Joule Thief VS Boost Converter ..#2


Joined Jan 27, 2019
Welcome to AAC.

The answer to your question depends on what you mean by “the same”.

A Joule Thief depends on the same principle as boost converters. It uses the collapsing magnetic field of an inductor to increase the voltage over the input, in that way, they are the same.

The major difference contrasting it with a proper boost converter, that is, what an engineer would imagine if you mentioned “boost converter” is the oscillator. In a Joule Thief, the circuit that boosts the voltage is also the oscillator, while in a practical boost converter, the oscillator is a separate circuit.

This schematic of a Joule Thief circuit shows its simplicity, and its reliance on self-oscillation.


The circuit uses the rising and collapsing magnetic field in the primary of the transformer to create a higher voltage on the secondary. The rise time is proportional to the battery voltage and primary inductance. The rising current turns on the transistor fully, but the transistor needs more base current to stay on as the voltage increases (this is very simplified) and so it turns off and the cycle starts again. In this way, one circuit is both the oscillator and the inductive booster.

On the other hand, in a proper practical boost converter there are additional requirements. The Joule Thief has no critical parameters. It doesn’t have to maintain a particular voltage. Its output doesn’t have to have particular noise or waveform characteristics, and it doesn’t have high current requirements.

For a boost converter, though, all these and other parameters must be carefully controlled. In this block diagram of a TPS6108x boost converter IC, you can easily see the complexity. And there are many subsystems not present in the Joule Thief. But I am focusing on the oscillator, in the red box.
You can see that the inductor in this circuit (L1) is separate from the oscillator circuit and a current sensor is used to provide feedback, combined with the oscillator and a ramp generator to drive a PWM circuit to manage the voltage of the output, which is considered critical in this boost converter but not even a concern in the Joule Thief.

So, the principle of storing a charge in the form of a magnetic field around the inductor at a low voltage, then allowing it to collapse, producing a higher voltage, is the same.

It is not entirely unlike a transformer which works with AC because of the constantly changing polarity causing the magnetic field on the primary to rise and then fall, the changing field inducing a voltage in the secondary proportional to the ratio of the windings.

But, in the case of DC, the polarity isn’t changing and so we interrupt the DC (the ramp waveform) in a carefully timed way, and the current from the inductor is taken directly, at a higher DC voltage at a lower current through the diode you see just before OUT.

[EDIT: fixed a typo and some formatting]
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