6 ms·

Those properties are all positive, but there is one glaring negative: rf radiation. I know some ham radio station builders that will not allow any switched mode

by wglb 5y ago

Those properties are all positive, but there is one glaring negative: rf radiation. I know some ham radio station builders that will not allow any switched mode power supply on their property, including wall warts and smartphone chargers.

Animats 5y ago
That's a problem, especially with low-end power supplies. I designed a switching power supply a few years ago for a specialist application - driving 1930s Teletype machines which need 120VDC 60mA.[1] A switching power supply is a spike generator. Here's the schematic.[2] This is reasonably RF-quiet. The transformer of the switching power supply is a toroid in a metal can, so you don't get too much RF from the transformer itself. But that's not where it usually comes from. The important thing is to keep spikes out of both the input and output wiring. That's dealt with by using LTSpice to simulate the circuit, and adding small capacitors and inductors until the spikes have disappeared in both voltage and current. At the output end, note the snubber C7, R1, D10, and D11, to soak up any spikes from the inductive load, and L2, to soak up output side current spikes. There's L1 and C12, to soak up kickback from the switched input side of the power supply. Plus C10 at the power input, which is from a USB port. They're all tiny surface mount components. The inductors are ferrite beads in surface mount form. So there are eight extra components, just to prevent unwanted RF generation. The LTSpice simulation shows that they're all needed. The simulation was used to choose the values. This is why good switching power supplies have more parts than bad ones. You see that in teardown videos. [1] https://github.com/John-Nagle/ttyloopdriver https://github.com/John-Nagle/ttyloopdriver [2] https://github.com/John-Nagle/ttyloopdriver/blob/master/board/images/schematic.png https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...
- wglb 5y ago
  > This is reasonably RF-quiet I wonder if you have actual measurements? >to prevent unwanted RF generation Perhaps actually "to reduce unwanted RF generation"

by wglb 5y ago

Animats 5y ago

That's a problem, especially with low-end power supplies. I designed a switching power supply a few years ago for a specialist application - driving 1930s Teletype machines which need 120VDC 60mA.[1] A switching power supply is a spike generator. Here's the schematic.[2] This is reasonably RF-quiet. The transformer of the switching power supply is a toroid in a metal can, so you don't get too much RF from the transformer itself. But that's not where it usually comes from. The important thing is to keep spikes out of both the input and output wiring. That's dealt with by using LTSpice to simulate the circuit, and adding small capacitors and inductors until the spikes have disappeared in both voltage and current. At the output end, note the snubber C7, R1, D10, and D11, to soak up any spikes from the inductive load, and L2, to soak up output side current spikes. There's L1 and C12, to soak up kickback from the switched input side of the power supply. Plus C10 at the power input, which is from a USB port. They're all tiny surface mount components. The inductors are ferrite beads in surface mount form. So there are eight extra components, just to prevent unwanted RF generation. The LTSpice simulation shows that they're all needed. The simulation was used to choose the values. This is why good switching power supplies have more parts than bad ones. You see that in teardown videos. [1] https://github.com/John-Nagle/ttyloopdriver https://github.com/John-Nagle/ttyloopdriver [2] https://github.com/John-Nagle/ttyloopdriver/blob/master/board/images/schematic.png https://github.com/John-Nagle/ttyloopdriver/blob/master/boar...

wglb 5y ago
> This is reasonably RF-quiet I wonder if you have actual measurements? >to prevent unwanted RF generation Perhaps actually "to reduce unwanted RF generation"