Negative resistance and why your DC-DC converter might not work properly

If your DC-DC converter is not working and you check for obvious problems (not plugged in, not opened, blown input fuse, etc.), the problem may be input impedance caused by negative resistance. A common solution is to AC couple resistors to the circuit, or separate resistors from the circuit. This can be done by placing the correct capacitor as close to the input pin of the DC converter as possible.

introduce

Many of us have experience buying DC-DC converters, and it works just fine; Ideally, this is the norm. Unfortunately, most of us have also had the experience of a DC-DC converter not working properly, and we can identify the source of the problem and find an appropriate solution. The problem may be fundamental; The converter is not plugged in, the power is not turned on, or the input fuse is blown. The basic problems are relatively easy to spot and solve. For DC input switching power supplies, the situation is often more complicated; Everything is connected correctly and simple problems have been eliminated, but the converter still cannot produce the correct output voltage. In this case, the problem may be that the input impedance of the DC-DC converter interacts with the output impedance of the power supply supplying the converter. When this impedance problem exists, the output voltage of the converter will have a significant AC component, and the input voltage of the converter will also have a significant AC signal.

To simplify this discussion, we will consider the constant impedance load applied to the output of the DC-DC converter. The power provided to a constant impedance load is also constant because the output voltage of the dc-dc converter is constant. Constant power provided to the output load means that constant power is drawn from the input end of the converter. Now the fun begins! We create a power load (a dc-dc converter with a constant output power load) that absorbs constant power independent of the applied voltage (the load on a dc-dc converter is constant, the power conversion efficiency is constant, and therefore the power drawn by the input to the converter is constant). As we shall see, having a load draw constant power independent of the applied voltage is an interesting situation.

Negative resistance

For this type of load, the input current drops when the input voltage applied to the dc-dc converter rises, and rises when the input voltage applied to the converter drops. The case where the incremental current and voltage move in reverse is the behavior of negative incremental resistance. For the more familiar positive incremental resistance, a rise in the applied voltage causes the applied current to increase and a fall in the applied voltage causes the applied current to decrease.

oscillation

We will now discuss one of the potential effects of negative incremental input resistance. It would be helpful to know the behavior of a circuit made up of capacitors and inductors. If energy is applied to the capacitor and inductor circuits, the energy will be exchanged between the electric field associated with the capacitor (the voltage at both ends of the capacitor) and the magnetic field associated with the inductor (the current through the inductor). This energy exchange will be expressed as the sinusoidal voltage at both ends of the element and the current flowing through the element, that is, the behavior of the oscillating circuit.

In real circuits with capacitors and inductors there are also parasitic resistances associated with components. This parasitic resistance consumes energy, and the oscillation eventually stops.

Negative drag and oscillation

If a negative resistance is added to the RLC circuit, it can eliminate the positive resistance and create a circuit with zero resistance, and can continue to oscillate. A circuit with capacitance, inductance, and zero resistance may occur correctly (or incorrectly) at the input of a DC-DC converter under certain operating conditions. Oscillations will persist when the negative input impedance of the DC-DC converter cancels out the positive impedance of the associated capacitors and inductors.

It should be understood that the capacitance and inductance of a circuit can be physical elements (intentional or parasitic) or can be synthesized from the output impedance of the power supply and the input impedance of the dc-dc converter. The resultant negative resistance and possible resultant reactance elements have the property that the value of the resultant element changes as the operating conditions of the system change. One challenge of this situation is that it is difficult to model systems associated with DC-DC converters precisely to determine under what operating conditions oscillations will occur. Another challenge in this case is that oscillations are present in some operating conditions and not in others.

Active resistance to damped oscillations

Although it may be difficult to predict operating conditions that will cause oscillations, it is relatively easy to add a positive resistance between the output of the power supply and the input of the DC-DC converter so that the oscillations do not persist. The two options for increasing resistance are to connect a resistance element in series between the power supply and the DC-DC converter, or to connect a resistance element in parallel between the output of the power supply and the input of the DC-DC converter. Unfortunately, the power consumed by any one of these solutions is usually too large to be acceptable. A third common solution is to pair resistors AC into circuits or DC block resistors in circuits. The benefit of this coupling or blocking is that the resistance will affect the AC signal (oscillation) and not the DC signal (the desired power flow). One way to achieve this desired resistance is to place electrolytic capacitors with large capacitance values (tens to hundreds of microfarads, depending on the power level) as close as possible to the input pins of the DC-DC converter. The equivalent series resistance (ESR) of a capacitor is ac-coupled into the circuit and is used to dissipate enough energy to prevent sustained oscillations, but is isolated from the DC power supply path and therefore does not dissipate the energy associated with the DC power supply. Depending on power level) as close to the input pins of the DC-DC converter as possible. The equivalent series resistance (ESR) of a capacitor is ac-coupled into the circuit and is used to dissipate enough energy to prevent sustained oscillations, but is isolated from the DC power supply path and therefore does not dissipate the energy associated with the DC power supply. Depending on power level) as close to the input pins of the DC-DC converter as possible. The equivalent series resistance (ESR) of a capacitor is ac-coupled into the circuit and is used to dissipate enough energy to prevent sustained oscillations, but is isolated from the DC power supply path and therefore does not dissipate the energy associated with the DC power supply.

The ESR of the capacitor needs to be small enough not to consume excessive power, and large enough to effectively suppress oscillations. The most commonly used electrolytic capacitors have the correct number of ESRs and can be used in this application. Very low cost electrolytic capacitors may have too high ESR, resulting in too much power consumption. The ESR values of very high cost electrolytic capacitors, film capacitors and ceramic capacitors may be too low to suppress oscillations properly.