4 ms·

I think they are asking this: why not use the transistor to chop the 50/60 Hz AC into potentially-megahertz AC? Then pass that chopped AC to the transformer, wh

I think they are asking this: why not use the transistor to chop the 50/60 Hz AC into potentially-megahertz AC? Then pass that chopped AC to the transformer, which could still be small. The motivation would be to try to eliminate the full bridge rectifier and the high-voltage capacitors. I'm not an EE, but I would guess the simple answer is it's because you can't pass a AC through a transistor, since a transistor not only switches but also acts like a diode. I think you could work around that (build a circuit that switches AC with transistors), but there must be a reason why you wouldn't want to. Perhaps it's because it's simpler to just go ahead and rectify, or perhaps there's an advantage to having capacitors on the high-voltage side.

kens 5y ago
One problem is that when your 60 Hz AC goes to 0, your chopped AC will go to 0 too. You'll need to store energy somewhere to get across the milliseconds of no power. And your transistor, transformer, etc will need to be larger to handle the peaks.
dfox 5y ago
Circuit that tries to do this directly from AC is going to be more or less equivalent to active rectifier followed by traditional SMPS power stage (including some kind of DC bus with some nontrivial smoothing capacitor). In case when the whole thing is only supposed to work in one direction and input is low fixed frequency AC such construction is simply not worth it.
tedd4u 5y ago
Nowadays it’s not an issue as much since the advent of digital seat-back entertainment but often, listening to analog passenger aircraft audio loops you can hear the 400Hz come through. 400Hz is noticeably sharper than G but between G (392 Hz) and G# (415 Hz) on the standard musical scale (A = 440Hz). So the 400 Hz clashes with everything.
- bonzini 5y ago
  Is this a standard or does it only apply to one aircraft maker?
  - kens 5y ago
    400 Hertz is the standard for aviation; it allows smaller components than 50 or 60 Hertz. For military aviation, the spec is MIL-STD-704.
    - tedd4u 5y ago
      Hey Ken! Thanks. Yep that spec (dated 6 October 1959) says: 4.1.1 A-c power. The a-c power system shall be a 3-phase, 4-wire “Y” system, having a nominal voltage of 115/200 and a nominal frequency of 400 cps. The neutral point of the source of power is connected to ground (see 3.2), and the ground is considered the fourth conductor. and 5.1.5 Steady-state frequency. The a-c power systems frequency shall be maintained at 400 ± 20 cps for steady-state operation.
R0b0t1 5y ago
You can do this but it is more expensive. There are two stages: power factor correction, and the forward converter. The forward converter uses a transformer mainly for isolation. The power factor corrector exists because the transformer is an inductive load. If you have too much of an inductive or capacitive load on a distribution network you affect is performance. You can make the power factor corrector, which does the rectification, out of active components that switch in time with the AC signal. This eliminates the voltage drop of the diodes which can be a few percentage points of efficiency and a fair bit of heat. If you didn't need to isolate the power supply you could chop mains directly but you need to manage the radiated noise. The current designs try to maximize safety and then economy and then efficiency. Coincidentally, the power factor correction, done correctly, also helps reduce radiated noise by keeping the system synchronized with the incoming power. (This is not precisely accurate but is an interesting way to think of it.)

kens 5y ago

One problem is that when your 60 Hz AC goes to 0, your chopped AC will go to 0 too. You'll need to store energy somewhere to get across the milliseconds of no power. And your transistor, transformer, etc will need to be larger to handle the peaks.

dfox 5y ago

Circuit that tries to do this directly from AC is going to be more or less equivalent to active rectifier followed by traditional SMPS power stage (including some kind of DC bus with some nontrivial smoothing capacitor). In case when the whole thing is only supposed to work in one direction and input is low fixed frequency AC such construction is simply not worth it.

tedd4u 5y ago

Nowadays it’s not an issue as much since the advent of digital seat-back entertainment but often, listening to analog passenger aircraft audio loops you can hear the 400Hz come through. 400Hz is noticeably sharper than G but between G (392 Hz) and G# (415 Hz) on the standard musical scale (A = 440Hz). So the 400 Hz clashes with everything.

bonzini 5y ago
Is this a standard or does it only apply to one aircraft maker?
- kens 5y ago
  400 Hertz is the standard for aviation; it allows smaller components than 50 or 60 Hertz. For military aviation, the spec is MIL-STD-704.
  - tedd4u 5y ago
    Hey Ken! Thanks. Yep that spec (dated 6 October 1959) says: 4.1.1 A-c power. The a-c power system shall be a 3-phase, 4-wire “Y” system, having a nominal voltage of 115/200 and a nominal frequency of 400 cps. The neutral point of the source of power is connected to ground (see 3.2), and the ground is considered the fourth conductor. and 5.1.5 Steady-state frequency. The a-c power systems frequency shall be maintained at 400 ± 20 cps for steady-state operation.

R0b0t1 5y ago

You can do this but it is more expensive. There are two stages: power factor correction, and the forward converter. The forward converter uses a transformer mainly for isolation. The power factor corrector exists because the transformer is an inductive load. If you have too much of an inductive or capacitive load on a distribution network you affect is performance. You can make the power factor corrector, which does the rectification, out of active components that switch in time with the AC signal. This eliminates the voltage drop of the diodes which can be a few percentage points of efficiency and a fair bit of heat. If you didn't need to isolate the power supply you could chop mains directly but you need to manage the radiated noise. The current designs try to maximize safety and then economy and then efficiency. Coincidentally, the power factor correction, done correctly, also helps reduce radiated noise by keeping the system synchronized with the incoming power. (This is not precisely accurate but is an interesting way to think of it.)