MP6002DN datasheet(10/15 Pages) MPS

10 / 15 page

MP6002 – MONOLITHIC FLYBACK/FORWARD DC-DC CONVERTER

MP6002 Rev. 1.0

www.MonolithicPower.com

12/4/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

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:



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:







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 steady-state reverse voltage of the

Schottky diode D2 is estimated as:





The output current is calculated as:

)

(







Where ID is the average current through

Schottky diode when it is conducting.

The input current is calculated as:





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

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

DMAX

VDS

(V)

VDS/0.9

(V)

VD2

(V)

VD2/0.9

(V)

0.36

119

132

0.41

125

139

0.45

131

146

0.49

138

153

0.53

144

160

0.56

150

167

0.58

156

174

0.60

163

181

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.

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