Today’s continuation of a DIY signal generator based on an AD9833 is going to be about a dual rail 12V power supply based on TPS65131 IC. In the previous part I have talked about a 5V dual power supply used to power the variable gain amplifier and other circuity. This dual 12V PSU will be mainly used to power the offset control amplifier.
Links to project’s all posts
- VCA822 Gain Amplifier Circuit
- LM7171 Offset Circuit
- Gain and Offset Control Filter Circuit
- Dual 5V Power Supply
- Dual 12V TPS65131 Power Supply (this post)
- Battery Charging Circuit with BQ24295
- Basic WEB Interface
- IPS Capacitive LCD on an ESP32
- IPS LCD, ESP32 with eSPI library and Touch screen
- Final PCB Design for the DIY Waveform Generator
- Custom Design PCBs and How To Get Them Manufactured
- Soldering the PCB
- AD9833 Library and Further Output Noise Reduction
- Arduino BQ24295 Battery Charger Library
- LCD GUI with LVGL on ESP-32
- 3D Printed Enclosure
- Finished DIY generator
The Circuit
So, as mentioned earlier, for this circuit I have used TPS65131 made by Texas Instruments. It has 2.7 – 5.5V input range, so it is a great chip for a battery powered device. Its output voltages can go up to +- 15V – which is more than I need. In this case output values will be set to around +- 12V. Finally, full initial schematic looked like this:
The schematic was taken from a TPS65131 datasheet. The problem is that I have looked into an example schematic witch was used for IC’s parameter measurement instead of an application’s schematic. It was not a big problem. Two additional 6.8 and 7.5 pF capacitors were placed in parallel respectively to R4 and R3. Also, I have made a mistake in the schematic – Q1 MosFet is placed backwards. So, at the end, I have just shorted it out.
A PCB again was self-made. I have written a tutorial how it can be done either by etching with chemicals (used in this case) or with a CNC router.
The chosen IC is in a tiny footprint which is hard to solder – I had to use a soldering hot air gun. After soldering everything in, I have connected all pins ENP, PSP, ENN and PSN to Vin – which enabled negative and positive outputs and it also put them into a power save mode. Also, in this mode both converters have better efficiency with low output currents.
R3 and R4 were chosen 1MOhm, while R2 – 100kOhm and R5 – 110kOhm. These values theoretically should give -12.13V and +12.24V output voltages.
Testing it out
After powering the PSU up, I have noticed yet another problem. When negative output was with no load it outputted around -12.16V, but when a load was applied, it changed output voltage to around -15V… The culprit of the problem was capacitor in parallel to R3 resistor (which is not shown in the initial schematic). It is possible that instead of using a single 7.5pF capacitor and using two capacitors of 3pF and 4.7pF in parallel, made the circuit to misbehave. After desoldering the capacitors, the problem was gone. So, in the future I should test if it will work with a single 7.5pF capacitor as I didn’t have it during the circuit test.
Circuit’s real output voltages were:
- From -12.1 to -12.17V (1kOhm to 100 Ohm load) on the negative output
- From +12.47 to +12.55V on the positive output
It was noticeable, that output value modulus was higher when additional capacitor in parallel to R4 or R3 was used.
Efficiency measurement
Eficiency was measured similarly to dual 5V PSU, which such connection:
Note, that input voltage was in the range of 3.2 – 3.7V. Below is efficiency graph:
So, average efficiency of this circuit is around 80% which is quite good, knowing that the circuit boosts the input voltage more than 3 times. Usually, the higher the difference between input and output voltages, the harder for a converter to keep high efficiency and output current.
Note, that NRS5024T4R7MMGJ inductors were used. Efficiency will depend on chosen inductors (and Schottky diodes). As I am going to search for smaller inductors than the ones used in this PCB, I should also measure efficiency values again in the future.
Summary
So, to sum up, yet another part of DIY Signal generator’s circuit has been made. This circuit – the dual 12V regulator will be used to supply all ICs which need this kind of voltages to operate.
The regulator could supply more than a needed 100mA at the DIY signal generator’s output.
Also, in my opinion, this circuit has quite high efficiency and is suitable for a battery use.
Before designing a final full schematic for the signal generator, I should test the circuit with other inductors and/or Schottky diodes which I also should write about (or add additional info to this post).
