AN4070 datasheet(7/53 Pages) STMICROELECTRONICS

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AN4070

DC-DC converter design

Doc ID 022934 Rev 1

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DC-DC converter design

The topology chosen for the DC-DC section of the MIC is an isolated, interleaved boost

converter. The basic electrical scheme of such a converter is shown in Figure 3.

Figure 3.

DC-DC converter topology

It consists of an input capacitor C1, two inductors, L1 and L2, two ground referenced

MOSFETs, MOS1 and MOS2, a high frequency transformer, T1, two rectifier diodes, D1 and

D2, and three output capacitors, C2, C3 and C4. The two rectifier diodes, together with C2

and C3, form a voltage doubler circuit which is used to further boost the HF transformer

output voltage. The advantage of such a solution, compared to a standard one using four

rectifier diodes, is to halve the transformer turns ratio, with consequent realization of a

shorter secondary winding which, in turn, means better primary to secondary flux coupling

and lower secondary winding conduction losses. The two driving signals used to command

the two MOSFET gates are 180 degree phase shifted with respect to each other and always

have a duty cycle >0.5.

This introduces the problem of controlling the DC-DC operation at very low output power.

Two solutions can be used to overcome this problem. With the first solution the DC-DC

converter is controlled by keeping the duty cycle at the minimum allowable value and the

switching frequency is used as a control variable. The second solution uses a constant

switching frequency and enables burst mode operation. This second case is the preferred

method of control at low power. The implementation of burst mode is based on the definition

of a suitable hysteresis bandwidth for the output voltage which is used to turn on and off the

modulation of the DC-DC converter.

Figure 3 shows the theoretical gate driving signals used to control MOS1 and MOS2. The

180 degree phase shift produces two phase shifted inductor currents (Figure 4) which in

turn produce the beneficial effect of input and output current ripple cancellation. This allows

the use of small capacitor values to filter the high frequency content of input and output

currents.

AN4070 Datasheet(PDF) 7 Page - STMicroelectronics