Circuit Design Guide for DC/DC Converters（1/10）
What is DC/DC Converter?
This manual provides tips for designing the circuits of DC/DC converters. How to design DC/DC converter circuits that satisfy the required specifications under a variety of constraints is described by using concrete examples as much as possible.
The properties of DC/DC converter circuits (such as efficiency, ripple, and load-transient response) can be changed with their external parts. Optimal external parts are generally dependent of operating conditions (input/output specifications). The power supply circuit is often used as a part of the circuits of the commercially available products and must be designed so that it satisfies the constraints such as size and cost as well as the required electrical specifications. Usually, the standard circuits listed on the catalogs have been designed by selecting such parts that can provide reasonable properties under the standard operating conditions. Those parts are not necessarily optimal under individual operating conditions. Therefore, when designing individual products, the standard circuits must be changed according to their individual specification requirements (such as efficiency, cost, mounting space, etc.). Designing the circuit satisfying the specification requirements usually needs a great deal of expertise and experience. In this manual, which parts to be changed and how to change them to implement required operations, without expertise and experiences, are described by using concrete data. You will be able to operate your converter circuits quickly and successfully without performing complicated circuit calculations. You may verify your design either by carefully calculating later by yourself or having personnel with expertise and experience review for you if you feel uncertain.
Types and Characteristics of DC/DC Converters
DC/DC converters are available in two circuit types:
- Non- Isolated types:
- Basic (one coil) type
- Capacity coupling (two-coil) type ―― SEPIC, Zeta, etc.
- Charge pump (switched capacitor/coil less) type
- Isolated types:
- Transformer coupling types―― Forward transformer type
- Transformer coupling types―― Fly-back transformer type
Characteristics of individual types are shown in Table 1.
|Circuit type||No. of parts
With the basic type circuit, the operation is limited to either stepping up or stepping down to minimize the number of parts, and the input side and the output sides are not insulated. Figure 1 shows a step-up circuit and Figure 2 shows a step-down circuit. These circuits provide advantages such as small size, low cost and small ripples, and the demand for them is increasing in accordance with the needs for downsizing of equipment.
Figure 1: Step-up Circuit
Figure 2: Step-down Circuit
With SEPIC and Zeta, a capacitor is inserted between VIN and VOUT of the step-up circuit and the step-down circuit of the basic type, and a single coil is added. They can be configured as step-up or step-down DC/DC converters by using a step-up DC/DC controller IC and a step-down DC/DC controller IC, respectively. However, as some DC/DC controller ICs do not assume to be used with these circuit types, make sure your DC/DC controller ICs can be used with these circuit types. The capacitor coupling two-coil type has an advantage to allow insulation between VIN and VOUT. However, the increased coils and capacitors will reduce the efficiency. Especially, at the step-down time, the efficiency is substantially reduced, usually to about 70% to 80%.
The charge pump type requires no coil, enabling to minimize the mounting area and height. On the other hand, this type is not liable to provide high efficiency for the applications that need a wide variety of output powers or larger currents, and is limited to applications for driving white LED or for the power supply of LCD.
The insulated type circuit is also known as the primary power supply (main power supply). This type is widely used for the AC/DC converters that generate DC power mainly from a commercially available AC source (100V to 240V) or for the applications that require the insulation between the input side and the output side to eliminate noises. With this type, the input side and the output side are separated by using a transformer, and the stepping up, stepping down, or reverse operation can be controlled by changing the turns ratio of the transformer and the polarity of the diode. Therefore, you can take out many power supplies from a single power circuit. If fly-back transformer is used, the circuit can be composed of a relatively small number of parts and may be used as a secondary power supply (local power supply) circuit. Fly-back transformer, however, requires void to prevent magnetic saturation in the core, increasing its dimensions. If forward transformer is used, a large power source can be easily retrieved. This circuit, however, requires a reset circuit on the primary side to prevent magnetization of the core, increasing the number of parts. Also, the input side and the output side of the controller IC must be grounded separately.
Basic Operation Principles of DC/DC Converters
The operating principles of stepping up and stepping down in DC/DC converter circuits will be described using the most basic type. Circuits of other types or those using coils may be considered composed of a combination of step-up circuit and step-down circuit or their applied circuits.
Figure 3 and Figure 4 illustrate the operations of a step-up circuit. Figure 3 shows the current flow when the FET is turned on. The broken line shows a slight leak current that will deteriorate the efficiency at the light- load time. Electric energy is accumulated in L while the FET is turned on. Figure 4 shows the current flow when the FET is turned off. When the FET is turned off, L tries to keep the last current value and the left edge of the coil is forcibly fixed to VIN to supply the power to increase the voltage to VOUT for step-up operation. Therefore, if the FET is being turned on longer, much larger electric current is accumulated in L, allowing retrieval of larger power. However, if the FET is being turned on too long, the time to supply the power to the output side becomes too short, and the loss during this time is increased, deteriorating conversion efficiency. Therefore, the maximum duty (ratio of on/off time) value is generally determined to keep an appropriate value.
With step-up operation, the current flows shown in Figure 3 and Figure 4 are repeated:
Figure 3: Current flow when the FET is turned on in a step-up circuit
Figure 4: Current flow when the FET is turned off in a step-up circuit
Figure 5 and Figure 6 illustrate the operations of a step-down circuit. Figure 5 shows the current flow when the FET is turned on. The broken line shows slight leak current that will deteriorate the efficiency at the light-load condition. Electric energy is accumulated in L while the FET is on and is supplied to the output side. Figure 6 shows the current flow when the FET is turned off. When the FET is turned off, L tries to keep the last current value and turns on the SBD. At this time, the voltage at the left edge of the coil is forcibly dropped below 0V, reducing the voltage at VOUT. Therefore, if the FET is being turned on longer, much larger electric current is accumulated in L, allowing retrieval of larger power. With a step-down circuit, while the FET is being turned on, power can be supplied to the output side, and the maximum duty needs not to be determined. Therefore, if input voltage is lower than output voltage, the FET is kept on. However, as the step-up operation is disabled, the output voltage is also lowered to the input voltage level or less.
With the step-down operation, the current flows shown in Figure 5 and Figure 6 are repeated:
Figure 5: Current flow when the FET is turned on in a step-down circuit
Figure 6: Current flow when the FET is turned off in a step-down circuit
4 Critical Points in Designing DC/DC Converter Circuits
Among specification requirements for DC/DC converter circuits, the following are considered critical:
- Stable operation (Not to be broken down by operation failure such as abnormal switching, or burnout or over-voltage)
- High efficiency
- Small output ripple
- Good load-transient response
These properties can be improved to some extent by changing the DC/DC converter IC and external parts. Weightings of these four properties vary with individual applications. In the following, let’s consider how to select individual parts to improve these properties.