DC/DC Converters Research Guide - Serial 1
Time：2022.11.22
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DC/DC Converter Research Guide
1. Preface
This document provides some tips for the design of DC/DC converter circuits, and tries to illustrate with specific examples how to design a DC/DC converter circuit that is closest to the required specifications under various constraints.
The various characteristics of a DC/DC converter circuit (efficiency, ripple, load transient response, etc.) can be changed according to the change of peripheral components. Generally, the optimal peripheral components vary according to the conditions of use (input/output specifications). For example, when you ask "How can I improve efficiency?" ", "it depends on the conditions of use" or "it depends on the specific situation", feel like a clever pond in the past, I guess you have encountered such a situation. So why the answer? The reason is that most power circuits use commercially available commodities as part of the circuit, so we must both consider the size, cost and other constraints and consider the electrical requirements to design specifications. The components selected by the standard circuit in the general product catalog are mostly those that can play the general characteristics under the standard service conditions. Therefore, it is not necessarily the best selection of components under various service conditions. Therefore, in each design, design changes must be made from the standard circuit according to the respective required specifications (efficiency, cost, mount space, etc.). But to be able to design a circuit to the required specifications, you need enough knowledge and experience. This document uses specific values for those who do not have such knowledge and experience to explain which components and how to change to achieve the required action, so that the DC/DC converter circuit can quickly work without complicated circuit calculation. As for the inspection of the design after regular work, you can calculate it carefully later, or you can ask people with rich knowledge and experience to inspect it at the beginning.
2. Types and characteristics of DC/DC converter
DC/DC converter circuit according to its circuit mode mainly has the following:
(1) Non-insulated type -- basic (single coil) type
-- Capacitive Coupling Double Coil -- SEPIC, Zeta,...
-- Charge pump (switching capacitor/wireless coil) type
(2) insulation type - transformer coupling type - positive
-- Back sweep
Table 1 shows the strengths of each method.
Basic type means a type that minimises the number of components by limiting circuit operation to boost or drop voltage only, with no electrical insulation on the input and output sides. Figure 1 shows the booster circuit and Figure 2 shows the step-down circuit. These circuits have the advantages of small size, cheap, small ripple and so on. With the miniaturization of equipment, the need for them is increasing.
SEPIC and Zeta insert capacitor and add a coil between VIN-VOUT of basic booster circuit and step-down circuit respectively. Moreover, both can be used by boosting DC/DC converter to control IC, step-down DC/DC converter to control IC to form a lift voltage DC/DC converter. But some DC/DC converter control IC is not designed to be used in these circuits, so it needs to be paid attention to when choosing. These capacitive coupling double coil types have the advantage that VIN-VOUT can be insulated. However, due to the increase of coils and capacitors, the efficiency will become low, especially in the voltage reduction, the efficiency is also greatly reduced, which is usually about 70% ~ 80%.
Charge pump type because it does not need coil, so its advantage is that the mount area, mount height are small, but because of its variety of output voltage and large current is not easy to make a circuit with good efficiency, so it is also used to be limited in the white LED driver and LCD power supply side.
The insulated type is also known as the primary power supply (main power supply), which is widely used in AC/DC converter from commercial power supply (AC100V ~ 240V) to DC power supply, and when the input side and output side need insulation due to noise removal and other reasons. Because they use a transformer to separate the input side and the output side, it can be by changing the transformer turns ratio and diode polarity to form the boost/step-down/reverse control, so that multiple power supplies can be obtained from a power supply circuit. In particular, the energy used in a sweep back transformer consists of fewer components and is sometimes used as a secondary power supply (local power supply) circuit. However, because the sweep transformer needs to be used to prevent the internal nuclear magnetic saturation gap, so the external size is large. Although the forward transformer is easy to obtain high power supply, it needs a recovery circuit to prevent the core magnetization in the primary side, so the number of components increases. The converter control IC also requires GND separation on the input side and the output side.
The above content is from Tris Semiconductor Co., LTD. In serial...
