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Can you (or someone else) write more about the advantages of DC/DC voltage conversion? If I understand correctly this was the main advantage of AC, and the reas

Can you (or someone else) write more about the advantages of DC/DC voltage conversion? If I understand correctly this was the main advantage of AC, and the reason why was it chosen.

londons_explore 3y ago
In the olden days, AC allowed easy voltage conversion with a transformer. The transformer converts the electricity into magnetism in a steel core, and then back into electricity at a different voltage. This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields... Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter. Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency. However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.
- Kirby64 3y ago
  Citation needed on DC/DC converters being as efficient as AC transformers. I'd agree they can cost less, but AC transformers are wildly efficient. 99% efficiency is not unheard of. Full load efficiency can be extremely high too. There are basically no DC/DC converters that hit that efficiency at any load.
  - mjevans 3y ago
    Give the benefit of doubt and read the surrounding context more carefully. Not only did they mention (rephrasing) for unlimited money / resources AC / AC transformers could be far more efficient, but the part you are critiquing is simultaneously comparing COST for SAME efficiency parts (I assume typical) and resulting efficiency for similar COST.
  - namibj 3y ago
    AC transformers have fixed thermal power density, but load power scales with mass to the 4/3rd power (i.e., scales with linear size by the 4th power, while volume and mass and loss only scale to the 3rd power). And no, 99% isn't hard for a resonant switched capacitor converter. They just happen to be restricted to integer voltage ratios. (With sometimes a few percent regulation around this ratio without substantial efficiency loss.)
velosol 3y ago
Changing properties of electricity (ie current to voltage or vice versa) requires a trip through the magnetic field and that means varying the current through a conductor. With AC that's easy as it's already changing and so you can trivially (passively) convert between voltages at the same frequency with the efficiency cost of heating the conductors of the coils used in the transformer. With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another. I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).
_trampeltier 3y ago
- AC motor are very easy to build and are very, very durable. - AC / AC voltage transformation is very easy and very effective. - Switching AC is much more easy than DC. - The grid we have today was not made for small small cellphone chargers. It was made for light and motors.
cogman10 3y ago
One big advantage is that you don't have a wave you have to sync with (Which is why DC is used for connection between grids). One of the harder parts of AC grid management is that every generator on the grid has a timing component to make sure it's producing power in the same waveform as the grid. It's not enough to produce AC at 60hz, if that 60hz is misaligned then you generator turns into a load on the grid. It also means that if 2 grids aren't in sync, they can't connect (even if they are both 60hz) without some expensive equipment to sync the waveform. A 60hz grid cannot connect with a 50hz grid without a DC phase.

Can you (or someone else) write more about the advantages of DC/DC voltage conversion? If I understand correctly this was the main advantage of AC, and the reason why was it chosen.

londons_explore 3y ago

In the olden days, AC allowed easy voltage conversion with a transformer. The transformer converts the electricity into magnetism in a steel core, and then back into electricity at a different voltage. This process is quite efficient but requires a lot of steel, since in a 60 Hz AC system, 1/120th of a second of the energy being converted has to be stored as a magnetic field in steel - and steel isn't a particularly good 'store' of magnetic fields... Modern DC/DC systems actually have similarities! But instead of operating at 60Hz, they tend to operate at more like 1,000,000 Hz. That means far less copper and steel is needed. Unfortunately, 1,000,000 Hz power has a habit of leaking out of cables and becoming radio waves, so we can't send it long distances like that - so we convert it to DC before and afterwards. The conversion to DC is done with electronic switches switched at 1 Mhz or more - usually MOSFETS are used, and one promising but expensive type is a GaN MOSFET. It turns out that the DC->AC, transformer, and AC->DC setup can also be combined and simplified a bit, and we call the result a buck/boost converter. Overall, buck/boost converters can normally convert DC voltages for less money and at higher efficiencies than their AC transformer counterparts - mostly due to the higher operating frequency allowing use of far less steel and copper, and allowing other engineering tradeoffs be made in the direction of efficiency. However, neither DC/DC nor transformers have any theoretical cap on efficiency - and with an unlimited budget, you could make either with an almost arbitrarily high efficiency.

Kirby64 3y ago
Citation needed on DC/DC converters being as efficient as AC transformers. I'd agree they can cost less, but AC transformers are wildly efficient. 99% efficiency is not unheard of. Full load efficiency can be extremely high too. There are basically no DC/DC converters that hit that efficiency at any load.
- mjevans 3y ago
  Give the benefit of doubt and read the surrounding context more carefully. Not only did they mention (rephrasing) for unlimited money / resources AC / AC transformers could be far more efficient, but the part you are critiquing is simultaneously comparing COST for SAME efficiency parts (I assume typical) and resulting efficiency for similar COST.
- namibj 3y ago
  AC transformers have fixed thermal power density, but load power scales with mass to the 4/3rd power (i.e., scales with linear size by the 4th power, while volume and mass and loss only scale to the 3rd power). And no, 99% isn't hard for a resonant switched capacitor converter. They just happen to be restricted to integer voltage ratios. (With sometimes a few percent regulation around this ratio without substantial efficiency loss.)

velosol 3y ago

Changing properties of electricity (ie current to voltage or vice versa) requires a trip through the magnetic field and that means varying the current through a conductor. With AC that's easy as it's already changing and so you can trivially (passively) convert between voltages at the same frequency with the efficiency cost of heating the conductors of the coils used in the transformer. With DC it's harder because you don't have the time changing nature necessary for the magnetic field so you have to turn the DC on and off which requires a switch. Nowadays we have very fast switches (transistors) that allow us to tune a circuit to the power required and temporary energy storage (capacitors and inductors) available. Ignoring (or shielding) the RF interference that's created with fast switching we have systems that can efficiently convert between one DC voltage and another. I'm not so sure we'll have DC to the home for supply, a zero-crossing is helpful to keep circuit breakers small and reduce damage in brief, accidental contact (eg broken insulation on a lamp etc).

_trampeltier 3y ago

- AC motor are very easy to build and are very, very durable. - AC / AC voltage transformation is very easy and very effective. - Switching AC is much more easy than DC. - The grid we have today was not made for small small cellphone chargers. It was made for light and motors.

cogman10 3y ago

One big advantage is that you don't have a wave you have to sync with (Which is why DC is used for connection between grids). One of the harder parts of AC grid management is that every generator on the grid has a timing component to make sure it's producing power in the same waveform as the grid. It's not enough to produce AC at 60hz, if that 60hz is misaligned then you generator turns into a load on the grid. It also means that if 2 grids aren't in sync, they can't connect (even if they are both 60hz) without some expensive equipment to sync the waveform. A 60hz grid cannot connect with a 50hz grid without a DC phase.