LT3798

14

3798f

voltage and the MOSFET switching frequency to determine

the expected power dissipation:

Decreasing the value of the capacitor will reduce the dis-

sipated power in the snubber at the expense of increased

peak voltage on the MOSFET drain, while increasing the

value of the capacitance will decrease the overshoot.

Transformer Design Considerations

Transformer specification and design is a critical part of

successfully applying the LT3798. In addition to the usual

list of caveats dealing with high frequency isolated power

supply transformer design, the following information

should be carefully considered. Since the current on the

secondary side of the transformer is inferred by the current

sampled on the primary, the transformer turns ratio must

be tightly controlled to ensure a consistent output current.

A tolerance of ±5% in turns ratio from transformer to

transformer could result in a variation of more than ±5% in

output regulation. Fortunately, most magnetic component

manufacturers are capable of guaranteeing a turns ratio

tolerance of 1% or better. Linear Technology has worked

OPERATION

Table 1. Predesigned Transformers—Typical Specifications, Unless Otherwise Noted

TRANSFORMER

PART NUMBER

SIZE

(L × W × H)

(μH)

(mΩ)

(mΩ)

MANUFACTURER

TARGET

APPLICATION

JA4429

21.1mm × 21.1mm × 17.3mm

400

1:0.24:0.24

252

126

Coilcraft

22V/1A

7508110210

15.75mm × 15mm × 18.5mm

2000

6.67:1:1.67

5100

165

Würth Elektronik

10V/0.4A

750813002

15.75mm × 15mm × 18.5mm

2000

20:1.0:5.0

6100

25

Würth Elektronik

3.8V/1.1A

750811330

43.2mm × 39.6mm × 30.5mm

300

6:1.0:1.0

150

25

Würth Elektronik

18V/5A

750813144

16.5mm × 18mm × 18mm

600

4:1:0.71

2400

420

Würth Elektronik

28V/0.5A

750813134

16.5mm × 18mm × 18mm

600

8:1:1.28

1850

105

Würth Elektronik

14V/1A

750811291

31mm × 31mm × 25mm

400

1:1:0.24

550

1230

Würth Elektronik

85V/0.4A

750813390

43.18mm × 39.6mm × 30.48mm

100

1:1:0.22

150

688

Würth Elektronik

90V/1A

750811290

31mm × 31mm × 25mm

460

1:1:0.17

600

560

Würth Elektronik

125V/0.32A

X-11181-002

23.5mm × 21.4mm × 9.5mm

500

72:16:10

1000

80

Premo

30V/0.5A

750811248

31mm × 31mm × 25mm

300

4:1.0:1.0

280

25

Würth Elektronik

24V/2A

S001621

25mm × 22.2mm × 16mm

820

16:1.0:4.0

1150

10

Renco

5V/4A

750312872

43.2mm × 39.6mm × 30.5mm

14

1:1:0.8

11

11

Würth Elektronik

28V/4A

with several leading magnetic component manufacturers

to produce predesigned flyback transformers for use with

the LT3798. Table 1 shows the details of several of these

transformers.

Loop Compensation

The voltage feedback loop is a traditional GM error ampli-

fier. The loop cross-over frequency is set much lower than

twice the line frequency for PFC to work properly.

The current output feedback loop is an integrator con-

figuration with the compensation capacitor between the

negative input and output of the operational amplifier.

This is a one-pole system therefore a zero is not needed

in the compensation. For offline applications with PFC,

the crossover should be set an order of magnitude lower

than the line frequency of 120Hz or 100Hz. In a typical

application, the compensation capacitor is 0.1μF.

In non-PFC applications, the crossover frequency may be

increased to improve transient performance. The desired

crossover frequency needs to be set an order of magnitude

below the switching frequency for optimal performance.