AC->DC power spike with bridge rectifier + capacitor

Thread Starter

mm3

Joined Oct 11, 2020
4
Please imagine this portion of a normal power supply where AC is converted into DC:
AC input -> bridge rectifier -> capacitor
The power is drawn from the input only during a small fraction of time when the capacitor is charging to the peak value.
Is this configuration used also for high current power supplies let's say 30A or is there a better way to draw power for a bigger portion of time avoiding small bursts of high power?
 

AlbertHall

Joined Jun 4, 2014
11,129
I used to repair 20V, 1000A DC power supplies and the circuitry on the secondary of the mains transformers (3 phase) was a capacitor, choke, capacitor pi filter. The transformer input was triac switched to regulate the output. Lord knows what the peak rectifier current was! There was about 1/3F total capacitance.
 

Thread Starter

mm3

Joined Oct 11, 2020
4
runsimpson: I believe power factor is different effect, it's about the phase between voltage and current.
Thanks AlbertHall, so I guess on one phase there is no way around these short bursts of power.
I was thinking may an inductor could help, but at low frequency it's not practical
 

AlbertHall

Joined Jun 4, 2014
11,129
That supply I referred to did have an inductor - a very large laminated iron cored one - as part of the pi filter but I suspect it was there to reduce the output ripple.
 

crutschow

Joined Mar 14, 2008
26,775
I was thinking may an inductor could help, but at low frequency it's not practical
That supply I referred to did have an inductor - a very large laminated iron cored one - as part of the pi filter but I suspect it was there to reduce the output ripple.
Large inductor can be, and have been used (as Albert noted) in high power rectifier power supplies.
They do indeed increase the conduction time of the rectifiers as well as reduce the output ripple.
There operation is similar to their use in buck regulators-- they store energy during the peak part of the AC waveform from the rectifier and continue to transfer it to the output capacitor, even when the diode voltage drops below the inductor output voltage.

Here is one discussion of the design of such a supply.
 

Thread Starter

mm3

Joined Oct 11, 2020
4
thanks crutschow for sharing that link, pity those Input Choke Power Supply and Resonant Choke Power Supply are not common nowadays as lower cost seems more important
 
As others have mentioned, a DC choke/inductor is an effective way to even out the AC current flow during steady-state operation of a power supply. However, it is worth noting that you'll want to include some means of suppressing transient voltage spikes on the output of your power supply as that choke will try to keep current flowing into your capacitor after the load has been removed.

The other consideration is that you're talking about a considerable amount of iron at line frequency. The simplicity yields excellent durability and reliability, but also presents cost issues along with poor load response times and the aforementioned voltage stability issues. Sags during increase in load, surges during decrease, voltage drop associated with the inductor's DC resistance, etc.

Here's a rough example of what sort of size choke you could be looking at in relation to the size of your power transformer - roughly 1/5 to 1/2 the mass seems to be typical:


Bigger choke, smoother output, poorer load response, increased weight & expense and vise-versa.
 

Thread Starter

mm3

Joined Oct 11, 2020
4
good point @Just Another Sparky indeed, so something in theory simple like AC->DC conversion can become complicated very easily. I guess an overvoltage protection circuit could be added at the end like zener+BJT but for high currents it's probably the case of an all or nothing mosfet controlled by a microcontroller
 
Depending on how large of a scale we're talking, what sort of a load you're going to be powering and how frequently it could be expected to see that sort of a spike, a properly sized MOV might even suffice. Otherwise you might be able to calculate the necessary voltage and charge ratings for your capacitor to allow it to safely absorb the maximum energy your inductor is capable of storing and then add a linear/switching regulator to stabilize the output.
 
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