How to choose the best switch mode DC-DC converter?

How to choose the best switch mode DC-DC converter?

By utilizing the energy storage characteristics of capacitors and inductors, high-frequency switching is performed through controllable switches (MOSFETs, etc.) to store input electrical energy in capacitors (inductors). When the switch is turned off, the electrical energy is released back to the load, providing energy as a switching power supply. The ability of its output power or voltage is related to the duty cycle (the ratio of the switch conduction time to the entire switch period). Switching power supplies can be used for boosting and lowering voltage.

DC-DC converters store energy in inductors and generate current when MOSFET switches are closed. When the switch is turned off, the stored inductor energy is output to the load through the diode. As shown in the following figure:

The working principle of the three converters shown is to first store energy and then release it in a controlled manner to obtain the required output voltage. For a certain job, the optimal switch mode DC-DC converter can meet the overall system needs with minimal installation cost. This can be measured by a set of parameters that describe the performance of switch mode DC-DC converters, including high efficiency, small installation size, small static current, small operating voltage, low noise, high functional integration, sufficient output voltage regulation capability, and low installation cost.

Work efficiency

① Inductive DC-DC converter: The conversion efficiency of battery powered inductive DC-DC converters is 80%~85%, and their losses mainly come from external diodes and modulator switches.

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

③ Charge pump with voltage regulation: It is a linear regulator that adds a low voltage drop after the output of the basic charge pump. Although voltage regulation is provided, its efficiency decreases due to the power consumption of the backend 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.

The best choice is: a non voltage regulated charge pump (in applications that do not require strict output regulation), or a voltage regulated charge pump (if the pressure difference across the back-end regulator is small enough).

Installation size

① Inductive DC-DC converters: Although many new types of inductive DC-DC converters can provide SOT packaging, they typically still require physically larger external inductors. Moreover, the circuit layout of inductive DC-DC converters also requires a large board level space (additional decoupling, special ground handling, shielding, etc.).

② Charge pump without voltage regulation: The charge pump does not require an inductor, but requires an external capacitor. The new charge pump device adopts SOP packaging and operates at higher frequencies, so it can use small capacitors (1 μ F) with less space occupation. The space occupied by the charge pump IC chip and external capacitors combined is not as large as the inductance in inductive DC-DC converters. It is also easy to obtain a combination of positive and negative output voltages using a charge pump. The TCM680 device can support an output voltage of+2 UIN using only external capacitors. 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 structure is needed.

③ Charge pump with voltage regulation: Adding a separate backend voltage regulator takes up more space, however, many of these regulators come in SOT form factor packaging, which relatively reduces the occupied space. The new type of charge pump device with voltage regulation, such as TCM850, integrates a charge pump, backend voltage regulator, and shutdown control in a single 8-pin 50lC package.

The best choice is: no voltage regulation or a charge pump with voltage regulation.

Static current

① Inductive DC-DC converter: Frequency Modulation (PFM) Inductive DC-DC converter is a switch mode DC-DC converter with the minimum static current. Voltage regulation through frequency modulation can minimize the supply current at low load currents.

② Charge pump without voltage regulation: The static current of the charge pump is proportional to the operating frequency. Most new charge pumps operate at frequencies above 150kHz, allowing for the use of capacitors as small as 1 μ F or even smaller. To overcome the problem of high static current caused by this, some charge pumps have input pins that are turned off to shut down the charge pump in case of prolonged inactivity, thereby reducing the supply current to near zero.

③ Charge pump with voltage regulation: The backend voltage regulator increases the static current, so the charge pump with voltage regulation is inferior to the basic charge pump in terms of static current.

The best choice is an inductive DC-DC converter, especially a frequency modulation (PFM) switch mode.

Minimum operating voltage

① Inductive DC-DC converter: A battery powered inductive DC-DC converter (such as TC16) can start working at voltages as low as 1V or even lower, making it very suitable for electronic devices powered by a single battery.

② Charge pumps without voltage regulation/charge pumps with voltage regulation: Most charge pumps have a minimum operating voltage of 1.5V or higher, making them suitable for applications with at least two batteries.

The best choice is an inductive DC-DC converter.

Noise generated

① Inductive DC-DC converters: Inductive DC-DC converters are a source of power supply noise and switch radiated noise (EMI). The broadband PFM inductive DC-DC converter generates noise over a wide frequency range. It is possible to increase the operating frequency of the inductive DC-DC converter so that the noise generated falls outside the frequency band of the system.

② Charge pumps without voltage regulation/charge pumps with voltage regulation: Charge pumps do not use inductors, so their EMI effects can be ignored. The pump input noise can be eliminated by a small capacitor.

The best choice is a charge pump without voltage regulation or with voltage regulation.