Electronics > Beginners
Switching frequencies and noise in DC-DC converters
analogo:
What produces less noise (ripple) in a switching DC-DC converter: higher or lower frequencies?
The switching frequency of DC-DC converter affects its efficiency and the size of the filtering components: the higher the frequency the lower the efficiency, the higher the frequency the smaller the external filtering components. https://www.sunpower-uk.com/glossary/what-is-switching-frequency/
But what about the noise?
Naively I would say that higher frequency converters produce the biggest ripple, but I am not so sure.
Is there a thorough but understandable guide (app note) to the tradeoffs involved in choosing a switching frequency?
Ice-Tea:
Higher frequency wih same components = lower ripple. At least with a regular buck.
In simple terms: it's essentialy a PMW with a filter after it. You charge coil and cap during the 'on' part and you drain it while the switch is 'off'. The lower the frequency, the longer you are discharging coil and cap and the more the voltage will drop. At higher frequency your energy buffer get charged sooner or you can get away with less energy storage/lower value components/smaller components.
T3sl4co1l:
Higher frequencies are more prone to radiating, but are also easier to filter (requires smaller filter components). :-//
Tim
ocset:
--- Quote ---What produces less noise (ripple) in a switching DC-DC converter: higher or lower frequencies?
--- End quote ---
Probably the best way to kill noise in SMPS is to damp the switching transition....ie make the dv/dt and di/dt of the fet switch on and switch off slower......you do this by eg adding series fet gate resistance....but this does increase switchiong losses.
Once i did a 150w led driver with 1r in series with fet gate.........when turned on , the lab radio turned off.......we increased gate resistor to 4r7 and problem was solved.
MaxSimmonds:
As Treez mentioned, this is very true. You can also have different turn on/turn off times by using a series gate resistor, and a parallel diode + series resistor. Like this:
+----D-----R---+
_______ | |
| Driver |------------R--------+--------- MOSFET GATE
----------
The diode would be reversed biased during turn on, and forward biased during turn off. I usually add this is as a precaution when laying out the pcb, incase the driver discharges the gate too fast for my likely and I want to dampen the resultant ringing with a different value resistor. It would be interesting to know if anyone thinks this is over kill/not needed?
Either way, like Treez said, it's a fine line between high di/dt (causing voltage spikes and ringing) and a series resistor to dampen the ringing, which then increases the turn on/off times of the FETs (by an R*C time constant, where C is the gate capacitance) and hence increased switching losses.
Usually a value between 1-5 ohms is good enough, it's best to leave space for a resistor on the PCB (if building one), you can always use a 0 ohm resistor if you see that you don't need it - but at frequencies of around 100-150k, I think they're going to be needed.
I'm still very new myself with switch mode power supplies, I'm fresh out of uni, but that's my experience with them!
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