'0V' Dropout Heater Regulator

Converting existing heaters to DC can be problematic because, after diode drops, ripple and AC line minimums, the rectified DC of a 12.6VAC transformer isn't much more than the needed 12.6VDC but single chip voltage regulators require at least an extra volt or two or they drop out of regulation. This regulator has '0V' drop out, or close to it. The drop out voltage is only the P-MOSFET Rdson x current.

Quite simple. The 'secret' is the P-MOSFET with everything else simply a voltage reference and single supply error opamp. The offset and R7/R8 around the trimmer pot causes Vout to drop to a safe voltage if the wiper breaks contact.

Output ripple is obscenely low.

Note that the values are for demonstration and would likely need trimming in an actual implementation. While spice modeling doesn't indicate the need it might be a good idea to compensate the opamp to prevent oscillations.


High Voltage Version


A discrete amp is used due to the higher voltage and gate control is by current through R8. A not immediately obvious feature is that more than half the ripple reduction is due to R8 since it's far end is connected to the input ripple, which adds to the current derived signal and is applied directly to the gate. That means the gate signal follows the source ripple before any feedback is employed.

Reference voltage for the Q4 current source is stolen off the TL431 reference.





(c) 2004-2008 All Rights Reserved