Invertering converter is a special kind of switching power supply that is rarely used. Its output voltage polarity is opposite to that of the input voltage, and the absolute value of the output voltage can be higher or lower than the input voltage. Therefore, in some In terms of classification, it is also classified as a "buck-boost converter", that is, a power converter that can step up or step down, but generally speaking, it is still more commonly used to generate a voltage that is opposite in polarity to the input voltage.

The so-called opposite polarity is actually aimed at the same reference ground potential. If the input voltage is positive, the output voltage is negative.

So when will negative voltage be used?

In the very early days, some DRAM chips required several sets of different power supply voltages, one of which was a negative voltage. For example, the DRAM chip 4116 used in the Apple II computer (one with only 16 K bits, that is, 2 K bytes) requires three sets of power supplies: +12 V, +5 V and -5 V, so the Apple II motherboard power supply There is a -5 V output on it. Of course, this was a requirement a long time ago. With the progress of semiconductor manufacturing process, even if the later ICs have negative voltage requirements, they can use the charge pump circuit on the chip to generate the required negative voltage, and there is no need to connect an external negative power supply. .

In addition to DRAM, there was a very fast logic circuit IC called ECL in the early days (its logic level is very different from that of ordinary logic ICs, and its operating voltage is negative, so a negative power supply is required). The transfer delay of CMOS circuits is more than ten or twenty ns, and the transfer delay below 2 ns can be achieved.

In the 1980s, many supercomputers were designed using this transistor logic IC called ECL (emitter-coupled logic). However, with the advancement of semiconductor manufacturing processes, the speed of CMOS logic is getting faster and faster, and it has the advantage of low power consumption. ECL It is also seen less and less.

As for analog circuits, common bipolar amplifier circuits also require negative voltages, so that both input and output signals can swing between positive and negative voltages, but since analog circuits are sensitive to power supply noise, generally if it is an operational amplifier A dual power supply is required, and a switching power supply will not be used, because the switching power supply must have output ripples. The common practice is to offset the input and output to 1/2 VCC, and use VCC as the positive power supply and GND as the negative power supply. standard to use.

Because of the above reasons, there are fewer and fewer occasions where negative power supplies are used in current circuit designs, so there are fewer and fewer opportunities for inverting converters to be used, so we put it at the end of this series.

Circuit configuration 

The figure above is a simplified inverting converter circuit. If you have read this series of readers, you must be familiar with this circuit. It looks a bit like a buck converter and a bit like a boost converter.

In fact, whether it is a buck, boost, or an inverting converter, the working principle is the same: in a certain cycle, the energy is stored in the inductor first, and in another cycle, the energy is released from the inductor, and when the energy is stored, The load is temporarily powered by the output filter capacitor.

Let's see how the inverting converter works. 

In the initial energy storage cycle, the switch is closed, the current flows from the input to the inductor, and the energy is stored in the inductor, while the diode in the figure limits the current charging the inductor, which can only flow to the inductor and not to the output.

Next, the toggle switch is disconnected. At this time, because the inductance has the characteristic of "not allowing the current to change discontinuously", it will make the above current continue to flow in the same direction, and because the rate of change of the current will change from "increase" to "decrease", according to our previous The principle of the switching power supply circuit that has been deduced many times, when the rate of change of the current changes, the direction of the voltage on the inductor will also reverse.

Therefore, in this cycle, the current on the inductor continues to flow in the same direction, but the voltage on the inductor is reversed, and this reversed voltage flows to the output, while charging the output filter capacitor.

Back to the energy storage cycle of the inductor. At this time, since the output filter capacitor has been fully charged in the previous cycle, the charge in the filter capacitor can continue to supply the current required by the load during the energy storage cycle.

By repeating the second and third states above, the power with reversed polarity can be continuously supplied to the load.

As we said before, the reason why the output voltage of the boost circuit must be higher than the input voltage is because when the inductor is in the discharge cycle, the voltage on the inductor is connected in series with the input voltage, but in the inverting structure, when the inductor discharges, it only passes through the diode It is connected in series with the load without the intervention of the input voltage, so the output voltage range of the inverting converter is from 0 to negative infinity. As for how negative it is, it is also determined by the duty cycle of the inductor charging and discharging.

output voltage control

As usual, I skip the derivation process and tell the reader the result directly:

VOUT / VIN = -D /(1-D)

D is our previous duty cycle: D = TON/ (TON + TOFF)

If we draw the ratio of the output voltage to the input voltage above, we will get:

It can be seen that this curve is very similar to the control curve of the boost converter, because they both have a term (1-D) in the denominator, so the drawn curve will be part of the hyperbola.

Like the boost converter, although the output voltage "seems" to reach infinite or negative infinity, when the duty cycle exceeds a certain ratio (about 0.9), the voltage will rise rapidly. In this interval, as long as the duty cycle changes a little, the output voltage will change greatly, which will cause difficulties in the design of the control circuit. Therefore, like the boost converter, when we design the inverting converter, we will generally limit the duty cycle to 0.8, and even lower ratios below.

Circuit Combat

When we introduced the boost or buck converter before, we used MC34063A to demonstrate the actual working circuit. Whether it is buck or boost, their control characteristics are "the longer the TON time, the higher the output voltage", so the two can be controlled by the same control logic: "The output voltage is lower than the target voltage, and the TON will be longer. A little; if the output voltage is higher than the target voltage, TON will be turned on a little bit or even not turned on.”

So can MC34063A be used as an inverting converter? Let's take a look at the control curve of the inverting converter. It is actually very similar to the boost converter. The larger the duty cycle, the larger the output voltage, but the larger the "negative".

Therefore, MC34063A can also be used as an inverting converter design, but special attention should be paid to the arrangement of the feedback voltage. Because when the output is a negative voltage, the feedback voltage generated by the feedback voltage divider resistor will also be negative, but MC34063A cannot accept negative feedback voltage, and the reference voltage of the comparator inside it is still 1.25 V, not -1.25V.

what can we do about it? In fact, we only need to use the internal comparator of MC34063A as the fourth pin GND of the reference ground potential, and connect it to the output voltage. Because VOUT is the lowest voltage in the entire circuit, the voltage of GND in the circuit is actually higher than VOUT, so for MC34063, when connected in this way, the voltage obtained by VFB will become "negative - 1.25 V". positive, the MC34063A will "see" the 1.25 V feedback voltage.

The picture above is the demonstration circuit of the inverting converter in the datasheet of MC34063A. It can be seen that the fourth pin connected to GND in the boost or buck converter circuit is connected to VOUT in the inverting converter circuit, and another point to note is that the oscillator control capacitor used to set the switching frequency CT will also refer to the comparison voltage of 1.25 V, so the negative pole of this capacitor should also be connected to the fourth pin, sharing the same reference potential with the entire control circuit.

Summary

This is the end of this series of articles on switching power supplies. In the past half a year, we have taken readers to see boost converters and buck converters, and also demonstrated power circuits designed with a single IC, or controller circuits that require additional transistors to increase output power. The inverting converter is introduced, which is used to generate a negative voltage opposite to the input voltage.