We compare different types of DC to DC converters, including linear vs. switch mode, buck, boost, buck-boost, and transformer-based isolated so you can pick the right one.
What is a DC Converter? Linear vs. Switch Mode Converters
A DC/DC converter is class of power supply that converts a source of direct current (DC) from one voltage level to another. There are two types of DC/DC converters: linear and switched. A linear DC/DC converter uses a resistive voltage drop to create and regulate a given output voltage, a switched-mode DC/DC converts by storing the input energy periodically and then releasing that energy to the output at a different voltage. The storage can be in either a magnetic field component like an inductor or a transformer, or in an electric field component such as a capacitor. Transformer-based converters provide isolation between the input and the output.
Switch mode DC/DC converters offer three main advantages:
- The power conversion efficiency is much higher.
- Because the switching frequency is higher, the passive components are smaller and lower losses simplify thermal management.
- The energy stored by an inductor in a switching regulator can be transformed to output voltages that can be smaller than the input (step-down or buck), greater than the input (boost), or buck-boost with reverse polarity (inverter).
Unlike a switching converter, a linear converter can only generate a voltage that is lower than the input voltage. While there are many advantages, there are also some disadvantages with switching DC/DC converters. They are noisy as compared to a linear circuit and require energy management in the form of a control loop. Fortunately, modern switching-mode controller chips make the control task easy.
Non-Isolated DC/DC Converters
In essence, a switching DC/DC converter or a regulator is a circuit that uses a power switch, an inductor, a diode and a capacitor to transfer the energy from the input to the output. These can be arranged in a variety of ways to realize the buck, boost or buck-boost (inverter) types identified earlier.
Step-down/Buck Converter
In a typical non-isolated step-down or buck converter the output voltage VOUT depends on the input voltage VIN and the switching duty cycle D of the power switch.
Figure 1: A fundamental step-down or buck DC/DC converter topology.
Step-up/Boost Converter
A fundamental boost DC/DC converter uses the same number of passive components but arranged to step-up the input voltage so that the output is higher than the input.
Figure 2: A fundamental boost DC/DC converter topology.
Buck-Boost Converter
A typical buck-boost DC/DC circuit allows the input DC voltage to be either stepped-up or stepped-down, depending on the duty cycle. The output voltage is given as:
VOUT = -VIN *D/(1-D)
As seen from the equation above, the output voltage is always reversed in polarity with respect to the input. Hence, a buck-boost converter is also known as a voltage inverter.
Figure 3: A typical buck-boost DC/DC converter topology.
Isolated DC/DC Converters
There are two main types of transformer-based isolating DC/DC converters: flyback and forward. In both these types, the transformer provides the isolation between the input and the output.
The flyback type operates like a buck-boost but uses a transformer to store energy:
Figure 4: A fundamental transformer based flyback DC/DC converter topology.
In forward topology, the transformer is used in a traditional manner to transfer energy from the primary to the secondary when the switch is closed.
For these examples, the switching MOSFET is represented by an ideal switch and the control circuitry is omitted. These are non-synchronous type converters. However, when the traditional diode rectifier is replaced by a MOSFET for rectification, it is called synchronous rectification and the converter is termed synchronous DC/DC converter.
Looking for more power supply knowledge? To learn more about how switching power supplies can optimize your design, see our break down of how switch mode power supplies work. Also, you can figure out if your project needs an uninterruptible power supply. Need an AC/DC solution? This article can help you choose the right one.