Improve existing designs using inductors

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Improve existing designs using inductors

Posted Date: 2024-01-25

Figure 1 A simple local low-noise voltage converter that can be used when a simple low-voltage negative supply is required.

Here is the circuit from the previous DI in Figure 1:

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It's simple and can be made more efficient with very quick changes. If the values ​​of R1 and R1′ are fixed (so that the input power is also fixed), the output voltage will have an extreme value on the graph Eo=Eo(R2). To facilitate this extreme, the circuit of Figure 1 can be modified as shown in Figure 2; here you can use a single potentiometer (R2) to change the value of R2/R2'.

Figure 2 adds an inductor to the output of Figure 1 to increase converter efficiency.

But the main change is the addition of inductor L at the output. Rather low inductance values ​​(0.1…1.0 mH) are sufficient. (This low value may be contrary to the low frequency of the multivibrator, which is less than 1 KHz.)

The negative output voltage increases slowly with increasing inductance: from -0.36 V @ L=0.1 mH to -0.4 V @ L=1 mH.

The main advantage is that the output current (voltage) increases by approximately 25%. The circuit in Figure 1 has an output voltage of -0.31 V, while the circuit in Figure 2 can deliver more than -0.39V into the same load (910 Ω).

This increase is due to...well...we'll see the explanation in the comments...

The second improvement is in output noise: the same inductor L significantly reduces output noise - the output capacitor in Figure 2 has only half the capacity, but here the output noise amplitude is half.

Component values ​​are: L=0.1…1.0 mH, R1=R1’=5.6 k, R2 =~22 k, C1=C1’=0.1 ?F. The output capacitor should have low impedance.

This circuit consumes less than 1.5 mA at +5 V and produces more than -0.39 V into a 910 Ω load. A circuit ("Photocell enables op amp to achieve true zero output") has the same output current, consumes approximately 10 times the power, but has approximately 100 times the output noise.

However, there can be a problem with all these circuits: they produce low voltages, which may not matter to the host system, but if the voltage drops in some way, the results will be distorted, and this may go unnoticed.

To ensure that any drop in this voltage is detected, the circuit in Figure 3 can be used. It is useful for monitoring the consistency of the power supply in any dual power supply system.

Figure 3 ensures that the circuitry detects any voltage drops that may distort the results of the converters in Figures 1 and 2.

The green LED indicates "Power Good" and can be used as a "power on" light for the entire host system. Resistors R1 and R2 should be at least 1% stable. The LED should light up when the output voltage grows to e= -20…-100 mV, depending on your buffer parameters.

For the values ​​of R1 and R2, let:

v1 = Vref + |e|,
v2 = 2.5 + |e|, then
R1 = R2 * ((v1 / v2)

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