SMART Group - Surface Mount and Related Technologies

Dc-dc converter replaces fan drive in fridges

Rohm BD9227F block

It has a PWM input, usually from a local MCU, which, rather than turning on and off the output, is internally smoothed to change the dc voltage at the output – output voltage is a fraction of the input voltage, with the fraction set by PWM duty cycle (within limits at low duty cycles, see below).

Called BD9227F, it switches at 1MHz to allow small capacitors and a small inductor to be used. Compared with Rohm’s discrete solution, footprint is down 75% and power conversion efficiency is up by 19% (at 300mA output). Efficiency is ~95% at 300-400mA 16-20V.

Rohm BD9227F discrete alternative“Until now, dc fan motor power supply blocks used in refrigerators and other equipment are primarily configured using discrete components, making it difficult to provide high accuracy control or carry out high frequency drive. As a result larger coils and output capacitors are required for the peripheral circuit, increasing mounting area considerably,” claimed the firm.

The internal switch is a 20V 200mΩ p-fet, so the converter can operate at 100% duty cycle so Vout=Vin.

Restrictions in minimum on-time for the converter mean that minimum output voltage is something like 25%Vin.

Under-voltage lock-out operates at an input of 5.3V, with some hysteresis.



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