MOSFET-based Joule Thief steps up voltage

A simple blocking oscillator circuit can be used to step up voltage using properties of coil inductance (V = L di/dt). Such a circuit is shown in Figure 1, which is more commonly called a Joule thief.

MOSFET-based Joule Thief steps up voltage


The output will be pulses of voltage that can be filtered using a diode and capacitor. As there is no regulation, the output voltage will vary with the input voltage or load. As this circuit uses a BJT, the supply voltage needs to be at least 0.7V to work, and with enhancement-mode MOSFETs, the supply voltage must usually be even higher.

In this Design Idea, a low-threshold MOSFET (Q1) is used with coupled coils L1 and L2 to act as a Joule thief circuit.

A toroidal ferrite core is used to make the coupled coil or transformer. A photovoltaic optocoupler, TLP191B, is used to circumvent the problem with the threshold voltage. Part of the output is used to power TLP191B to make an isolated voltage source in series with the gate of Q1 (AOI508). This isolated voltage can be controlled by the pot R5; capacitor C1  is used to pass the pulses produced by L1 to the gate for switching.

For regulation, another MOSFET, Q2 (IRLU3103), is used. The threshold voltage of Q2 is used to regulate the output voltage. When the output reaches 5V, the divider formed by R1 and R2 makes Q2 conduct, and the oscillation ceases, causing the output voltage to drop. As the threshold voltage is not very sharp, the regulation is not usually that good.

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