4 ms·

Not totally on topic, but can someone explain why switched mode power supplies don't seem to have any type of inrush current limiting? Often when I turn on pow

Not totally on topic, but can someone explain why switched mode power supplies don't seem to have any type of inrush current limiting? Often when I turn on power right after a power loss, the circuit breaker immediately trips. I assume it's because all those power supplies have such a big inrush current that they overload the circuit. All these devices use less than an amp in total on average, but the inrush current is enough to immediately trip a 13A circuit breaker.

fake-name 5y ago
Because that's another part that adds to the BOM cost and assembly cost. SMPSes all have input stages that effectively directly rectify the incoming mains to provide the SMPS internal bus voltage. This is also a great way to basically design an inrush current simulator (diode(s) + big capacitor). Adding stuff like active PFC correction in your common wall-wart is a non-starter. Those things are already massively cost-optimized.
namibj 5y ago
No they have enough that they don't vote a breaker when you plug them in the first time. Also toroidal cores are worse than modern active PFC SMPSs. Also note that there are different breaker classes for different loads, with the more-forgiving ones having tighter demands on the insurance of the downstream wiring, because they have to still reliably trip instantly from a short.
Ballas 5y ago
They should have an NTC for inrush limiting.
- newaccount74 5y ago
  How long do those NTCs take to reset? Maybe that explains why the breaker trips after I turn it back on after a power failure -- if those NTCs are still warm, they would not limit the inrush current.
  - formerly_proven 5y ago
    Modern high efficiency PSUs should have no problem with this, because their active PFC limits input current. NTCs also reduce efficiency and don't work when toggling power, unless you add a relay to short it out. They're a hack.
    - asddubs 5y ago
      if you're gonna short it out, you can just use a resistor though
      - formerly_proven 5y ago
        You need to use an NTC that'll stay within power limit under load to avoid a fire hazard when the relay doesn't work; if you try to do the same with a normal resistor you need large and expensive power resistors.
  - deleted 5y ago
    [deleted]
zeroflow 5y ago
Regarding the breaker side, there are different characteristics available. For example, I've switched the breaker of my workspace from B13 to C13, which both trip at above 13A (plus some) continuous, but C13 has a higher limit for short pulsed loads. If you take a look here, you can see that a "B" breaker accepts 1 second of 5x load, while C allows for 1 second of 10x load, so up to 130A surge and more for shorter periods. https://www.klocknermoeller.com/faz/faz.characteristics.htm https://www.klocknermoeller.com/faz/faz.characteristics.htm
xxs 5y ago
> Not totally on topic, but can someone explain why switched mode power supplies don't seem to have any type of inrush current limiting? They absolutely do - NTC, and they even have a relay to bypass the NTC after the PSU has booted up. If you hear a click after booting up, there is a bypass relay. >Often when I turn on power right after a power loss, the circuit breaker immediately trips. The circuit breakers (domestic) tend to be B or C class, B triggers much easier than C, the latter tend to be used for motors, stuff with high inrush current. If you believer you have too much inrush current, you might wish to replace the breaker. Also, the PSU datasheets specify how many of then can be connected in parallel to the same breaker. Last but not least - if you have too much inrush current, it's likely there is no power factor correction either, so the high value caps are connected straight after the bridge rectifier.

fake-name 5y ago

Because that's another part that adds to the BOM cost and assembly cost. SMPSes all have input stages that effectively directly rectify the incoming mains to provide the SMPS internal bus voltage. This is also a great way to basically design an inrush current simulator (diode(s) + big capacitor). Adding stuff like active PFC correction in your common wall-wart is a non-starter. Those things are already massively cost-optimized.

namibj 5y ago

No they have enough that they don't vote a breaker when you plug them in the first time. Also toroidal cores are worse than modern active PFC SMPSs. Also note that there are different breaker classes for different loads, with the more-forgiving ones having tighter demands on the insurance of the downstream wiring, because they have to still reliably trip instantly from a short.

Ballas 5y ago

They should have an NTC for inrush limiting.

newaccount74 5y ago
How long do those NTCs take to reset? Maybe that explains why the breaker trips after I turn it back on after a power failure -- if those NTCs are still warm, they would not limit the inrush current.
- formerly_proven 5y ago
  Modern high efficiency PSUs should have no problem with this, because their active PFC limits input current. NTCs also reduce efficiency and don't work when toggling power, unless you add a relay to short it out. They're a hack.
  - asddubs 5y ago
    if you're gonna short it out, you can just use a resistor though
    - formerly_proven 5y ago
      You need to use an NTC that'll stay within power limit under load to avoid a fire hazard when the relay doesn't work; if you try to do the same with a normal resistor you need large and expensive power resistors.
- deleted 5y ago
  [deleted]

zeroflow 5y ago

Regarding the breaker side, there are different characteristics available. For example, I've switched the breaker of my workspace from B13 to C13, which both trip at above 13A (plus some) continuous, but C13 has a higher limit for short pulsed loads. If you take a look here, you can see that a "B" breaker accepts 1 second of 5x load, while C allows for 1 second of 10x load, so up to 130A surge and more for shorter periods. https://www.klocknermoeller.com/faz/faz.characteristics.htm https://www.klocknermoeller.com/faz/faz.characteristics.htm

xxs 5y ago

> Not totally on topic, but can someone explain why switched mode power supplies don't seem to have any type of inrush current limiting? They absolutely do - NTC, and they even have a relay to bypass the NTC after the PSU has booted up. If you hear a click after booting up, there is a bypass relay. >Often when I turn on power right after a power loss, the circuit breaker immediately trips. The circuit breakers (domestic) tend to be B or C class, B triggers much easier than C, the latter tend to be used for motors, stuff with high inrush current. If you believer you have too much inrush current, you might wish to replace the breaker. Also, the PSU datasheets specify how many of then can be connected in parallel to the same breaker. Last but not least - if you have too much inrush current, it's likely there is no power factor correction either, so the high value caps are connected straight after the bridge rectifier.