A low cost step-up converter by IC 555

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This is a power source that creates a DC voltage higher than the voltage feeding the it. It might be used to power your laptop (+18V) from a car battery (12V ~ 13.8V). It is built using very ordinary parts that should be readily available. Here are some specs:

Input voltage	9V ~ 15V
Output voltage	12V ~ 25V
Output current	Max. 3A @18V
Switching frequency	20kHz - 40kHz
Efficiency	65% - 95%

If you know these circuits, please skip the following. The circuit is a step-up converter, and it’s workings can be explained by the main property of a coil: its inductance. Imagine a current flowing through a coil. The inductance will cause the current to continue running, even if the power supply that caused this current is taken away. The voltage across the coil will take any value required to keep the current running. This is why you can easily create sparks with a coil, a spark is simply a current flowing at a very high voltage.

This circuit flips between two states: ‘charging’ the coil with current, and dumping this current into the output capacitor. The frequency at which this happens is so high that only a small ripple remains at the output capacitor (10000uF). Here is the circuit.

The 555, a timer circuit, generates pulses of specific width that control T1. T1 either grounds the coil, allowing current to build-up, or disconnects it, allowing current to flow through D1 into the buffer capacitor. The combination of R1 (10K) and the zener of 15V plus T2 is actually a simple voltage monitor. When the output voltage is higher than approx. 16.5V the output signal will drop low. As this signal is fed to the 555 RESET input, no pulses will be generated if the output voltage is above approx. 16.5V using a 15V zener. The 5V zener is necessary for proper startup of the circuit at powerup.

Use a coil with 1 mm wire diameter, inductance approx. 200uH. T1 is a power darlington. A TIP141 works fine. A MOSFET would be better, use rdsON < 0.1 Ohm. T2 is an ordinary signal NPN, eg. BC547B. Use a power scottky diode (D1) for highest efficiency, but an ordinary silicon type will do. Take care: at 18V @3A quite a bit of power will be dissipated in the rectifier. Cool it. If you have the choice, use “low ESR” type of capacitors.

This article was written by Arian van Dorsten jwasys@xs4all.nl

source http://www.xs4all.nl/~jwasys/old/diy4.html#stepup

This entry was posted on Saturday, July 21st, 2007 at 4:53 pm and is filed under Power supply, switching power supply. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.