User`s guide

Hardware Design

The peak current in the primary is fixed for a given output power and the switch on-time

varies as a function of the DC input voltage, as shown in Equation 2-36.

EQUATION 2-36:

From the TNY277 data sheet, the maximum switching frequency is 140 kHz, and a

sensible maximum on-time is 4.5 μs. The minimum DC voltage is 120V, and the power

at the transformer primary is 17W, assuming ~ 80% efficiency. It is worth noting that this

is only a transient requirement, since once the dsPIC DSC device rails are established,

the PFC will boost the DC input voltage to 400V. Equation 2-35 and Equation 2-36 can

be rearranged to find the required primary magnetizing inductance of the transformer,

as shown in Equation 2-37.

EQUATION 2-37:

Therefore, the target primary inductance is 1.2 mH and the peak primary current is

0.45A. The primary turns must be selected to ensure that the ferrite core losses are

around 0.5W for thermal reasons. The EF20 core has a cross-sectional area of 32 mm

and a volume of 1490 mm

The core loss can be computed from Equation 2-38.

EQUATION 2-38:

Therefore, the peak flux density is about 150 mT. The required number of primary turns

can be computed using Equation 2-39.

EQUATION 2-39:

Using the above formula, N

is set at 110 turns, which requires a 0.5 mm air-gap in the

EF20 core to give 1.2 mH inductance. To minimize the leakage inductance of the

transformer, and hence the loss in the snubber/clamp, the primary winding is split into

two layers of 55 turns each. The rms current in the primary is given by Equation 2-40.

EQUATION 2-40:

on DC sw

tV f

0.365

41.59

1.36 10

core

sw e

−

⎛⎞

⎜⎟

⎝⎠

where f

is in kHz and B is in mT.