STCMB1 datasheet(32/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

32/59

DocID028958 Rev 4

(Figure 16, right-hand side), it is

 = 0, the operating frequency reaches the critical value

fswcm, the switching period is Tswcm = 1/fswcm and TSH = Tswcm / 2.

Therefore, in a properly designed converter the range of TSH is included between Tswmin / 4

and Tswmax / 2, with Tswmax < Tswcm.

Conceptually, with TSC an inner loop is closed, and the outer loop that regulates the output

voltage provides the reference TSH for the inner loop. This is schematically illustrated in

Figure 17.

The feedback variable in the inner loop is the tank current iR, whose zero crossings are used

as the starting point for TSH, while the input quantity to the tank circuit is the applied square

wave voltage vHB. From the control point of view, converter's input voltage Vin can be seen

as a disturbance that affects the inverter block (the half-bridge leg that generates the square

wave voltage applied to the resonant tank).

Figure 17. Dual-loop structure of a resonant converter with TSC

Since the tank circuit responds primarily to the first harmonic of the applied square wave

voltage vHB and negligibly to the higher order harmonics, the inner loop can be analyzed

using the describing function approach. The result of the stability analysis is that the loop is

absolutely stable as long as the control variable is included in the range:

(Tswmin / 4, Tswcm / 2).

The open-loop dynamic behavior of an LLC converter with TSC has been found to be that of

a low Q second-order system, that is, a system with two real, well separated poles.

Therefore, it is possible to use dominant pole approximation at frequencies below crossover

and, from the practical point of view, treat it as a first-order system.

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