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ADuM3470/ADuM3471/ADuM3472/ADuM3473/ADuM3474

Data Sheet

TRANSFORMER DESIGN

Custom transformers were designed for use in the circuits shown

in Figure 38, Figure 39, and Figure 40 (see Table 18). The trans-

formers designed for use with the ADuM347x differ from other

transformers used with isolated dc-to-dc converters that do not

regulate the output voltage. The output voltage is regulated by a

PWM controller in the ADuM347x that varies the duty cycle of

the primary side switches in response to a secondary side feed-

back voltage, VFB, received through an isolated digital channel.

The internal controller has a maximum duty cycle of 40%.

TRANSFORMER TURNS RATIO

To determine the transformer turns ratio—taking into account

the losses for the primary switches and the losses for the secondary

diodes and inductors—the external transformer turns ratio for

the ADuM347x can be calculated using Equation 1.

)

(

MIN

DD1

ISO

(1)

where:

NS/NP is the primary to secondary turns ratio.

VISO is the isolated output supply voltage.

VD is the Schottky diode voltage drop (0.5 V maximum).

VDD1 (MIN) is the minimum input supply voltage.

D is the duty cycle = 0.30 for a 30% typical duty cycle (40% is

the maximum duty cycle).

2 is a multiplier factor used for the push-pull switching cycle.

For the circuit shown in Figure 38 using the 5 V to 5 V reference

design in Table 18 and with VDD1 (MIN) = 4.5 V, the turns ratio is

NS/NP = 2.

For a 3.3 V input to 3.3 V output isolated single power supply

and with VDD1 (MIN) = 3.0 V, the turns ratio is also NS/NP = 2.

Therefore, the same transformer turns ratio, NS/NP = 2, can be

used for the three single power applications: 5 V to 5 V, 5 V to

3.3 V, and 3.3 V to 3.3 V.

The circuit shown in Figure 39 uses double windings and diode

pairs to create a doubler circuit; therefore, half the output voltage,

VISO/2, is used, as shown in Equation 2.

)

(

MIN

DD1

ISO

(2)

where:

NS/NP is the primary to secondary turns ratio.

VISO is the isolated output supply voltage. VISO/2 is used because

the circuit uses two pairs of diodes, creating a doubler circuit.

VD is the Schottky diode voltage drop (0.5 V maximum).

VDD1 (MIN) is the minimum input supply voltage.

D is the duty cycle = 0.30 for a 30% typical duty cycle (40% is

the maximum duty cycle).

2 is a multiplier factor used for the push-pull switching cycle.

For the circuit shown in Figure 39 using the 5 V to 15 V reference

design in Table 18 and with VDD1 (MIN) = 4.5 V, the turns ratio is

NS/NP = 3.

The circuit shown in Figure 40 also uses double windings and

diode pairs to create a doubler circuit. However, because a

positive and negative output voltage are created, VISO is used,

and the external transformer turns ratio can be calculated using

Equation 3.

)

(

MIN

DD1

ISO

(3)

where:

NS/NP is the primary to secondary turns ratio.

VISO is the isolated output supply voltage.

VD is the Schottky diode voltage drop (0.5 V maximum).

VDD1 (MIN) is the minimum input supply voltage.

D is the duty cycle = 0.35 for a 35% typical duty cycle (40% is

the maximum duty cycle).

2 is a multiplier factor used for the push-pull switching cycle.

For the circuit shown in Figure 40, the duty cycle, D, is set to 0.35

for a 35% typical duty cycle to reduce the maximum voltages seen

by the diodes for a ±15 V supply.

For the circuit shown in Figure 40 using the +5 V to ±15 V refer-

ence design in Table 18 and with VDD1 (MIN) = 4.5 V, the turns

ratio is NS/NP = 5.

Table 18. Transformer Reference Designs

Part No.

Manufacturer

Turns Ratio,

PRI:SEC

ET Constant

(V × µs Min)

Total Primary

Inductance (µH)

Total Primary

Resistance (Ω)

Isolation

Voltage (rms)

Isolation

Type

Reference

JA4631-BL

Coilcraft

1CT:2CT

255

0.2

2500

Basic

Figure 38

JA4650-BL

Coilcraft

1CT:3CT

255

0.2

2500

Basic

Figure 39

KA4976-AL

Coilcraft

1CT:5CT

255

0.2

2500

Basic

Figure 40

TGSAD-260V6LF Halo Electronics 1CT:2CT

389

0.8

2500

Supplemental Figure 38

TGSAD-290V6LF Halo Electronics 1CT:3CT

389

0.8

2500

Supplemental Figure 39

TGSAD-292V6LF Halo Electronics 1CT:5CT

389

0.8

2500

Supplemental Figure 40

TGAD-260NARL

Halo Electronics 1CT:2CT

389

0.8

1500

Functional

Figure 38

TGAD-290NARL

Halo Electronics 1CT:3CT

389

0.8

1500

Functional

Figure 39

TGAD-292NARL

Halo Electronics 1CT:5CT

389

0.8

1500

Functional

Figure 40

Rev. B | Page 26 of 36

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