4 ms·

Power engineer here. Something that is not apparent is that many small power supplies are limited in power handling not by the components, but by maximum safe t

Power engineer here. Something that is not apparent is that many small power supplies are limited in power handling not by the components, but by maximum safe touch temperatures for meeting regulations (probably around the 10W level). Therefore making a more efficient power supply means that you can package it smaller since the heat needs lower surface area to dissipate safely. Right now, gallium nitride MOSFETs are gaining popularity allowing for higher efficiencies in power conversion, and some wildly high power densities for USB chargers. I'm a fan of ravpower's 30W USBc supply which is about one cubic inch with folding prongs.

londons_explore 6y ago
For context, the "safe touch temperature" in some countries is 50 celsius, in a place where air temperatures can reach 40C. That means the charger can't rise more than 10C above ambient temperature, which is tricky to achieve without a cooling fan.
baybal2 6y ago
Yep, you are completely right. It is a triangle of size, efficiency and cost. Simplest flybacks, and half bridges can't compare to topologies with extra magnetics on efficiency, but you need to find space for extra xformers. The appeal of high end 3-5 switches is that you can get high efficiencies even in simplest topologies, and employ no extra magnetics. My engineers too hit icebergs few times when we tried making power supplies "the smart way." PZT xformers are smaller than iron, but their output may not be completely sinusoidal. Fancy vitroperm, finemet, and other similar magnetics can have rather low tolerances for magnetic properties, and some even drift over time as their metallurgy changes at relatively low temperatures. GaN switches can be easily damaged without carefully made gate driver. Capacitive isolation is not foolproof, and substandard safety caps are everywhere. SiC switches are said to be as reliable as galvanic iso, are not so reliable in practice.
- kens 6y ago
  > It is a triangle of size, efficiency and cost. It's interesting to consider that the reason there are tradeoffs is because uniformly bad technologies get discarded, so the remaining choices have tradeoffs. For chargers, consider the 1990s-style line transformer and linear regulator (big wall warts). These are larger, less efficient, and cost more than a modern switching power supply, so essentially nobody uses them. So it may seem like you're stuck in a triangle of choices that are bad in some way, but in the historical picture, these choices are all very good.
  - milesvp 6y ago
    Yeah, you’re sort of describing engineering in a nutshell. It’s all about tradeoffs, and there’s so much min-maxing going on. Then eventually you discover some property or design where the tradeoffs aren’t as bad while still mostly maxing the desired property and it becomes a common solution. What’s interesting, is that periodically you can go back to what looked like a bad solution, and because of advancements in related designs, this solution can actually end up being superior in some ways.
solarkraft 6y ago
Yet there seem to be no designs with any ventilation. Why is that? Are heat sinks not allowed because the case is (usually) not grounded?
- lloeki 6y ago
  Useful ventilation means holes through which someone could insert a thin metallic part (wire, needle...) Never underestimate the ingenuity of fools. The safest design is one without holes.
  - dogma1138 6y ago
    You don’t even need people inserting anything. Dust is a problem on its own it can ignite from ESD and the charing causes carbon built ups that eventually cause a short. Even when it doesn’t ignite from ESD it can change the capacitance of circuits which can cause a flash. This is compounded even more by the fact that many dust particles in cities can be conductive carbon particles form combustion, platinum and other metals form catalytic converters and fine metal particles from things like car brakes, roads and wheels are all over the place. A lot of that black/dark grey dust that can settle around windows and other leaky parts of your house in a city isn’t dead skin and dirt.
- occamrazor 6y ago
  The general rule is that on the mains side one needs either grounding and single insulation, or no grounding and double insulation. In Europe (except UK and Ireland) there us the Europlug standard plug with two prongs, that works everywhere, but no single standard three-pronged plug with grounding. For economy if scale and simplicity of logistics, double insulation is the best option, but as you note it makes a heatsink less practical.
deleted 6y ago
[deleted]

londons_explore 6y ago

For context, the "safe touch temperature" in some countries is 50 celsius, in a place where air temperatures can reach 40C. That means the charger can't rise more than 10C above ambient temperature, which is tricky to achieve without a cooling fan.

baybal2 6y ago

Yep, you are completely right. It is a triangle of size, efficiency and cost. Simplest flybacks, and half bridges can't compare to topologies with extra magnetics on efficiency, but you need to find space for extra xformers. The appeal of high end 3-5 switches is that you can get high efficiencies even in simplest topologies, and employ no extra magnetics. My engineers too hit icebergs few times when we tried making power supplies "the smart way." PZT xformers are smaller than iron, but their output may not be completely sinusoidal. Fancy vitroperm, finemet, and other similar magnetics can have rather low tolerances for magnetic properties, and some even drift over time as their metallurgy changes at relatively low temperatures. GaN switches can be easily damaged without carefully made gate driver. Capacitive isolation is not foolproof, and substandard safety caps are everywhere. SiC switches are said to be as reliable as galvanic iso, are not so reliable in practice.

kens 6y ago
> It is a triangle of size, efficiency and cost. It's interesting to consider that the reason there are tradeoffs is because uniformly bad technologies get discarded, so the remaining choices have tradeoffs. For chargers, consider the 1990s-style line transformer and linear regulator (big wall warts). These are larger, less efficient, and cost more than a modern switching power supply, so essentially nobody uses them. So it may seem like you're stuck in a triangle of choices that are bad in some way, but in the historical picture, these choices are all very good.
- milesvp 6y ago
  Yeah, you’re sort of describing engineering in a nutshell. It’s all about tradeoffs, and there’s so much min-maxing going on. Then eventually you discover some property or design where the tradeoffs aren’t as bad while still mostly maxing the desired property and it becomes a common solution. What’s interesting, is that periodically you can go back to what looked like a bad solution, and because of advancements in related designs, this solution can actually end up being superior in some ways.

solarkraft 6y ago

Yet there seem to be no designs with any ventilation. Why is that? Are heat sinks not allowed because the case is (usually) not grounded?

lloeki 6y ago
Useful ventilation means holes through which someone could insert a thin metallic part (wire, needle...) Never underestimate the ingenuity of fools. The safest design is one without holes.
- dogma1138 6y ago
  You don’t even need people inserting anything. Dust is a problem on its own it can ignite from ESD and the charing causes carbon built ups that eventually cause a short. Even when it doesn’t ignite from ESD it can change the capacitance of circuits which can cause a flash. This is compounded even more by the fact that many dust particles in cities can be conductive carbon particles form combustion, platinum and other metals form catalytic converters and fine metal particles from things like car brakes, roads and wheels are all over the place. A lot of that black/dark grey dust that can settle around windows and other leaky parts of your house in a city isn’t dead skin and dirt.
occamrazor 6y ago
The general rule is that on the mains side one needs either grounding and single insulation, or no grounding and double insulation. In Europe (except UK and Ireland) there us the Europlug standard plug with two prongs, that works everywhere, but no single standard three-pronged plug with grounding. For economy if scale and simplicity of logistics, double insulation is the best option, but as you note it makes a heatsink less practical.

deleted 6y ago

[deleted]