A Guide to DC-DC Converters

One of the necessities of custom system power supplies is the conversion of DC-DC current to output regulated voltage. So, what are DC-DC converters? 

They are high-frequency power conversion circuits that use high frequency switching and inductors to smooth out the noise from switching into DC voltages. Closed feedback loops maintain constant voltage output even with changing input voltages and current outputs. 

They are much more efficient and smaller than linear regulators – provided they are 90% efficient. The less sunny side is they come with a lot of noise and complexity.

Let’s discuss the different types of these custom system power supplies.

Types of DC-DC Converters

Before getting into the more customized converters like the military DC-DC converter that are specially ruggedized and customized for military DC-DC power supply, let’s look at the most basic types of DC-DC converters. 

Isolated DC-DC Converters

These types of converters can be further categorized into two: flyback and forward. Though they have differences, in both, the transformer provides the isolation between the input and output.

Forward 

In a forward setup, the transformer is used rather traditionally for energy transfer between the primary and secondary coil when the switch is closed.

Flyback

The flyback setup operates like a buck boost only that it uses a transformer to store energy. A typical buck boost DC-DC circuit allows the input DC voltage to be stepped up or down depending on the duty cycle. 

Non-Isolated DC-DC Converters

Stripped down to the basics, a switching DC-DC converter 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. Under this, you can also play around with some configurations for military DC DC converters and otherwise:

Buck-boost converter

Like I had mentioned earlier, a typical DC-DC circuit allows the input DC voltage to be stepped either up or down depending on the duty cycle. This converter can also be referred to as a voltage inverter because the output voltage is always reversed in polarity with respect to the input. 

Buck converter

In a buck converter, also called a step down converter, the output voltage is dependent on the input voltage as well as the switching duty of the power switch.

Boost converter

A basic boost DC-DC converter uses the same number of passive components arranged to step up the input voltage to achieve higher output voltage. This stepping up of voltage is why this converter can also be referred to as a step-up converter.

Modes of Operation

So far, we’ve learnt that DC-DC converters convert DC-DC current, only changing the current to whatever voltage the system power supply requires. DC-DC converters operate in either of two modes:

Linear Mode

In linear mode, the converter uses a resistive voltage drop to create and regulate a given output voltage. Linear converters are limited in that they can only generate a voltage that is lower than the input voltage, which makes them impractical for buck converters along with any applications that may require the same.

Switching Mode

A switched mode DC-DC converter, on the other hand, converts power by storing the input energy periodically and then releasing that energy to the output at a different voltage. The energy can be stored in either a magnetic field component like a transformer or an electric field component like a capacitor. 

Converters in this category have some advantages over their linear counterparts:

• The switching frequency is high so the passive components are small and lower losses simplify heat management. 
• Power conversion efficiency is high, making it suitable for sensitive applications like military DC DC power supply. 
• The energy stored by an inductor in a switching regulator can be transformed into either of the converter setups i.e buck, boost and buck-boost.