Linear or Switch Mode Power supply for PWM Driven Stepper Motors?

Hi Cabe,

You have nothing to worry about in many applications there is no performance cost to be paid by using a Linear Regulator vs. a Switching Regulator. Enrico brings up good points if you are using an off the shelf supply, since in many cases they may not meet transient specifications, not because its a switching regulator but because of shotty workmanship on a device that is inherently more complicated than a Linear Regulator.

The only application I can think of off the top of my head in which the choice between Linear and switching is important is powering high performance ADCs. Linear regulator simply clamp a high DC voltage to a lower DC voltage, it can be modeled as a resistor that can adjust its resistance depending on the load. In this case the Regulator does not introduce any ripple into the power supply rails, high performance ADCs are sensitive to ripple so it's important to avoid any interference in the form of voltage ripple which raises the ADC noise floor.

In this application a switching regulator is not so advantageous since it introduces ripple with a frequency equal to the switching frequency of the supply. Switching regulators can operate with frequencies over 1 MHz and the noise they introduce is generally not acceptable. Today switching regulators can be designed to minimize switching ripple and in some cases are acceptable for ADCs but the conventional approach is to use a linear regulator.

I know I went off topic, but I just wanted to show you when the choice between linear and switching regulators is important. Outside of noise and EMI considerations most applications can take either one.

Hope this helps.

Regards,

Jorge Garcia

Hi Jorge,

Let's continue the off-topic discussion for a bit ;-) I think that ripple&noise and EMI is indeed a problem for delicate analog circuits but this can be avoided when the power supply is custom build with a specific application in mind. I know, this is costly. I think it is more acceptable to use a good standard power supply and add a filter stage on the application board for the delicate circuits. Replacing the elcaps on a smps with low ESR types helps greatly. Another critical application is audio. Switching frequences might enter the audio stage. Switch mode and audio is not uncommon anymore. An switch mode audio stage is also interesting and works really nice, but for high end audio a no-go. High end audio has also to do with religion :-)

Best regards,

Enrico Migchels

Hi Enrico,

I definitely agree now and days SMPS can be designed specifically for low ripple on the output stage, I've read several articles on the use of SMPS for powering high precision ADCs.

I believe your solution is quite workable and in fact is often implemented.

I would like to mention a minor point on the Audio App. I'm not specialized in Audio but I believe that in Analog amplifiers high frequency switching noise would not be that large an issue. The human ear can detect signals from 20hz-20khz approximately, any noise with a frequency outside of this range would be undetected by a listener, this is handy for the design of simple amplifiers.

Class D(or C I don't recall) amplifiers I believe make use of this idea. In their simplest for the the audio signal is the input to a PWM circuit whose output is then filtered to to retrieve an amplified version of the original signal. Any high frequency residuals are ignored due to the human ear's natural cutoff.

A digital audio system would be adversely affected due to aliasing from sampling(Nyquist). I'm splitting hairs I know, you are completely correct in your comment, but I just felt like ranting a little;-).

Regards,

Jorge Garcia

Hi Enrico,

I definitely agree now and days SMPS can be designed specifically for low ripple on the output stage, I've read several articles on the use of SMPS for powering high precision ADCs.

I believe your solution is quite workable and in fact is often implemented.

I would like to mention a minor point on the Audio App. I'm not specialized in Audio but I believe that in Analog amplifiers high frequency switching noise would not be that large an issue. The human ear can detect signals from 20hz-20khz approximately, any noise with a frequency outside of this range would be undetected by a listener, this is handy for the design of simple amplifiers.

Class D(or C I don't recall) amplifiers I believe make use of this idea. In their simplest for the the audio signal is the input to a PWM circuit whose output is then filtered to to retrieve an amplified version of the original signal. Any high frequency residuals are ignored due to the human ear's natural cutoff.

A digital audio system would be adversely affected due to aliasing from sampling(Nyquist). I'm splitting hairs I know, you are completely correct in your comment, but I just felt like ranting a little;-).

Regards,

Jorge Garcia