|
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 from a step-down
transformer and employs an n-channel MOSFET IRF 540 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 resistor 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 output. With adequate heatsink for the MOSFET, the circuit can
provide up to 3A output at slightly elevated minimum voltage drop.
Trimpot VR1 in the circuit is used for fine adjustment of the
output voltage. Combination of capacitor C5 and resistor 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 output. 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 output voltage of 12V. However, during output short-circuit
condition, the output 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 output current.
The circuit will remain latched in this state, and input voltage
has to be switched off to reset the circuit. 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 output 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. These
circuits above will help reduce power-loss by allowing to keep
lower input voltage range to the regulator 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
|
