[Heath] Switching Power Supply

[vouty at comcast.net]

Hi Folks, 
I am forwarding from Rod Holt, who worked for Steve Jobs on the Apple II switching power supply. I forwarded your emails on this topic to him. 

"I have been using switching power supplies for many years. My current application is the design of very, very low noise (sub-nV/√Hz) amplifiers in the 0 to 25MHz range. I have recently been using off the shelf switchers ("wall-mount" Meanwell and Phihong) stocked by Mouser. Larger bench top types good for up to 100W are available. These supplies meet current FCC Part 15 regulations for conducted and radiated noise. I use them right out of the box. Of course, the low noise amplifiers are themselves shielded but the switching supplies are unmodified and their DC outputs are fed directly to the outer case of the amplifiers — that is, the DC Low wire from the supply is connected to the amplifier case (and consequently to the low level input ground) and the DC hot wire is fed directly to a special low noise regulator. You will need a good spectrum analyzer to actually see the noise contribution from the external switching supply. 

An off line switching power supply is basically a DC to DC converter. Typically, the AC line input is rectified and chopped into a square wave at 30 to 100kHz which is in turn fed to a small transformer which provides the necessary isolation from the power line. The reason for doing this is to save size and weight, improve power efficiency and regulate the output voltage(s) all at the same time. Apple Computer introduced the Apple II which had a switching supply in mass production in 1969. This power supply would handle up to 40W with 4 regulated DC outputs and dissipated one-tenth of what a conventional supply would have done with the same specs. 

I don't recommend anyone designing their own switching regulating power supply. You will need a lab full of very expensive and very sophisticated equipment. You can buy well-designed medium and small switchers that meet the FCC specs for less than $1 per watt. (Meanwell number GS60A18-P1J that is 18V at 3.3A for only $45) 

Yes, a person can build a switching regulating power supply which is as clean as you want, but it is a non-trivial task. 

--rod holt 
( rholt at planeteria.net ) 

On Nov 2, 2015, at 7:08 PM, vouty at comcast.net wrote: 

> 
> 
> Sent from XFINITY Connect Mobile App 
> 
> 
> ------ Original Message ------ 
> 
> From: Ron 
> To: Heathkit List 
> Sent: November 2, 2015 at 5:47 PM 
> Subject: [Heath] Off Topic Switching Power Supply 
> 
> I built an 18vdc 1.5 amp power supply using a transformer, bridge rectifier and a 7818 regulator. Works fine, but I need to go to 3 amps. I have been trying laptop switching power supplies that put out 3 amps that produce noise on my hf rig. If I unplug the laptop power supply, the noise goes away. The power supply is used for charging batteries through a solar charger. 
> 
> Questions: what is the difference between a regular power supply and a switching power supply? Can I filter out the noise on a switching power supply? 
> 
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