[time-nuts] Low noise power supplies - dont' use Electrolytics

Jose Camara camaraq1 at quantacorp.com
Mon Nov 28 19:42:35 EST 2011

A switcher at 2.2MHz does not have single frequency spikes in its power
spectrum, but a rather wideband distribution. It is not a fixed frequency,
fixed duty cycle oscillator, but a load-dependent feedback loop that will
change duty cycle or frequency (depending on device) to maintain a target
output. Check for yourself.

Try to get >90% efficiency for instance, from 24V down to 3.5V 5A. Unlikely
you can work at 2MHz.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of SAIDJACK at aol.com
Sent: Monday, November 28, 2011 3:06 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Low noise power supplies - dont' use Electrolytics

Hi Attila,
I like >2MHz switchers because they use small components (inductors  and 
capacitors) and are easier to filter out at our usual frequencies of
The LT3502A for example works at 2.2MHz, which gives harmonics at 8.8MHz  
and 11MHz, far enough away from 10.0MHz to avoid beating and causing a spur

issue. The 1.1MHz part uses a 15uH inductor, the 2.2MHz part only a 4.7uH  
inductor for the same 1.8V, 500mA application example, a huge difference in 
size  and cost. The capacitor is also 22uF vs. 47uF, again a large size and 
cost  savings for the 2.2MHz part.
Not much of an efficiency difference between the 1MHz and 2MHz parts  
anymore, they are usually around 80% to 85% at 2.2MHz and one or two percent

better at 1.1MHz. With proper layout these parts are extremely well behaved
 don't have any noticeable ringing etc on the switch as older units did, 
and they  don't create a lot of noise when using shielded inductors.
In a message dated 11/28/2011 14:22:58 Pacific Standard Time,  
attila at kinali.ch writes:

> Use  ceramic caps where vibration is not an issue. Use high frequency   
> (>2MHz) switchers wherever efficiency is required, otherwise use  linear  
> regulators.

Not really. If you want to have good  efficiency, then you want to stay
below 1MHz, otherwise switching loses get  too high. But the disadvantage
is that you have big and bulky capacitors  and inductors. 

Staying somewhere between 1MHz and 1.5 is usually a  good compromise if
you can tolerate PFM (aka pulse skip) for low load  conditions. Going up
to 2MHz (and beyond) is only recomended if you are  severly space limited.

Attila Kinali

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