AN2644 datasheet(9/64 Pages) STMICROELECTRONICS

9 / 64 page

AN2644

The LLC resonant half-bridge converter

9/64

and, in certain conditions, the initial imbalance of the V·s applied to the transformer at start

up (see "Converter's start-up" section). Additionally, it makes the input current to the

converter look like that of a full-bridge converter, as shown in Appendix E, with a resulting

reduction in both the input differential mode noise and the stress of the input capacitor.

Obviously, the currents through Q1 and Q2 will be unchanged. It is easy to recognize that

the two Cr/2 capacitors are dynamically in parallel, so that the total resonant tank's

capacitance is again Cr.

The system appears quite bulky, with its three magnetic components. However, the LLC

resonant topology lends itself well to magnetic integration. With this technique inductors and

transformers are combined into a single physical device to reduce component count, usually

with little or no penalty to the converter's characteristics, sometimes even enhancing its

operation. To understand how magnetic integration can be done, it is worth looking at the

well-known equivalent schematics of a real transformer in Figure 5 and comparing them to

the inductive component set of Figure 3.

Lp occupies the same place as the magnetizing inductance LM, Ls the same place as the

primary leakage inductance LL1. Then, assuming that we are going to use a ferrite core plus

bobbin assembly, Lp can be used as the magnetizing inductance of the transformer with the

addition of an air gap into the magnetic circuit and leakage inductance can be used to make

Ls.

Figure 5.

Equivalent schematic of a real transformer (left, tapped secondary; right,

single secondary)

To do so, however, a leaky magnetic structure is needed, which is contrary to the traditional

transformer design practice that aims at minimizing leakage inductance. The usual

concentric winding arrangement is not recommended here, although higher leakage

inductance values can be achieved by increasing the space between the windings.

Figure 6.

Example of high-leakage magnetic structures (cross-section)

i1(t)

n : 1

v1(t)

LL1

M(t)

ideal

LL2 i

2(t)

v2(t)

i1(t)

n : 1

v1(t)

LL1

M(t)

ideal

LL2 i

2(t)

v2(t)

i1(t)

n : 1 : 1

v1(t)

LL1

M(t)

ideal

LL2 i

2(t)

LL2

v2(t)

i1(t)

n : 1 : 1

v1(t)

LL1

M(t)

ideal

LL2 i

2(t)

LL2

v2(t)

Windings on separate legs of an EE core

Primary

winding

Secondary

winding

Primary

winding

Secondary

winding

Side-by-side windings (EE or pot core)

Windings on separate legs of an EE core

Primary

winding

Secondary

winding

Primary

winding

Secondary

winding

Side-by-side windings (EE or pot core)

AN2644 Datasheet(PDF) 9 Page - STMicroelectronics

AN2644 Datasheet(HTML) 9 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datasheet Download

Link URL