CS1600

12

DS904A5

5.1.3

PFC Boost Inductor

Equation 3 can be rewritten to calculate the PFC boost

The RMS current rating for the inductor is estimated using an

scaling factor used to account for variations in the input

current shape across the AC line cycle, over and above the

nominally calculated value. The nominal value before using

the scaling factor is as follows:

where

β = inductor scaling factor

following equation:

Inductor tolerances should be considered when estimating the

peak currents present in the application.

The internal control algorithm of the controller dictates that the

peak inductor current seen in the application could be as high

as a pre-defined threshold of 0.001984 times the inverse of

the inductor, which in this example amounts to 4.72 A. Care

needs to be taken to ensure that the saturation current rating

of the PFC boost inductor factors in this threshold used for the

protection schemes.

5.1.4

PFC MOSFET

The peak voltage stress on the PFC MOSFET is a diode drop

above the output voltage. Accounting for leakage spikes, for

the 460 V output application, a 600 V FET is recommended.

The FET should be able to handle the same peak current as

that seen through the inductor. This would amount to 3.96 A.

The scaling factor to determine the RMS current through the

MOSFET for a 108 V input is about 1.15, and the minimum

calculated as follows:

where

γ = FET scaling factor

5.1.5

PFC Diode

The PFC diode peak current is equal to the inductor peak

current:

The PFC diode average current is calculated as follows:

5.1.6

PFC Output Capacitor

The output capacitor needs to be designed to meet the voltage

ripple and hold-up time requirements. In the case of a cost-

sensitive ballast application, the hold-up requirement is not a

key requirement.

To address the output ripple requirements, the following

equation may be used as a guide:

where

For a 40 V ripple and minimum line frequency of 45 Hz, the

α

V

link

400V

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

90V

V

in min

()

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

×





2

V

link

V

link

400V

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

90V

×

2

×





–

V

link

V

in min

()

2

×

–

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

×

=

[Eq.6]

α

V

link

400V

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

90V

V

in min

()

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

×





V

link

400V

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

90V

×

2

×





–

V

link

V

in min

()

2

×

–

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

×

0.937

==

L

B

αη

V

in min

()

()

2

×

×

V

link

V

in min

()

2

×

()

–

2f

max

P

O

V

link

×

×

×

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

×

=

[Eq.6]

L

B

0.937

0.95 108

×

2

×

460

108

2

×

–

()

27010

3

×

115

460

×

×

×

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

×

431

μH

==

I

LB rms

()

P

O

V

in min

()

2

×η

×

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

β

×

=

I

LB rms

()

1.07A

=

I

LB rms

()

115

108

2

×

0.95

×

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

1.35

×

=

[Eq.7]

I

LB pk

()

4

P

O

×

η V

×

in min

()

2

×

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

=

I

LB pk

()

3.17 A

=

I

LB pk

()

4

115

×

0.95

108

×

2

×

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

=

[Eq.8]

I

FET rms

()

P

O

V

in min

()

2

×η

×

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

γ

×

=

I

LB rms

()

0.91A

=

I

LB rms

()

115

108

2

×

0.95

×

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

1.15

×

=

[Eq.9]

I

Dpk

()

I

LB pk

()

=

I

Dpk

()

3.17 A

=

[Eq.10]

I

Davg

()

P

O

V

link

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

=

I

Davg

()

0.25 A

=

I

Davg

()

115

460

----------

=

[Eq.11]

[Eq.12]

C

out

P

O

2

π f

line min

()

×

V

link

×ΔV

link rip

()

×

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

=