Wednesday, February 1, 2023

# 5V DC To 48V DC Converter For Phantom Power Supplies

By PETRE TZV PETROV

This circuit presents a simple and low-cost DC-to-DC converter for phantom power supplies used in mixing consoles, microphone preamplifiers, telephone systems and similar equipment. There are many application circuits that require 24V and 48V phantom power supplies, but these DC power supplies are costly and not easily available. 5V DC power supplies are easily available at low cost. This circuit presents a solution for converting 5V DC to 48V DC using a popular MC34063A IC.

## Circuit and working

The circuit diagram of the 5V DC to 48V DC converter is shown in Fig. 1. It is built around boost/buck/inverting switching regulator MC34063A (IC1), two rectifier diodes 1N4007 (D1 and D3), Schottky diode SR1100 (D2), two 5mm LEDs (LED1 and LED2), two inductors (L1 and L2) and a few other components.

Input power supply ranging from 5V to 7V can be applied to connector CON1. Fuse F1 protects the circuit from any continuous over-current input. Diode D1 protects the circuit from reverse input voltage. Resistor R2 limits the maximum switching current to around 1.5A, as per relationship:

Ipeakmax = 0.3V/R2

Resistor R3 limits the current for the inbuilt driver transistor of IC1. This current should be below 100mA. Operating frequency of MC34063A is in the range of 40kHz to 60kHz, and depends mainly on capacitor C3. Inductor L1 is in the range of 100µH to 150µH for current above 1.8A.

Output voltage is around 48V and can be adjusted with R4. It can be calculated using the approximate relationship:

Vout = 1.25Vx(1+R4/R5)

Output current is up to around 50mA, which is enough for several applications including microphone amplifiers. Voltage on pin 5 of IC1 should be equal to internal reference voltage or around 1.25V (1.21V to 1.29V). Schottky diode D2 should preferably be rated at least 100V. Schottky or ultrafast diode can either be SB1A0, SR1100, SS110 or SR180.

C4 and C5 are the first filtering capacitors. Ripples at OUTPUT1 are around 50mV at 50mA load, which may be too high for some applications. So, second output (OUTPUT2) is provided after LC filter. L2 (usually from 22µH to 220µH for current above 100mA) and C6 produce additional filtration of the voltage produced by the DC-to-DC converter.

We can use both the outputs, OUTPUT1 and OUTPUT2, at the same time under the condition that total current is below 60mA. The circuit does not need any adjustment.

## Construction and testing

A PCB layout of the DC-to-DC converter is shown in Fig. 2 and its components layout in Fig. 3. After assembling the circuit on the PCB, connect the 5V regulated power supply across CON1. Connect LED1 and LED2 on the front panel to indicate power status.

Petre Tzv Petrov was a researcher and assistant professor in Technical University of Sofia (Bulgaria) and expert-lecturer in OFPPT(Casablance), Kingdom of Morocco. Now he is working as an electronics engineer in the private sector in Bulgaria.

#### This article was first published on 13 August 2019 and was updated on 4 March 2021.

1. I want the output voltage to be stepped up to 110v from 24v and current should be 2 amps . Can you suggest some ideas using this circuit.

2. Vce of 34063A output transistor is rated for 40V max. So this circuit can be used for output voltage less than 40V, by the diode drop and the output ripple voltage.

3. Hii Sir, This is vijay. I am an Engineering Student. We have microprojects . So we decided to do your project . So you have uploaded the pdf format of pcb layout . But i need Gerber file format . Kindly help me

4. If rather than designing PCB from layout if I directly use wire on zero PCB, will my circuit get complicated?

5. Hello,
I am not an engineer, but can this design affect the audio sampling frequency of 96kHz? Somewhere I have read that if the the switch of the regulator is below the desired audio sampling frequency it will degrade the performance of the sampled signal. The datasheet of MC34063A states the max sampling frequency is 100kHz. If this is true, will this regulator and whole design be enough for 24bit/96kHz audio recording with microphone?

Thank you.

6. How can we increase current up to 500 mA for 40V output. If yes can you help me with the range of Inductor value to be used.

7. Sadly, this IC isn’t suitable for use at 48V output – it’s practical maximum is about 36V. There are other, better, simpler circuits for 48V phantom supplies. Mine uses a few discrete transistors and an inductor, and will provide up to 2A at precisely 48V with a ripple of ~220µV at full load. It costs less to build than the price of the Motorola IC you’re mis-using!