AN3095 datasheet(9/55 Pages) STMICROELECTRONICS

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AN3095

DC-DC converter

Doc ID 16555 Rev 3

9/55

input of the HF transformer which varies between +Vin and -Vin, while the effect of the

modulation on the active rectifier is to generate, on the secondary of the HF transformer, a

square wave varying between +Vbus and -Vbus, where Vbus is the voltage on capacitor C2,

phase shifted with respect to the primary one of an angle

δ, equal to the phase shift of the

modulating signals, as shown in the equivalent circuit of figures 5, 6, 7, 8, 9, 10, and 11.

Figure 5.

DC-DC converter equivalent circuit

As a result, the primary voltage and the secondary transformer voltage reflected to the

primary determine the rising and falling slope of the current in the leakage inductance.

According to leakage inductance current waveforms, two operating modes may be

distinguished for the converter:

●

Discontinuous current mode DCM

●

Continuous current mode CCM

Both in CCM and DCM, three main operating modes or intervals may be distinguished in

half the switching period. Considering the modulation shown in Figure 9, in CCM the

leakage inductance current may be calculated as follows:

●

Mode 1, interval (t0 - t1):

At t0 M1 and M4 are turned on at ZVS, M6 is also on. The voltage across the leakage

inductance is:

Equation 1

and the current may be written as follows:

Equation 2

Since this current is negative, as shown in Figure 9, it flows in the circuit as demonstrated in

Figure 6.

bus

()(

)

(

)

(

bus

⋅

−

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