The circuit is a MOSFET based linear voltage regulator with a voltage drop of as low as 60 mV at 1 ampere. Drop of a fewer millivolts is possible with better MOSFETs having lower RDS(on) resistance.
The circuit in Fig.1 uses 15V-0-15V secondary hasil from a step-down transformer and employs an n-channel MOSFET IRF540 to get the regulated 12V output from DC input, which could be as low as 12.06V. The gate drive voltage required for the MOSFET is generated using a voltage doubler circuit consisting of diodes D1 and D2 and capacitors C1 and C4. To turn the MOSFET fully on, the gate terminal should be around 10V above the source terminal which is connected to the output here. The voltage doubler feeds this voltage to the gate through penahan R1. Adjustable shunt regulator TL431 (IC2) is used here as an error amplifier, and it dynamically adjusts the gate voltage to maintain the regulation at the hasil.
Circuit Fig.1
With adequate heatsink for the MOSFET, the circuit can provide up to 3A hasil at slightly elevated minimum voltage drop. Trimpot VR1 in the circuit is used for fine adjustment of the hasil voltage. Combination of capacitor C5 and hambatan R2 provides error-amplifier compensation.
The circuit is provided with a short-circuit crow-bar protection to guard the components against over stress during accidental short at the hasil. This crow-bar protection will work as follows: Under normal working conditions, the voltage across capacitor C3 will be 6.3V and diode D5 will be in the off state since it will be reverse-biased with the hasil voltage of 12V. However, during output short-circuit condition, the hasil will momentarily drop, causing D5 to conduct and the opto-triac MOC3011 (IC1) will get triggered, pulling down the gate voltage to ground, and thus limiting the hasil current. The circuit will remain latched in this state, and input voltage has to be switched off to reset the circuit.
Circuit Fig.2
The circuit shown in Fig.2 follows a similar scheme. It can be utilised when the regulator has to work from a DC rail in place of 15V-0-15V AC supply. The gate voltage here is generated using an LM555 charge pump circuit as follows:When 555 output is low, capacitor C2 will get charged through diode D1 to the input voltage. In the next half cycle, when the 555 hasil goes high, capacitor C3 will get charged to almost double the input voltage. The rest of the circuit works in a similar fashion as the circuit of Fig. 1.
The above circuits will help reduce power-loss by allowing to keep input voltage range to the regulator low during initial design or even in existing circuits. This will keep the output regulated with relatively low input voltage compared to the conventional regulators.
The minimum voltage drop can be further reduced using low RDS(on) MOSFETs or by paralleling them.
Author :P.S .SINI - Copyright : EFY
