Basic Switching Circuits
Understanding the fundamental operation of switching power supplies is an introduction into
the dark arts of engineering. On paper, they are no more than the introductory college courses
of electronics, but when you add in the parasitic behaviors of the components, then the chal-
lenge becomes much greater. For me, it is exciting.
I remember the first switching power supply I ever attempted. It was a 30-kHz flyback supply. I
designed the control section using an NE556 and wound my own transformer on an MPP tor-
roid core. When I powered it up, I made a shocking discovery. I had made a wonderful 3-MHz,
AM modulated RF transmitter. At which I said, “ Oh my—(fill in your preferred deity). I need to
do a lot more learning. ”
To do a thorough design and analysis of a switching power supply you will need a new set of
instruments, such as: oscilloscope voltage and current probes, a spectrum analyzer and a net-
work analyzer. Welcome to a world where few engineers have ventured.
Switchmode power supplies first became practical in the 1970s, where one used improvised
control circuits, bipolar transistors and slow diodes. They operated at less than 50 kHz. The first
power MOSFETs emerged in the late 1970s. They were easier to drive and switched much faster
than bipolar transistors. This allowed the switchmode power supplies to now exceed 100 kHz
and even go as high as 300–500 kHz. Today, with the much improved diode performances, bet-
ter magnetic materials, resonant techniques, and surface mount packaging, the switching power
supply can easily operate over 1 MHz and have a much smaller size and high efficiency.
Switching power supply design will always involve a certain amount of “ customization. ”
Whether it is the number of outputs, size, height, efficiency or noise, some aspect of the design
always needs to be tailored to the surrounding application. I have always said, “ Once you put
an AC flux into a magnetic core, you can do anything. ”
In this chapter Ray Mack provides an intuitive introduction to the basic operation of the basic
building blocks of switching power supplies. I think you will find it very informative.
— Marty Brown