ISL6721 datasheet(13/23 Pages) RENESAS

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ISL6721

FN9110 Rev 9.00

Page 13 of 23

May 20, 2016

Circuit Element Descriptions

The converter design may be broken down into the following

functional blocks:

• Input storage and filtering capacitor: C1, C2, C3

• Isolation transformer: T1

• Primary voltage clamp: CR6, R24, C18

•Start bias regulator: R1, R2, R6, Q3, VR1

• Operating bias and regulator: R25, Q2, D1, C5, CR2, D2

• Main MOSFET power switch: Q1

• Current sense network: R4, R3, R23, C4

• Feedback network: R13, R15, R16, R17, R18, R19, R20, R26,

R27, C13, C14, U2, U3

• Control circuit: C7, C8, C9, C10, C11, C12, R5, R6, R8, R9,

R10, R11, R12, R14, R22

• Output rectification and filtering: CR4, CR5, C15, C16, C19,

C20, C21, C22

•Secondary snubber: R21, C17

Design Criteria

The following design requirements were selected:

•Switching frequency, fsw: 200kHz

•VIN: 36V to 75V

•VOUT(1): 3.3V at 2.5A

•VOUT(2): 1.8V at 1.0A

•VOUT(BIAS): 12V at 50mA

•POUT: 10W

• Efficiency: 70%

• Maximum duty cycle, DMAX: 0.45

Transformer Design

The design of a Flyback transformer is a non-trivial affair. It is

an iterative process, which requires a great deal of experience

to achieve the desired result. It is a process of many

compromises, and even experienced designers will produce

different designs when presented with identical requirements.

The iterative design process is not presented here for clarity.

The abbreviated design process follows:

• Select a core geometry suitable for the application.

Constraints of height, footprint, mounting preference, and

operating environment will affect the choice.

• Select suitable core material(s).

• Select maximum flux density desired for operation.

• Select core size. Core size will be dictated by the capability

of the core structure to store the required energy, the

number of turns that have to be wound and the wire gauge

needed. Often the window area (the space used for the

windings) and power loss determine the final core size. For

Flyback transformers, the ability to store energy is the

critical factor in determining the core size. The cross

sectional area of the core and the length of the air gap in the

magnetic path determine the energy storage capability.

• Determine maximum desired flux density. Depending on the

frequency of operation, the core material selected, and the

operating environment, the allowed flux density must be

determined. The decision of what flux density to allow is

often difficult to determine initially. Usually the highest flux

density that produces an acceptable design is used, but

often the winding geometry dictates a larger core than is

required based on flux density and energy storage

calculations.

• Determine the number of primary turns.

• Determine the turns ratio.

• Select the wire gauge for each winding.

• Determine winding order and insulation requirements.

•Verify the design.

Input Power:

•POUT/efficiency = 14.3W (use 15W)

•Max On time: tON(MAX) = DMAX/fsw = 2.25µs

• Average input current: IAVG(IN) = PIN/VIN(MIN) = 0.42A

Peak Primary Current:

Maximum Primary Inductance:

Choose desired primary inductance to be 40µH.

The core structure must be able to deliver a certain amount of

energy to the secondary on each switching cycle in order to

maintain the specified output power.

Where

w is the amount of energy required to be transferred

each cycle and Vd is the drop across the output rectifier.

The capacity of a gapped ferrite core structure to store energy

is dependent on the volume of the airgap and can be

expressed in Equation 12:

Where Aeff is the effective cross sectional area of the core in

m2, lg is the length of the airgap in meters, µo is the

permeability of free space (4

10-7) and B is the change in

flux density in Tesla.

A core structure having less airgap volume than calculated will

be incapable of providing the full output power over some

portion of its operating range. On the other hand, if the length of

the airgap becomes large, magnetic field fringing around the

IPPK

2IAVG IN





fsw tON MAX





-----------------------------------------

1.87

(EQ. 9)

Lp max



VIN MIN

 tON MAX





IPPK

---------------------------------------------------------

43.3

H

(EQ. 10)

wP

OUT

VOUT Vd



fsw VOUT



------------------------------------



joules

(EQ. 11)

Aeff lg





o w



B

-----------------------------

(EQ. 12)

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