Reference design general description

STEVAL-ISA007V1

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Reference design general description

A dedicated reference design with ESBT has been developed, featuring the high power wide

input range requirement of a highly demanding industrial market.

The maximum output power is 150W. Generally speaking, the necessity to handle both high

output power and wide input voltage leads to the design of a fly-back stage working in mixed

operation mode: continuous and discontinuous:

The continuous current mode introduces a right half plan zero in the loop-transfer function

which makes the feedback stabilization difficult. At the input voltage range, and after a brief

description of the differences between continuous and discontinuous mode, it will soon be

clear that it is very difficult and inconvenient to design a fly-back converter working in

discontinuous mode. Finally, referring to the power spec of our demo, it is clear that

discontinuous mode cannot be used because it would determine a very high primary and

secondary peak current with a higher cost of all the main components involved: power

transistor, secondary diode and output capacitor.

Thanks to the adoption of the PWM L5991, the converter is able to operate in mixed mode.

For higher power levels with a switching frequency close to 90kHz, the converter operates in

Continuous Current Mode (CCM), while in stand-by and for power levels lower than 25W,

the converter works in Discontinuous Current Mode (DCM) at a switching frequency of

about 35kHz.