Buck Converter Bootcamp Information

What is a buck converter? What does it look like?

A buck converter is a circuit that decreases DC voltage while increasing current. Figure 1 shows a circuit diagram of a simple buck converter:

Figure 1: Simplified circuit diagram of a buck converter

Where The battery on the left is the input voltage source, and the load is the resistor on the right.

How does a buck converter work?

Buck converters lower voltage and increase current by controlling the flow of input current to the load by using a digitally-controlled switch (typically a transistor) with feedback, along with various circuit components to stabilize the output waveform to a DC signal. The combination of the inductor, capacitor, and diode in the arrangement shown in figure 1 is known as a flywheel circuit.

When the Switch is Closed

When the switch is closed, current flows from the input source into the flywheel circuit. During this time, the inductor opposes the flow of current by generating a magnetic field, storing energy for later, but soon the inductor is fully charged and can no longer oppose the current. The current increases gradually through the load once the switch is closed due to the inductor, and during this time the capacitor also charges. Since the voltage across the diode is in reverse bias, the diode does not play a part during this phase.

Figure 2: Buck converter circuit when the switch is closed

When the Switch is Opened

Once the switch is opened, the current no longer flows from the input source, changing the polarity across the inductor and causing the magnetic field of the inductor to begin collapsing. As the magnetic field collapses, the inductor will continue to discharge current for some time. Since the inductor now supplies the current, the circuit causes current to flow through the load and the diode, making the diode work in forwarding bias. As the energy from the inductor decreases, the load voltage will start to decrease as well, causing the capacitor to begin discharging its energy, sustaining the voltage for some time before the switch is closed again, from which the cycle repeats.

When the switch is toggled at a high enough frequency, the output waveform becomes more or less steady-state. Changing the duty cycle of the square wave controlling the switch will change the load voltage.

What is a duty cycle?

The duty cycle of a wave describes how long the wave stays “on” compared to “off”, expressed as a percentage. Thus, it only applies to digital signals (square waves). When a square wave is at a high signal for the same length of time as it is off, then it can be said that the duty cycle of the signal is 50%. If the wave signal is always high, then the wave will have a duty cycle of 100%, and conversely, if the wave is always low, then the duty cycle is 0%. Mathematically, the duty cycle is defined as the ratio of the on-time to the period of the signal.

What is the difference between a synchronous and asynchronous buck converter?

The main difference between a synchronous and asynchronous buck converter is how they complete the circuit when the switch is opened and the inductor and capacitor are discharging. The diagrams shown above are examples of asynchronous buck converters, where a diode is used to complete the circuit. In a synchronous buck, this diode is replaced with another switch that toggles inversely to the first one. When the first transistor is on (closed), the second transistor is off (open). Synchronous bucks are generally more efficient than asynchronous ones but also require more complicated control circuitry. Where asynchronous bucks can reach an efficiency of around 80%, synchronous ones can have an efficiency as high as 95%.

What is the voltage transfer function of a buck converter? Consequently, what determines the output voltage and why? How can you calculate the duty cycle given input and output voltage design requirements?

What happens at 100% and 0% duty cycle?
What do the waveforms for the inductor voltage and current look like
What do the waveforms for the capacitor voltage and current look like
What is the purpose of the output inductor?
What is the purpose of the output capacitor?
What direction does current flow in the inductor and capacitor in the two switching states?
Why is the inductor current a linear ramp? (Think back to the fact that buck converters are used for DC-DC conversion)
When does the output capacitor charge/discharge
Why is input capacitance needed? What happens if it’s removed?
What is voltage and current ripple? What parameters control the magnitude of the output voltage and current ripple? (important)
What determines the inductor current slope?
What is dead-time insertion? Why is it a must have?
How does a MOSFET’s parasitic internal body diode play a role in the workings of a buck converter?
Where is inductor current sourced from in the off-state?
Why do N-channel MOSFETs require additional circuitry to switch on? What are those circuits called? How do they work?
What happens to a bootstrap circuit when a half-bridge operates at too high of a duty cycle? What issues arise?
What is the difference between a buck converter vs LDO? What are the tradeoffs?
When can an LDO be more efficient than a buck converter?
What are the sources of power loss in a buck converter? What are they?
How can you improve buck converter efficiency?
How are Vgs and Rds(on) related?
How are Vgs and gate charge (or turn-on/turn-off switching times) related?
What are load and line regulation tests? How are they used to characterize buck converters in testing?