Chokes before DC DC converters

Thread Starter

Rscott9399

Joined Jan 13, 2017
51
I have a question on some DC DC converters

For example


I see in most applications a Choke is recommended on the line before hand.
Can anyone give me a quick reason as to why that is?
The source is DC soooo..??

Thanks
 
Last edited:

SamR

Joined Mar 19, 2019
4,301
It is a switch-mode converter and the capacitors are to smooth any ripple induced by the switching. Yes, a buck-converter is a DC-DC stepdown converter done by PWM switching.
 

Thread Starter

Rscott9399

Joined Jan 13, 2017
51
Capacitors? Yes i underderstand what a decoupling cap is my question is
why put a choke on the input side of a dc dc converter? Its DC in and DC out. There is no AC
 

Papabravo

Joined Feb 24, 2006
18,806
You only think there isn't. Chokes are bi-directional when it comes to suppressing HF noise. It is important that noise from the converter switching not make it back to the source.
 

dendad

Joined Feb 20, 2016
4,168
Read up on how the switch mode power supplies work.
http://www.learnabout-electronics.org/PSU/psu30.php
They often switch at quite high frequencies 1Mhz or more, and can cause significant radio interference. In fact, here in Oz, those cheap cigarette lighter DC-DC converters are banned in one fire brigade as they are disrupting the radios in the trucks. Those cheap supplies do not include RFI suppression.
 

SteveSh

Joined Nov 5, 2019
108
Like others have said, if you have switch mode power supply (SMPS), the input choke is recommended to prevent the high frequency noise from the switcher from making it back onto the input lines. To know the details of what that choke needs to be, a detailed analysis of your PS design needs to be done to know what the noise levels going back out onto the primary power are; what your conducted emissions (CE) requirement is; and so what level of attenuation at what frequencies is needed.
 

dendad

Joined Feb 20, 2016
4,168
it IS AC!
The circuit is switching on and off at a high frequency, producing an AC signal. If you were to add a wire to the switch circuit, it could make a crude radio transmitter! (not recommended)
Why do you not think there is any AC involved? Did you read any theory of how switch mode supplies work?
 

SteveSh

Joined Nov 5, 2019
108
So this is to protect the higher DC source then? Interesting

So why a choke? Its not AC, so why not a cap on the input
Well, "protect the higher DC source" is probably not the way I would describe it. The main purpose of the choke is to protect anything else that may be connected to that source. A DC source, such as a battery, is most likely not going to be bothered by KHz or MHz of noise down in the fraction of a volt region. But some other device being powered by that same DC source could be.

A simple cap across the input does not do anything for common mode noise - that is, noise that is on both the supply line and its return. A cap or multiple caps, may be part of the overall input filter design, but a single cap across the inputs will not do it.
 

Papabravo

Joined Feb 24, 2006
18,806
It goes up
Yes, the higher the frequency the higher the impedance. So an inductor naturally passes low frequencies, including DC, and it attenuates high frequencies, including fast rising and falling edges on a PWM signal. That is the motivation for using them on input and output.
 
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