The circuit described here is mostly aimed at development engineers
who are looking for an economical step-down converter which offers a
wide input volt-age range. As a rule this type of circuit employs a
step-down converter with integrated switching element. However, by using
a more discrete solution it is possible to reduce the total cost of the
step-down converter, especially when manufacturing in quantity. The
TL5001A is a low-cost PWM controller which is ideal for this project.
Low Cost Step Down Converter with Wide Input Voltage Range
The
input voltage range for the step-down converter described here is from 8
V to 30 V, with an output voltage of 5 V and a maximum output current
of 1.5 A.
When the input voltage is applied the PWM output of
IC1 is enabled, taking one end of the voltage divider formed by R1 and
R2 to ground potential. The cur-rent through the voltage divider will
then be at most 25 mA: this value is obtained by dividing the maximum
input voltage (30 V) minus the saturation voltage of the output driver
(2 V) by the total resistance of the voltage divider (1.1 kΩ). T1 and T3
together form an NPN/PNP driver stage to charge the gate capacitance of
P-channel MOSFET T2 as quickly as possible, and then, at the turn-off
point, discharge it again. The base-emitter junction of T3 goes into a
conducting state when the PWM output is active and a voltage is dropped
across R2. T3 will then also conduct from collector to emitter and the
gate capacitance of T2 will be discharged down to about 800 mV. The
P-channel MOSFET will then conduct from drain to source. If the
open-collector output of the controller is deactivated, a negligibly
small current flows through resistor R2 and the base of T1 will be
raised to the input voltage level.
The base-emitter junction of
T1 will then conduct and the gate capacitance of T2will be charged up
to the input voltage level through the collector and emitter ofT1. The
P-channel MOSFET will then no longer conduct from drain to source. This
driver circuit constructed from discrete components is very fast, giving
very quick switch-over times.
Diodes D2 and D3 provide voltage
limiting for the P-channel MOSFET, whose maximum gate-source voltage is
20 V. If the Zener voltage of diode D2 is exceeded it starts to
conduct; when the forward voltage of diode D3 is also exceeded, the two
diodes together clamp the gate-source voltage to approximately 19 V. The
switching frequency is set at approximately 100 kHz, which gives a good
compromise between efficiency and component size.
Finally, a
few notes on component selection. All resistors are 1/16 W, 1 %. Apart
from electrolytic C1 all the capacitors are ceramic types. For the two
larger values (C2 and C5) the following are used:
- C2 is a Murata type GRM21BR71C105KA01 ceramic capacitor, 1 µF, 16 V, X7R, 10 %;.
- C5 is a Murata type GRM32ER60J476ME20 ceramic capacitor, 47 µF, 6.3 V, X5R, 10 %.
- D1 (Fairchild type MBRS340T3) is a 40 V/3 A Schottky diode. Coil L1 is a Würth WE-PD power choke type 744771147, 47 µH, 2.21 A, 75 mΩ.
- T1 (BC846) and T3 (BC856) are 60 V, 200 mA, 310 mW complementary bipolar transistors from Vishay.
- The TL5001AID (IC1) is a low-cost PWM controller with an open-collector output from Texas Instruments. Source by Link
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