Active adjustable voltage limiting circuit

Question

I'm looking for a circuit that can clip the peaks of a sine wave source, typically the source is 40 V peak to peak and should be clipped at about 13.8 V. The circuit below does exactly this but to my knowledge there are no Zeners that'll be able to handle the 50 W or possible larger peaks. The idea will be use power MOSFETs or BJTs attached to a heatsink to clamp the peaks.

This will be used to clamp the generator output on a gasoline engine, the kind of regulators used on these generation of engines are pretty bad and cooks the battery, I'm attempting to fix this problem. The generator won't be damaged by clamping the output.

Edit: Added the half bridge diode for Rload to make everyone in the comments happy :D

Answer 1

The circuit to do this with a bipolar power transistor has been around since the 1950's. In your case, I would use a power Darlington.

If you replace the zener diode with a pot in series with a fixed resistor, you have an adjustable zener. The circuit often appears in audio power amplifier designs, where you can adjust the voltage spacing between the output transistors to set the no-signal static current through them.

http://www.seekic.com/circuit_diagram/Basic_Circuit/Active_Power_Zener.html

UPDATE:

This circuit replaces only D2 in your original circuit. D3 still is needed.

Answer 2

I'm looking for a circuit that can clip the peaks of a sine wave source, typically the source is 40 V peak to peak and should be clipped at about 13.8 V.

This is not a particularly great idea, as you'll be wasting energy as heat - and that's a rather expensive energy, since it comes from an ICE.

A switching buck DC/DC converter would be ideal, but a linear regulator can do the job as well, while wasting less heat than a shunt would, especially at lighter loads (e.g. when the battery has charged up).

In essence, all you need is a linear regulator with reasonably low dropout, set to 13.8V. The circuit below does this:

^{simulate this circuit – Schematic created using CircuitLab}

The voltage waveforms on named nodes are shown below.

The average power dissipated on the pass element and the Schottky series diode, with a 10 Ohm load, is a couple Watts total:

With two mosfets, rectification can be active, for lower dropout, but most likely this won't be of any advantage in this applicaiton:

^{simulate this circuit}

A battery charger of this kind also requires active current limiting, to protect itself, the load, and the generator's winding from short circuits and overload.

The easiest way to get such protection is with a thermal circuit breaker. It could also be implemented electronically.

One way uses the series rectifier diode as a current sensor. A second, matched diode, running at a small constant current, needs to be thermally coupled to the rectifier diode. The difference between these voltages is a measure of the logarithm of the current ratio between the two diode.

Another approach is to use the Rds(on) resistance of the mosfet. This is straightforward when the mosfet is fully turned on. With two mosfets, we can decouple their control and use one as an ideal diode, acting as a current sensor when turned on, and the other as the pass element. The ideal diode doesn't require a heatsink, since it has negligible losses - this way it also stays at a temperature that's relatively independent of the load current. Then we don't need a third reference mosfet to establish Rds(on) scaling.