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1. Transformer Turns Ratio
MP6003 – MONOLITHIC FLYBACK/SEPIC DC-DC CONVERTER
MP6003 Rev. 1.01 www.MonolithicPower.com 10
1/26/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
APPLICATION INFORMATION
Switching Frequency
The frequency (fS), has big effects on the
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, fS is usually 200kHz to 300kHz if
synchronous rectifiers are used and 300KHz to
500kHz if Schottky rectifiers are used.
Fundamental Equations
The transformer turns ratio N is defined as:
S
P
N
N
N=
Where NP and NS are the number of turns of the
primary and secondary side windings,
respectively.
The output voltage VO is estimated to be:
N
V
D1
D
VIN
O×
−
=
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:
OINDS VNVV ×+=
The steady-state reverse voltage of the
Schottky diode D2 is estimated as:
N
V
VV IN
O2D +=
The output current is calculated as:
)D1(II DO −×=
Where ID is the average current through
Schottky diode when it is conducting.
The input current is calculated as:
DII SIN ×=
Where IS is the average current through the
primary power switch when it is conducting.
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:
• Higher Duty Cycle
• Higher voltage stress on S (VDS), but
lower voltage stress on D2 (VD2).
• Lower primary side RMS current (IS(RMS)),
but higher secondary side RMS current
(ID2(RMS)).
• Use of a smaller input capacitor but
bigger output capacitor.
• Lower primary side peak current (IS(PEAK))
and lower primary snubber loss.
• Lower main switch (S) turn-on loss
For a 5V power supply design, with
VIN=36V~75V, below table shows the voltage
stresses of the power switch (S) and the
rectifier (D2).
Table 1—Main Switch (S) and Rectifier (D2)
Voltage Stress vs. Transformer Turns Ratio
N DMAX VDS
(V) VDS/0.9
(V) VD2
(V) VD2/0.9
(V)
4 0.36 119 132 38 42
5 0.41 125 139 32 36
6 0.45 131 146 28 31
7 0.49 138 153 25 28
8 0.53 144 160 23 26
9 0.56 150 167 21 24
10 0.58 156 174 20 22
11 0.60 163 181 19 21
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.