[Amps] Buck-Boost Transformer Selection

[Manfred Mornhinweg]

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.

Manfred



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