## Optimising the smallest negative power supply circuit

Series By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions Company

Many integrated circuits (ICs) still need both positive and negative voltage supplies (i.e., +VCC and -VCC). To obtain these, there are several options, including: using two batteries connected in series (Figure 1), and using a negative voltage supply generator circuit.

Option 1 is expensive as two batteries are required instead of one, but it also adds to the circuit’s size and weight, which is not a preferable option.

Option 2 circuits generate a negative voltage at their output when a positive voltage is applied to their input. Many electronic circuit designers prefer this option since it eliminates the need for a second battery set, it weighs less, it’s smaller and cheaper.

Several circuits can be used to generate negative voltage; however, the simplest, cheapest and lightest is the Schmitt-trigger inverter circuit, which can effectively serve as a negative voltage supply generator.

The circuit in Figure 2 is the perfect choice for generating negative voltage supply in small and low-cost circuits. However, the generated negative voltage supply ranges from -0.1V to 4.8V, depending on C1 and R1 values. Hence, our challenge is to select the best C1 and R1 values to obtain the maximum negative voltage at this circuit’s output.

Figure 1: Two batteries connected to generate negative voltage

Figure 2: Schmitt-trigger inverter circuit connected to generate negative voltage

Methodology

To maximise the generated negative voltage from the circuit in Figure 2, it is very important to understand its operation. Section 1 is a simple oscillator, with the rest mostly used to rectify the signal and charge capacitor C3 with a negative voltage (the output).

Our main challenge is to select C1 and R1 sizes where section 1 generates the maximum oscillation magnitude. The higher the oscillation, the more negative voltage the circuit can generate.

Since the signal shape is not important for our application, we can ignore the distortion of the oscillation circuit.

The magnitude of the signal generated in section 1 is measured with various C1 and R1 components. All C1 and R1 values were changed, and with value set, the oscillation signals (in section 1) were measured, and the best R1 and C1 values for a maximum oscillation signal identified.

Assumptions

• Throughout this experiment, 100 Ohms up to 100k Ohms were used for R1.
• Similarly, the C1 values used were between 1nF and 500nF.
• Capacitors C2 and C3 were set at 100nF and 300nF, respectively.
• Diodes D1 and D2 were selected to be 1N4001, which is a general-purpose diode.

Impacts

Figure 3 shows the oscillation signals for various C1 and R1 values, with the red trace applying to signals below 5V, yellow for signals of 5V, orange for signals between 5V and 5.4V, and green for signals above 5.4V.

Figure 3: Oscillation signal magnitude profile for various C1 and R1 values

From the figure it can be clearly seen that the maximum oscillation signal magnitude (5.44V) is at C1 = 5nF and R1 = 78k or 79k. Consequently, the circuit can generate a negative output voltage of 4.73V, which is its maximum. 