Column: Circuit drill
Optimising the smallest negative power supply circuit
By Sulaiman Algharbi Alsayed, Managing Director,
Smart PCB Solutions Company, Jordan
M any integrated circuits still need both
positive and negative voltage supplies, +VCC and -VCC
. To obtain these, there are several
options, including using two batteries connected in series (Figure 1) and a negative voltage supply generator circuit.
T e fi rst option is expensive, as two batteries are required instead
of one, and also it adds to the circuit’s size and weight, which is not preferable. T e option-two circuits generate a negative voltage when a
positive voltage is applied to their input. Many designers prefer this option since it eliminates the need for a second battery, it weighs less and it’s smaller and cheaper. Several circuits can be used to generate negative voltage; the
simplest, cheapest and lightest is the Schmitt-trigger inverter circuit. T e circuit in Figure 2 is the perfect choice for generating
negative voltages in small and low-cost circuits. Since the generated negative voltage ranges from -0.1V to 4.8V, depending on the C1 and R1 values, our challenge is to select the best ones to obtain the maximum negative output voltage.
Methodology To maximise the negative voltage from the circuit in Figure 2, it is very important to understand its operation. T e fi rst part is a simple oscillator, with the rest of the circuit 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 so that this
section generates the maximum oscillation voltage. T e 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. T e magnitude of the signal generated in section 1 is measured
with various C1 and R1 values. As their values were changed, the oscillation signals (in that section) were measured, helping identify the best R1 and C1 values for a maximum oscillation signal.
Assumptions • T roughout this experiment, R1 ranged from 100 Ohms to 100k Ohms.
• For C1, we used between 1nF and 500nF. • Capacitors C2 and C3 were 100nF and 300nF, respectively. • Diodes D1 and D2 were chosen to be 1N4001, a general-purpose diode.
12 July/August 2021
www.electronicsworld.co.uk
Figure 3: Oscillation signal magnitude profi le for various C1 and R1 values
Impacts Figure 3 shows the oscillation signals for various C1 and R1 values, with the red trace applying to signals below 5V, yellow to signals of 5V, orange to signals between 5V and 5.4V, and green for signals above 5.4V. From the fi gure it can be clearly seen that the maximum oscillation
signal magnitude (5.44V) is for C1 = 5nF and R1 = 78k or 79k. Consequently, the circuit can generate a negative output voltage of 4.73V, which is its maximum.
Figure 2: Schmitt-trigger inverter connected to generate negative voltage
Figure 1: Two batteries connected to generate negative voltage
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