Key points and methods for selecting the best DC/DC converter

1. Selection of components

1. Three components of DC-DC power converter

1) Switch: Regardless of the type of DC/DC converter, the components used in the main circuit are only electronic switches, inductors, and capacitors. Electronic switches have only two states: fast on and fast off. Only fast state conversion can reduce the loss. Currently, most electronic switches used are bipolar transistors and power field effect tubes. IGBT tubes are gradually becoming popular, and there are also various new high-power switch components with better characteristics.

2) Inductor: Inductor is a commonly used component in switching power supply. Since its current and voltage phases are different, the theoretical loss is zero. Inductor is often used as energy storage component. It is also often used with capacitor in input filter and output filter to smooth current. It is also called choke. Its characteristic is that the current flowing through it has "large inertia". In other words, due to "flux continuity", the current on the inductor must be continuous, otherwise a large voltage spike wave will be generated. Inductor is a magnetic component, so there is naturally a problem of magnetic saturation. In most cases, the inductor works in the linear region. At this time, the inductance value is a constant and does not change with the terminal voltage and the current flowing through. However, there is a problem that cannot be ignored in switching power supply, which is the phenomenon of two distributed parameters (or parasitic parameters) caused by the winding of the inductor. One is the winding resistance, which is inevitable; the other is the distributed stray capacitance, which depends on the winding process and materials. The stray capacitance has little effect at low frequency, and gradually becomes apparent as the frequency increases. When the frequency is above a certain level, the inductor may become a characteristic of the capacitor. If the stray capacitance is integrated into one, the capacitive nature after one corner frequency can be seen from the equivalent circuit of the inductor.

3) Capacitor: Capacitor is a commonly used component in switching power supply. Like inductor, it is also a component for storing and transmitting electric energy. But its frequency characteristics are just the opposite. In application, it mainly "absorbs" ripple and has the function of smoothing voltage waveform. In fact, capacitor is not an ideal component. Capacitor forms an equivalent series internal resistance ESR due to the dielectric, contact and lead. This equivalent series internal resistance plays an important role in the control of small signals in switching power supply and the design of output ripple suppression. In addition, there is a series inductor in the equivalent circuit of capacitor, which is very important when analyzing the filtering effect of circuit. Sometimes increasing the capacitance value does not make the voltage waveform straight, because this series parasitic inductance has a side effect. The series resistance of the capacitor is related to the contact and lead wire, and also to the electrolyte. The composition of common aluminum electrolytic capacitor is AL2O3, and the conductivity is seven times greater than that of air. In order to increase the capacitance, the surface of aluminum foil is made into a regular uneven shape to increase the surface area of ​​the oxide film, and the added electrolyte can flow on the convex and concave surface. When the high-frequency pulsating current of ordinary aluminum electrolytic capacitors increases significantly, the high-frequency impedance temperature rises greatly, which becomes the bottleneck of the long life of the switching power supply. So-called good capacitors are resistant to reverse current, temperature rise, and have small ESR values. The electrolyte of the capacitor is affected by temperature. When the temperature rises, the resistance decreases, that is, the series resistance of the capacitor decreases, which is ideal. When the temperature rises, the equivalent series resistance increases, resulting in a shortened capacitor life, which is a disadvantage of ordinary aluminum electrolytic capacitors. To improve this disadvantage, the electrolyte is covered on the surface of the oxide film and then dried to form a solid electrolyte capacitor, namely "tantalum capacitor".

2. Key points for device selection

If an external switch tube is used, it is best to choose a switch transistor or a power MOS tube, and pay attention to the withstand voltage and power consumption. If the switching frequency is very high, the inductor can be selected with multiple wires in parallel to reduce the influence of the skin effect. The freewheeling diode is generally a Schottky diode with a short recovery time and a small forward conduction voltage, but attention should be paid to the withstand voltage. If the output voltage is very small (a few tenths of a volt), a MOS tube must be used for freewheeling. The output filter capacitor generally uses a high-frequency capacitor, which can reduce the output ripple and reduce the temperature rise of the capacitor. A 0.1~1μf capacitor in parallel with the upper arm resistor of the sampling circuit can improve the transient response. The following points should be noted when selecting components for power supply design:

1) Choose a DC/DC converter with greater design flexibility to expand the scope of circuit design;

2) Low current consumption and high efficiency can extend the battery life;

3) Small external components can be used to achieve product miniaturization;

4) Powerful technical support tools.

2. Key points for selecting the best DC/DC converter

The best switching DC/DC converter is the one that can meet the overall needs of the system with the lowest installation cost. This can be measured by a set of parameters that describe the performance of the switching DC/DC converter, including: high efficiency, small installation size, small quiescent current, small operating voltage, low noise, high functional integration, sufficient output voltage regulation capability, and low installation cost.

1. Work efficiency

① Inductive DC/DC converter: The conversion efficiency of the battery-powered inductive DC/DC converter is 80%~85%, and its loss mainly comes from the external diode and modulator switch.

② Charge pump without voltage regulation: It is a basic charge pump (such as TC7660H). It has a very high power conversion efficiency (generally more than 90%), because the loss of the charge pump mainly comes from the ESR of the capacitor and the on-resistance (RDS-ON) of the internal switch tube, both of which can be made very low.

③ Charge pump with voltage regulation: It is a linear regulator with low voltage dropout added after the output of the basic charge pump. Although voltage regulation is provided, its efficiency is reduced due to the power consumption of the back-end regulator. To achieve the highest efficiency, the output voltage of the charge pump should be as close as possible to the voltage regulated by the back-end regulator.

2. Installation dimensions

① Inductive DC/DC converters: Although many new inductive DC/DC converters can provide SOT packages, they usually still require external inductors with larger physical size. In addition, the circuit layout of the inductive DC/DC converter itself also requires a larger board space (additional decoupling, special ground wire processing, shielding, etc.).

② Charge pump without voltage regulation: The charge pump does not use an inductor, but requires an external capacitor. The new charge pump device uses an SOP package and operates at a higher frequency, so a small capacitor (1μF) that takes up less space can be used. The space occupied by the charge pump IC chip and the external capacitor is not as large as the inductor in the inductive DC/DC converter. It is also easy to obtain a positive and negative combination of output voltages using a charge pump. For example, the TCM680 device can support an output voltage of +2 UIN with only an external capacitor. To obtain the same output voltage using an inductive DC/DC converter, two independent converters are required. If one converter is used, a transformer with a complex topology must be used.

③ Charge pump with voltage regulation: Adding a discrete back-end voltage regulator takes up more space, but many of these regulators have SOT packages, which relatively reduces the space occupied. New charge pump devices with voltage regulation, such as the TCM850, integrate a charge pump, back-end voltage regulator, and shutdown control in a single 8-pin 50lC package.

3. Quiescent current

① Inductive DC/DC converter: Frequency modulation (PFM) Inductive DC/DC converter is a switching DC/DC converter with the smallest quiescent current. Voltage regulation through frequency modulation can minimize the supply current under small load current.

② Charge pump without voltage regulation: The quiescent current of the charge pump is proportional to the operating frequency. Most new charge pumps operate at frequencies above 150kHz, so capacitors of 1μF or even less can be used. To overcome the problem of high quiescent current, some charge pumps have a shutdown input pin to shut down the charge pump when idle for a long time, thereby reducing the supply current to near zero.

③ Charge pump with voltage regulation: The back-end voltage regulator increases the quiescent current, so the charge pump with voltage regulation is worse than the basic charge pump in terms of quiescent current.

4. Minimum operating voltage

① Inductive DC/DC converter: Battery-powered dedicated inductive DC/DC converters (such as TC16) can start working at a voltage as low as 1V or even lower, so they are very suitable for single-cell battery-powered electronic devices.

② Charge pump without voltage regulation/charge pump with voltage regulation: The minimum operating voltage of most charge pumps is 1.5V or higher, so they are suitable for applications with at least two batteries.

5. Noise generated

① Inductive DC/DC converter: Inductive DC/DC converter is the source of power supply noise and switching radiation noise (EMI). Wideband PFM inductive DC/DC converter will generate noise in a wide frequency band. The operating frequency of the inductive DC/DC converter can be increased so that the noise it generates falls outside the frequency band of the system.

② Charge pump without voltage regulation/charge pump with voltage regulation: The charge pump does not use an inductor, so its EMI impact can be ignored. The pump input noise can be eliminated by a small capacitor.

6. Integration

① Inductive DC/DC converter: Chips that integrate switching regulators and other functions (such as voltage detectors and line regulators) have been developed. For example, the TC16 chip integrates a PFM boost converter, LD0 and voltage detector in an SO-8 package. Compared with discrete implementations, such devices provide excellent electrical performance and take up less space.

② Charge pump without voltage regulation: Basic charge pump, such as TC7660, has no additional functions integrated and occupies little space.

③ Charge pump with voltage regulation: The integration of more functions of charge pump chips with voltage regulation has become a development trend. Obviously, the functional integration of the next generation of charge pump with regulation will be comparable to that of the integrated chip of inductive DC/DC converter.

7. Output Regulation

① Inductive DC/DC converter: Inductive DC/DC converter has good output regulation capability. Some inductive DC/DC converters also have external compensation pins, allowing "fine tuning" of the output transient response characteristics according to the application.

② Charge pumps without voltage regulation: These devices have no voltage regulation at the output. They simply convert the input voltage to a negative or multiple output voltage. Therefore, the output voltage will drop as the load current increases. Although this is not a problem for some applications (such as LCD bias), it is not suitable for applications that require a stable output voltage.

③ Charge pump with voltage regulation: It provides voltage regulation (stabilization) through a back-end linear voltage regulator (on-chip or external). In some cases, it is necessary to increase the number of switching stages for the charge pump to provide enough headroom for the back-end regulator, which requires the addition of external capacitors, which has a negative impact on size, cost, and efficiency. However, the back-end linear regulator can make the output voltage stability of the regulated charge pump the same as that of the inductive DC/DC converter.

8. Installation costs

① Inductive DC/DC converter: In recent years, the cost of using inductive DC/DC converters has gradually decreased, and the demand for external components has also become less. However, an inductive DC/DC converter requires at least an external inductor, capacitor and Schottky diode. The total cost of the diode, inductor, and relatively expensive switch conversion chip is higher than that of a charge pump.

② Charge pump without voltage regulation: Charge pump without voltage regulation is cheaper than inductive DC/DC converter and only requires external capacitors (no inductor), saving board space, inductor cost, and shielding cost in some cases.

③ Charge pump with voltage regulation: The cost of a charge pump with voltage regulation is roughly equivalent to the cost of the inductive switching DC/DC converter itself. In some cases, an external back-end voltage regulator can be used to reduce costs, but it will increase the required installation space and reduce working efficiency.

3. Ways to choose the best DC/DC converter

1. For efficiency, the best DC/DC converter choice is: without voltage regulation charge pump (in applications that do not require strict output regulation) or with voltage regulation charge pump (if the voltage drop across the back-end regulator is small enough).

2. In terms of installation size, the best DC/DC converter choice is: without voltage regulation or with voltage regulation charge pump.

3. For the quiescent current, the best DC/DC converter choice is: inductive DC/DC converter, especially frequency modulation (PFM) switching type.

4. Regarding the minimum operating voltage, the best DC/DC converter choice is: inductive DC/DC converter.

5. Regarding the noise generated, the best DC/DC converter choice is: no voltage regulation or charge pump with voltage regulation.

6. Regarding the integration level, the best choice for DC/DC converter is: inductive DC/DC converter.

7. For output regulation, the best DC/DC converter choice is a charge pump with voltage regulation.

8. Regarding installation cost, the best choice of DC/DC converter is: when strict voltage regulation is not required, the best choice is a charge pump without voltage regulation; if output voltage regulation is required, the cost of a charge pump with voltage regulation and an inductive DC/DC converter are roughly the same.