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Switched mode power supplies

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To become a truly valuable microwave engineer, you will have to learn about other non-microwave electronics as well.  Power supplies are a critical component of systems, and efficiency and noise of a power supply can make or break your career.

As Tesla pointed out so long ago, AC voltages are easy to step up and step down in voltage, using transformers. But what if the available power form is direct current?  What if you wanted to make a laptop computer power a PIN diode phase shifter through the USB port for a trade show booth demo? The USB supplies +5V DC, which will be handy for diodes that you want to forward bias, but you might want -20V to turn them off.

Buck and boost converters

Switched mode power supplies (SMPS) are used to convert DC voltages, either up (boost) or down (buck), or invert the polarity.  Theoretical efficiency is 100% in a SMPS, but typically it is 90%.

Buck converters employ two switches, or a switch and a diode, to periodically charge up a capacitor though an inductor, using a clocked waveform of variable duty cycle (the duty factor determines the output voltage).  As you recall from year-one circuits class, capacitors can't change voltage instantaneously and inductors can't change current instantaneously; you use these magical properties to arrive at your desired voltage across the capacitor, which provides the output of the supply.  As you move the clock frequency up (which will allow you to shrink components), layout parasitics start to come into play and can add significant voltage ripple to your "DC" power supply. That Spice simulator that you use to model a switched mode power supply in steady-state does not model layout effects, and you need to up your game.

Here's a video from Keysight that you might find helpful. In it, Applications Engineer Andy Howard shows how to design a DC-to-DC converter.  The video provides and excellent explanation of how a buck converter works, dives into the layout effects that can cause voltage ripple, then shows how to use EM simulation to analyze what is going on and improve performance.

How to Design DC-DC Converters, by Andy Howard

The other way to convert voltages is with linear regulators.  Linear regulators are limited to converting voltages down only, and depending on how much overhead voltage you burn off they can be extremely inefficient.  However, linear power supplies can be remarkably noise free, while SMPS's will have a pile of frequency spurs on the output that you will have to deal with.  In many radar applications those spurs will result in false targets. In practice you might want to add a linear regulator at the output of a boost converter to clean up all that mess before it causes you to fail your phase noise specification.

Modeling nonlinear magnetics

Below is a video from Keysight.  First it provides a review of nonlinear magnetic theory such as saturation and hysteresis on the B-H curve, and Faraday's and Ampere's Laws. You will learn what is meant by hard and soft magnetics, and a simple buck converter design will be presented.

Colin Warwick presents"How to Model Nonlinear Magnetics in Power Electronics"

 

Author : Unknown Editor

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