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STEVAL-ISA147V3

2/10

DocID029092 Rev 1

1

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The DC-DC stage performs voltage step-down using an HF transformer with a primary to

secondary turn ratio chosen to maintain good efficiency and regulation over the entire

operating range. The transformer is supplied with a square wave voltage generated by the

primary side active switches. On the secondary side, this voltage waveform is rectified and

then smoothed by the output filter; on the primary side, switching losses are reduced

thanks to zero voltage switching (ZVS). On the secondary side, synchronous rectification

(SR) is used to ensure low conduction losses. The overall effect of these design choices is

high system efficiency, in line with the stringent requirements of the power supply industry.

The system is controlled by two microcontrollers from the STM32 product family. On the

primary side, an STM32F051K8 controls the bridgeless PFC by sampling the current of the

two MOSFETs, the input AC voltage and the output bus voltage. Two control signals,

PWM1 and PWM2, are then generated to drive two power switches. On the secondary

side, an STM32F334C8 microcontroller samples the output voltage and adjusts the

frequency of the LLC bridge control signals to ensure stable operation throughout the

overall load range. In addition, two ADC channels are used to sample the rising and falling

edges of the drain-to-source voltage of the SR MOSFETs. The two microcontrollers

exchange information about the status of the input and output power stage via bidirectional

serial communication.

Both the power stage and control stage are supplied by an offline flyback circuit which

provides a suitable, regulated voltage to the microcontrollers, the gate drive ICs and signal

conditioning circuits.