How do voltage boosters work

Although the charge C1 drains away through the load during this period, C1 is recharged each time the MOSFET switches off, so maintaining an almost steady output voltage across the load. Because the output voltage is dependent on the duty cycle, it is important that this is accurately controlled.

For example if the duty cycle increased from 0. Before this level of output voltage was reached however, there would of course be some serious damage and smoke caused, so in practice, unless the circuit is specifically designed for very high voltages, the changes in duty cycle are kept much lower than indicated in this example. See the current paths during the on and off periods of the switching transistor.

Note that the operation during the first "On" period is different to later periods becaust the Capacitor C is not charged until the end of the first "On" period. See the magnetic field around the inductor grow and collapse, and observe the changing polarity of the voltage across L. Watch the effect of ripple during the on and off states of the switching transistor. See the input voltage and the back e. Because of the ease with which boost converters can supply large over voltages, they will almost always include some regulation to control the output voltage, and there are many I.

In this circuit, an appropriate fraction of the output voltage V OUT , dependent on the ratio of R2:R3 is used as a sample and compared with a reference voltage within the I. This produces an error voltage that is used to alter the duty cycle of the switching oscillator, enabling a range of automatically regulated boost voltages between 5V and 28V to be obtained.

The LM contains an internal oscillator operating at a fixed frequency of about 1. Notice also that a Schottky diode with an appropriate voltage and current rating is used for D1 to keep losses due to the forward voltage drop of the diode as small as possible, and to enable high switching speeds to be achieved. The I. There was a lot left out, but it was worth it to make the working of the boost converter absolutely clear. So now that we have that understanding, we can move on to the finer details.

The Oscillator. We use our knowledge of inductors to calculate the time required to reach a sensible current one Amp, for example and then configure the on time of the oscillator accordingly. This results in the inductor current waveform looking like a saw edge, hence the name sawtooth.

If you look closely, during step 3, the MOSFET sees a voltage that is the supply voltage plus the inductor voltage, which means that the MOSFET has to be rated for a high voltage, which again implies a rather high on resistance.

The maximum output voltage of the boost converter is not limited by design but by the breakdown voltage of the MOSFET. The inductor. Inductors used in boost converters should be able to withstand the high currents and have a highly permeable core, so that the inductance for a given size is high. So we store some energy in the inductor from the input and transfer that same energy to the output though at a higher voltage power is conserved, obviously.

This happens many thousands of times a second depending on the oscillator frequency and so the energy adds up in every cycle so you get a nice measurable and useful energy output, for example 10 Joules every second, i. As the equation tells us, the energy stored in the inductor is proportional to the inductance and also to the square of the peak current. To increase output power, our first thought might be to increase the size of the inductor.

Of course, this will help, but not as much as we think! However, since energy is proportional to the square of the maximum current, increasing the current will lead to a larger increase in output energy! So we understand that choosing the inductor is a fine balance between inductance and peak current.

To begin with, we need a thorough understanding of what our load requires. It is highly recommended from experience that if you attempt to build a boost converter at the beginning it is very important to know the output voltage and current independently, the product of which is our output power. Once we have the output power, we can divide that by the input voltage which should also be decided to get the average input current needed. This new value is the peak input current. Also the minimum input current is 0.

Now that we have peak and minimum current, we can calculate the total change in current by subtracting the peak and minimum current. Now we calculate the duty cycle of the converter, i. Now it is time to decide upon the frequency of the oscillator. This has been included as a separate step because the signal source can be anything from a timer where the frequency and duty cycle are completely under your control or a fixed frequency PWM controller.

Since we have determined the on time, input voltage and change in current, we can plug those values into the inductor formula which has been rearranged a little:. With a little tweaking, the system should work just fine. Of course, the MOSFET is used in all applications these days, since they are very efficient, but there may be situations where a normal bipolar transistor may suffice because of simplicity. The lower this value is, the easier the driving requirements are.

If you stop the current, they will jack up the voltage until something gives and the current can flow again. In a real boost converter the switch is operated electronically. It switches on and off at a controlled rate to provide a steady output. For the simple booster, it was more important to make it simple than to make it conventional. It had to be simple to build, and simple to explain. Every additional part just obscures the basic principle. Well then, tear the cardboard tube out without disconnecting the wires.

Untangle to coil until you have just one long loop of wire. You may have noticed that I gave a value of 49 micro henries for the coil used in the simple booster. That is the calculated inductance given the wire I used 0. The value was chosen on purpose.

There is a commercial IC that does the same thing as the simple booster. I knew that the combination of inductance and available current would provide enough boost to work without a risk of burning up the LED. If you are an engineer, you can calculate that kind of thing. If you are a hobbyist like I am, you use shortcuts rather than slogging through a bunch of calculations to get an exact number.

I went back and recalculated the size of the inductor. Turns out I was off by quite a bit. If you are interested in what gave me the idea for this circuit and why the booster is built like it is, have a look here. Time to get abstract. The simple voltage booster - Table of Contents Last time around, I built a really simple switching power supply that boosts 1.

This is a table of the parts used in the simple voltage booster together with the symbols used in drawing circuit diagrams: Name Photo Symbol Designator Value Cell BT1 1. The number of loops in the inductor symbol has nothing to do with the number of turns on the real inductor. Current flows when pin 2 is at a higher voltage than pin 1. It opens and closes the circuit just the same, despite being made of a piece of wire and a file.

The LED is shorted by the inductor.