S
+
×
N
−
D
V
V
×
D
N
N
IN
IN
V
)
voltage
side
O
MP6001 – MONOLITHIC FLYBACK/FORWARD DC-DC CONVERTER
windings,
© 2007 MPS. All Rights Reserved.
of
www.MonolithicPower.com
the
Transformer (Coupled Inductor) Design
1. Transformer Turns Ratio
The transformer turns ratio determines the duty
cycle range, selection of the rectifier (D2),
primary side peak current, primary snubber loss,
and the current as well as voltage stresses on
the power switch (S). It also has effects on the
selection of C1 and C2. A higher transformer
turns ratio (N) means the following:
For
V
stresses of the power switch (S) and the
rectifier (D2).
Note:
The voltage spike due to the leakage inductance of the
transformer and device’s voltage rating/derating factors were
considered. See power switch selection and snubber design for
more information.
Voltage Stress vs. Transformer Turns Ratio
Table 1—Main Switch (S) and Rectifier (D2)
IN
10
11
=36V~75V, below table shows the voltage
N
4
5
6
7
8
9
•
•
•
•
•
•
a
Higher Duty Cycle
Higher voltage stress on S (V
lower voltage stress on D2 (V
Lower primary side RMS current (I
but higher secondary side RMS current
(I
Use of a smaller input capacitor but
bigger output capacitor.
Lower primary side peak current (I
and lower primary snubber loss.
Lower main switch (S) turn-on loss
D2(RMS)
D
0.36
0.41
0.45
0.49
0.53
0.56
0.58
0.60
5V
MAX
).
power
119
125
131
138
144
150
156
163
V
(V)
DS
supply
V
DS
132
139
146
153
160
167
174
181
(V)
/0.9
design,
V
(V)
38
32
28
25
23
21
20
19
D2
D2
).
DS
V
D2
), but
S(RMS)
S(PEAK)
(V)
42
36
31
28
26
24
22
21
/0.9
with
8
),
)