Avian’s Blog

Past two weeks I've been working on a small step-up switching power supply for a solar cell powered device (maybe I'll write some more about that thing later).

The basic design looks like this: Small solar array (for cca. 10 Euro) charges two NiMH batteries. Switcher then converts 2.0V - 2.4V voltage from the batteries to a more or less stable 9V that are needed by the device (which draws 100mA or so in peaks a couple of times per day, and around 1mA otherwise)). I designed the switcher around MAX641 chip which turned out to be a really good idea so far. In contrast with other switcher controller chips it has no feedback loop that varies the duty cycle of the pulses (duty cycle is constant 50%). This way a lot of the usual stability problems disappear and I got the basic circuit working in no time. The drawback is the circuit never works in the continuous mode with such a design and you get larger output voltage ripple (but that is not really a problem here)

After some experimenting with different inductors I got the more or less constant efficiency of the conversion of around 80% at full power over entire input voltage range, which is not bad. Selecting the right inductor was tricky though. The formulas for calculating the inductance in the manufacturers application do not help: our shops with electronic supplies either do not sell ferrite cores or sell them, but do not know the properties of the material they are made of. Any calculations are therefore completely out of the question and you are left with trial and error. A thing I really should remember for the next time I'll be doing something like this is not to even consider cores without an air gap. It turned out that after cut an air gap into the core, almost anything resembling a ferrite works.

Anyway the thing that has been really getting on my nerves since the last weekend is that when the switcher is working, analog electronics in the vicinity seems to go completely nuts. For example a simple circuit for low battery voltage detection (needed to prevent damaging NiMH cells by deep discharge) is simply ignoring all common sense any refuses to work correctly no matter what I do. I already redesigned the circuit board layout two times to minimize interference between the switcher's power electronics and control electronics with no luck...

So a couple of minutes ago when I was looking for some ideas in the Art of Electronics I found this quote:

Switchers are noisy - they will ruin your life.

Encouraging, isn't it?