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Re: [Amps] Buck-Boost Transformer Selection

Subject: Re: [Amps] Buck-Boost Transformer Selection
From: Manfred Mornhinweg <>
Date: Thu, 26 Jun 2014 16:43:44 +0000
List-post: <">>
Jim, Dick,

For example, let's say your actual line voltage is 250V and the amplifier
draws 14 Amps, which correspnds to a power rating for the amplifier of
3.5KVA. (That, by the way, is the peak power. The average power is probably
about half that.)

I have a different interpretation, even if I don't know the details of the Quadra. What power level is it? 1kW, or 1.5kW, or what? Assuming it's 1kW output, the power consumption from the line should be around 2kW, after combining the efficiency of the amplifier itself, with that of a switching power supply. If the power supply had a unity power factor, at 240V that would be 8.33A. The 14A rating most likely indicates that the Quadra does not have input power factor correction, and that the power factor at full load is around 0.6, which is typical for uncorrected switching power supplies.

By the way, the power factor of conventional transformer/rectifier/capacitor power supplies is worse than that, specially if they use big capacitors to achieve low ripple!

The actual peak current, drawn from the line during the current peaks, while the voltage waveform is nearing the peak value, is likely higher than 14A, so that the peak power during those parts of the waveform is well above 3.5kW.

In a "perfect" system, where the line voltage is a pure sine wave, and the load is a pure resistor, the peak power is already exactly twice the average power!

I agree with Jim that a simple filament transformer is enough to buck the line voltage a bit. And also I agree with others that one should complain to the power company if the line voltage is out of spec, and should run equipment directly off the line - it's made for it!

Switching power supplies typically use 1000V rectifier diodes, 400V filter capacitors, and MOSFETs rated for 400V. So the diodes are no problem, and the limit is given by a 400V bus voltage, which would allow an absolute maximum RMS line voltage of 283V, assuming a clean sine wave shape. Normally the waveform on a power line is distorted, with a strong third harmonic, that tends to reduce the peak-to-RMS ratio. For this reason, the limiting voltage at which things can go bang is usually even higher than 283V RMS.

On the other hand, of course, it's good to leave some safety margin, and stay comfortably below the absolute limit.


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