DC-DC Inverting Converter Circuit for 15V Output

A DC-DC Inverting Converter Circuit is a special kind of circuit that flips the input voltage around.

It takes a positive voltage and turns it into a negative one!

These circuits are super useful for things that need negative power like operational amplifiers, analog circuits and communication gadgets.

In this circuit, we used the uA78S40 IC which is a handy switching regulator.

It changes a +15V DC input into a -15V output and can handle up to 500mA of current.

The uA78S40 can even give a peak output current of 1.5A!

This circuit is great because it works well and does not cost too much making it perfect for low to medium power needs.

Circuit Working:

Parts List:

The uA78S40 IC is a special kind of regulator that helps manage power by using an external transistor Q1 like a switch.

This switch helps control the energy that gets stored in a part called the inductor L1

The circuit gets its power from an input voltage of +15V Vin.

Inside the uA78S40 there is a tiny oscillator that makes a switching pulse to control the transistor Q1.

When Q1 is turned ON it, allows current to flow through the inductor L1 which stores energy in its magnetic field.

When Q1 is turned OFF the inductor lets go of that energy and the diode D1 changes it into a negative output voltage of -15V.

To make the output voltage nice and smooth the capacitor C2 helps reduce any ripples.

There is also a little resistor R5 that checks the current to keep it steady.

R1 and R2 work together to set the output voltage by using the feedback feature of the IC.

The speed of the switching is managed by the capacitor C1

Formulas with Calculations:

Below mentioned are the formulas with calculations for DC-DC Inverting Converter Circuit for 15V Output:

Output Voltage Calculation:

The output voltage Vout is determined using the voltage divider formula:

Vout = -((R2 / R1) + 1) * Vref

where,

• Vref for uA78S40 is 1.25V
• R1 is 1.5k
• R2 is 18k

Now lets substitute the values:

Vout = -((18k / 1.5k) + 1) * 1.25V

Vout = -((12 + 1) * 1.25V)

Vout = -15V

Inductor Selection:

The inductor value L is calculated using:

L = (Vin * (Vout – VD)) / (Iout * f * k)

where,

• Vin is 15V
• Vout is -15V
• VD is 0.4V schottky diode voltage drop
• Iout is 0.5A
• f is 100kHz (Switching frequency)
• k is 0.5 ripple factor around 0.4 to 0.7

Substituting the values:

L = (15 * (15 – 0.4)) / (0.5 * 100000 * 0.5)

L = (15 * 14.6) / 25000

L = 50µH

Current Sense Resistor Calculation:

The current sense resistor Rsc is given by:

Rsc = 0.6V / Ilimit

For a current limit of 2A:

Rsc = 0.6V / 2A

Rsc = 0.3Ω

How to Build:

To build a DC-DC Inverting Converter Circuit for 15V Output following steps are required to follow for connections:

• Assemble all the components as mentioned in the above circuit diagram
• Connect pin 3 of IC1 uA78S40 to GND of the circuit
• Connect pin 8 of IC1 of to pin 9 of IC1 through resistor R1
• Connect pin 10 of IC1 to pin 11 of IC1 and GND.
• Connect pin 12 of IC1 to one terminal of capacitor C1 and other terminal connect to GND.
• Connect resistor R5 between pin 13 and pin 14 of IC1
• Connect pin 15 of IC1 to pin 16 of IC1
• Connect a base of transistor Q1 to one terminal of resistor R3 and other terminal connect to pin 15 and pin 16 of IC1.
• Connect one terminal of resistor R4 from pin 14 of IC1 and other terminal connect to base of transistor Q1
• Connect terminal of L1 inductor coil to collector of transistor Q1 and other terminal connect to GND.
• Connect cathode of D1 diode to collector of Q1 transistor and anode of transistor connect to negative of -15V Vout supply
• Connect one terminal of resistor R2 from pin 8 and 9 of IC1 and other terminal connect between diode D1 and Vout of -15V
• Connect a negative of capacitor C2 to -15V of Vout supply and positive of capacitor C2 connect to GND.
• Connect the positive of +15V Vin to the junction of pin 13 of IC1
• Connect a positive of capacitor C3 to +15V of Vin supply and negative of capacitor C3 connect to GND.

Conclusion:

This DC-DC Inverting Converter Circuit for 15V Output changes a +15V input into a -15V output using the uA78S40 switching regulator.

It is a simple and efficient circuit that gives a steady negative voltage for many electronic uses.

By adding an external transistor, it can handle more power and a current sense resistor is included to protect against overloads.

You can also easily tweak this design to get different output voltages by changing the resistor values.

References:

ADJUSTABLE INVERTING DC/DC CONVERTER