Victor Tikhonov [mailto:[EMAIL PROTECTED]] wrote:
> This is exactly how expensive Brusa chargers are built. An I > suspect, its HF isolation transformer is the reason why it > cannot put quite as > many Amps in the pack at low AC input voltage (120VAC) as PFC can. I suspect the isolation transformer is not the limiting factor. The more likely limitation is the current handling capability of the input/PFC boost stage. A typical wide-range PFC topology (and I haven't been inside a Brusa to confirm that it is similar) is for the PFC input stage to boost the rectified AC line voltage to something like a 400VDC bus; this bus is then fed through an isolation transformer by a DC/DC stage that switches at high frequency. Regardless of the AC line voltage, the isolation transformer is always fed from about 400VDC, so it is not the limiting factor. However, for the same output power (or power at the 400VDC internal bus), the input stage must switch higher currents and run at higher duty cycles when the AC line voltage is low; this is the likely limitation. The isolation transformer limitation is that the output voltage range of the charger must be restricted to a narrower band since the voltage is determined by the duty cycle of the DC/DC stage, and this duty cycle cannot vary over a very wide range if good efficiency is to be achieved. Cheers, Roger.
