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Switching frequency

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I am designing a smps. one of the two options is a resonant converter and the other is a dual active bridge. In papers they have listed that since DAB opeartes at a constant frequency in a pwm manner it is advantageous i.e. better than resonant converter. How is this better?
 

dknguyen

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In general, constant frequency operation in converters is desirable simply because it is more predictable since you you know always know the operating frequency under all operating conditions and loads. This makes filtering easier and meeting EMI and noise requirements easier.
 

ronsimpson

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I have had good results with resonant power supplies. The frequency does change with line and load. There are less switching losses and less high frequency noise.
One problem with resonant converters is they often do not work well with very light loads. (Load Min to Load Max is smaller)
 

Pommie

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I have a cheap Chinese usb car adaptor. If I listen to a particualr fm radio station I can't charge my phone due to interference. I'm guessing that the load my phone adds changes the frequency to be some harmonic of the radio station.

Mike.
 

ronsimpson

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I'm guessing that the load my phone adds changes the frequency to be some harmonic of the radio station.
It is more complicated than that.
--Probably true that the PWM changes frequency. So right on that.
--Many off line PWM have a requirement to reduce power when there is no load. And to operate at very light loads. So they do "cycle skipping".
-----example: Below 10% the PWM might not work well so it will move to 20% and skip half of the cycles. At no load it might be 20% and skip 99/100 cycles.
--What you are hearing at 100mhz is not a harmonic of the PWM but more likely caused by transistor switching and diodes switching.
----You can make a radio transmitter from a power diode turning off and a LC. The C might be mostly internal to the diode and transformer. The L comes form leakage inductance in the transformer and the traces on the PCB. The power transmitted comes from the current in the diode when turned off. The frequency is set by the LC which will change with power levels.
--The power transistor (MOSFET) might be turning on or off in a time period that is the same as your radio receiver. (100mhz) So not the PWM frequency but the on or off time of the transistor.
 
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