10-04-2017, 09:34 PM
Advanced Power Electronics
Summary
DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage). Additionally, the battery voltage declines as its stored energy is drained. Switched DC to DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.
Most DC to DC converters also regulate the output voltage. Some exceptions include high-efficiency LED power sources, which are a kind of DC to DC converter that regulates the current through the LEDs, and simple charge pumps which double or triple the output voltage.
DC to DC converters developed to maximize the energy harvest for photovoltaic systems and for wind turbines are called power optimizers.
Definitions
Hard switched - Transistors switch quickly while exposed to both full voltage and full current
Resonant An LC circuit shapes the voltage across the transistor and current through it so that the transistor switches when either the voltage or the current is zero
Magnetic DC-to-DC converters may be operated in two modes, according to the current in its main magnetic component (inductor or transformer):
Continuous - The current fluctuates but never goes down to zero
Discontinuous - The current fluctuates during the cycle, going down to zero at or before the end of each cycle
Step-down- A converter where output voltage is lower than the input voltage (like a Buck converter).
Step-up- A converter that outputs a voltage higher than the input voltage (like a Boost converter).
Continuous Current Mode- Current and thus the magnetic field in the inductive energy storage never reach zero.
Discontinuous Current Mode- Current and thus the magnetic field in the inductive energy storage may reach or cross zero.
Noise- Since all properly designed DC-to-DC converters are completely inaudible, "noise" in discussing them always refers to unwanted electrical and electromagnetic signal noise.
RF noise- Switching converters inherently emit radio waves at the switching frequency and its harmonics. Switching converters that produce triangular switching current, such as the Split-Pi, forward converter, or uk converter in continuous current mode, produce less harmonic noise than other switching converter. Linear converters produce practically no RF noise. Too much RF noise causes electromagnetic interference (EMI).
Input noise- If the converter loads the input with sharp load edges, electrical noise can be emitted from the supplying power lines as RF noise. This should be prevented with proper filtering in the input stage of the converter.
Output noise- The output of a DC-to-DC converter is designed to have a flat, constant output voltage. Unfortunately, all real DC-to-DC converters produce an output that constantly varies up and down from the nominal designed output voltage. This varying voltage on the output is the output noise. All DC-to-DC converters, including linear regulators, have some thermal output noise. Switching converters have, in addition, switching noise at the switching frequency and its harmonics. Some sensitive radio frequency and analog circuits require a power supply with so little noise that it can only be provided by a linear regulator. Many analog circuits require a power supply with relatively low noise, but can tolerate some of the less-noisy switching converters.