Low Loss Step Down Converter

This circuit arose from the need of the author to provide a 5 V output from the 24 V battery of a solar powered genera-tor. Although solar power is essentially free it is important not to be wasteful especially for small installations; if the battery runs flat at midnight you’ve got a long wait before the sun comes up again. The basic requirement was to make an efficient step-down converter to power low voltage equipment; the final design shown here accepts a wide input voltage from 9 to 60 V with an output current of 500 mA. The efficiency is very good even with a load of 1 mA the design is still better than a standard linear regulator. The low quiescent current (200 µA) also plays a part in reducing losses. 

Some of the components specified (particularly the power MOSFET) are not the most economical on the market but they have been deliberately selected with efficiency in mind.

Low Loss Step Down Converter  Circuit diagram :

Low Loss Step Down Converter Circuit Diagram

When power is applied to the circuit a reference voltage is produced on one side of R2. D1 connects this to the sup-ply (pin 7) of IC1 to provide power at start-up. Once the circuit begins switching and the output voltage rises to 5 V, D2 becomes forward biased and powers the IC from the output. Diode D1 becomes reverse biased reducing current through R1. When the circuit is first powered up the voltage on pin 2 of IC1 is below the reference voltage on pin 3, this produces a high level on output pin 6. The low power MOSFET T1 is switched on which in turn switches the power MOSFET T3 via R5 and the speed-up capacitor C4, the output volt-age starts to rise. 

When the output approaches 5 V the voltage fed back to the inverting input of IC1 becomes positive with respect to the non inverting input (reference) and switches the output of IC1 low. T1 and T3 now switch off and C3 transfers this negative going edge to the base of T2 which conducts and effectively shorts out the gate capacitance of T3 thereby improving its switch off time. 

The switching frequency is not governed by a fixed clock signal but instead by the load current; with no load attached the circuit oscillates at about 40 Hz while at 500 mA it runs at approximately 5 kHz. The variable clock rate dictates that the output inductor L1 needs to have the relatively high value of 100 mH. The coil can be wound on ferrite core material with a high AL value to allow the smallest number of turns and produce the lowest possible resistance. Ready-made coils of this value often have a resistance greater than 1 ? and these would only be suitable for an output load current of less than 100 mA. 

The voltage divider ratio formed by R4 and R3 sets the output voltage and these values can be changed if a different out-put voltage is required. The output volt-age must be a minimum of 1 V below the input voltage and the output has a minimum value of 4 V because of the supply to IC1. 

A maximum efficiency of around 90 % was achieved with this circuit using an input voltage between 9 and 15 V and supplying a current greater than 5 mA, even with an input voltage of 30 V the circuit efficiency was around 80 %. If the circuit is used with a relatively low input voltage efficiency gains can be made by replacing D4 with a similar device with a lower reverse breakdown voltage rating, these devices tend to have a smaller for-ward voltage drop which reduces losses in the diode at high currents. At higher input voltage levels the value of resistor R1 can be increased proportionally to reduce the quiescent current even further.

Converter 12 Vdc to 230 Vac or Inverter

As shown in the Inverter schema diagram obove , Its used as the oscillator stage astable multivibrator contained in IC1, a CMOS 4047 (this cult series 40xx series) by varying the resistance value of R1 trimmer (220 k total resistance) can vary the oscillation frequency of 40 Hz to 70 Hz square wave, phase shifted by 180 °,  Output pin 10-11 will drives two NPN transistors TR1-TR3, which in turn is fed to the TR2-TR4. 
Converter 12 Vdc to 230 Vac or Inverter Schematic 

The diodes DS2-DS3, mounted on the output transistors TR2-TR4 are used to protect against voltage surges appearing across the windings V 9 + 9 V transformer T1. For the transformer T1, I used an ordinary mains transformer (primary 230 V so) with a secondary dual 2 x 9 V. 

Parts List: 
R1 ……. 220 k trimmer 
R2 ……. 330 k 
R3 ……. 680 
R4 ……. 2.2 k 
R5 ……. 2.2 k 
C1 ……. 4.7 nF polyester 
C2 ……. 220 uF electrolytic 
DS1 ….. 1N4004 
DS2 ….. 1N4004 
DS3 ….. 1N4004 
DL1 ….. LED 
TR1 ….. BC184 NPN 
TR2 ….. NPN BDX53C 
TR3 ….. BC184 NPN 
TR4 ….. NPN BDX53C 
IC1 …… 4047 CMOS 
T1 …….. transformer sector 80 VA primary 230 V 0.35 A / Secondary 2 x 9 V 3.5 A 
S1 ……. switch 
Note : 

• Two final power TR2-TR4 should be mounted on the right size heatsink, otherwise they will overheat. You can choose from MJ4033 – MJ3007 or more, provided that the NPN.
• The maximum power output that can be used depending on the size of the core of the transformer T1, the VA is: with 50 VA can be taken in the secondary 230 V 0.2 A (current consumed by the end will be 4 A) with 90 VA can be taken on the secondary 230 V 0.4 a (current consumed by the end will be 7 A).
• To power the schema from the 12V battery, it will take over at least 1.8 millimeters in diameter, to avoid loss by Joule effect.