AN4070 datasheet(17/53 Pages) STMICROELECTRONICS

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AN4070

DC-DC converter design

Doc ID 022934 Rev 1

17/53

The second major contribution to transformer losses is given by core losses. The value of

specific core loss, Plm, for an RM14 core operating at 35 kHz, 100 °C and with a maximum

flux density of 200 mT, is equal to 141 kW/m^3.

The total core loss is therefore given by the following equation:

Equation 32

Transformer temperature rise:

Using the value of thermal resistance reported in Table 6, the maximum temperature

rise for the HF transformer is given by:

Equation 33

DC-AC converter design

The basic scheme of the DC-AC converter topology is depicted in Figure 9. MOS5 and

MOS6 are connected in one leg and switch at a frequency of 17.4 kHz. Two Schottky diodes

are connected to the drain of these two MOSFETs in order to inhibit the internal body diode.

Two external silicon carbide (SiC) diodes are therefore connected in anti-parallel for current

freewheeling while avoiding problems connected to reverse recovery at MOSFET turn-on.

MOS7 and MOS 8 are connected in the second leg and switch at grid frequency, either 50

Hz or 60 Hz. An LCL filter is connected to the mid-point of each inverter leg and is used to

interface the system to the grid. On the DC side a bank of four electrolytic capacitors is used

to store and deliver energy whenever this is required during normal operation.

Figure 9.

Basic scheme of the DC-AC converter

14000

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