Buck converters are non-isolated switching converters which output voltage is less than the input voltage thus the term “buck” means. Buck converter could be a conventional one with the diode in the bottom leg or the so-called synchronous buck converter which uses switching devices such as MOSFETs in both upper and lower legs. There are quick facts about synchronous buck converter that help design engineers or those aspiring to learn quickly understand the converter. Keep reading.
Here are the Quick Facts about Synchronous Buck Converter
1. The Upper and Lower Switches See the Same Voltage Level
This is true in ideal scenario. As you can see, when Q1 is off while Q2 is on, the voltage across Q1 is just Vin since Q2 is shorted to ground ideal. When Q1 is on and Q2 is off, the voltage across Q2 is also Vin since Q1 is shorted and ideally has zero voltage drop. We emphasized here ideal but when we consider the voltage drops of the devices, there could be a slight difference in the upper and lower FETs voltage, especially when the parasitic components are considered.
2. Both Switches are Controlled by the Same Switching Frequency
This is not an argument at all since there is only a single controller that controls the converter.
3. Both Switches will Not Operate at the Same Time
When the upper MOSFET operates, the lower MOSFET will not and vice versa. This is the nature of the circuit.
4. Both MOSFETs will Not See the Same Current
This is true except when the duty cycle is 50%. At 50% duty, the effective value of the current is the same though the waveforms are different.
5. When Duty Cycle is 50%, Both Devices will Draw the Same Current
At 50% duty cycle, the upper and lower MOSFETs have the same RMS and DC current though the current will not flow at the same time.
6. At Duty Cycles Below 50%, the Upper MOSFET Draw Less Current than the Lower MOSFET
This is correct because the upper MOSFET will only turn on at less time than the lower MOSFET. This results to a lower RMS and DC currents.
7. At Duty Cycles Above 50%, the Upper MOSFET Draws More Current than the Lower MOSFET
At duty cycle above 50%, the upper MOSFET turns on in more time than the lower MOSFET which results in higher RMS and DC currents.
8. A Big Difference Between Input and Output Voltage will Result in a Lower Duty Cycle while a Small Difference will Result to a Higher Duty Cycle
9. In Ideal Case, the Maximum Output Voltage is can be Equal to the Input Voltage
This is only applicable to an ideal scenario. This is explained by the ideal Duty cycle equation below. By re-writing the equation, Vout is just the product of Duty cycle and Vin where the maximum duty cycle is 100%.
Duty cycle = Vout / Vin
10. Conduction Loss is not Frequency Dependent
Conduction loss is only governed by the Rdson of the MOSFET and the drain current. Thus, to minimize conduction loss, choose a MOSFET with a low Rdson. Rdson is the on state resistance of the drain to source junction.
11. Switching Loss is Frequency Dependent
Switching losses occur with the switching activity. The faster the switching actions, the higher this loss is. To minimize switching losses, choose a MOSFET with low dynamic parameters or limit the switching frequency. Switching losses are complex to analyze but there are ways to quantify it. Read the article How to Compute MOSFET Switching Losses.
12. Conduction Loss and Switching Loss are Always Countering Each Other
In order to minimize conduction loss, the MOSFET Rdson must be very low. However, very low Rdson will result to higher dynamic parameters such as total gate charge, COSS and rise and fall times. On the other hand, to minimize switching losses, the MOSFET dynamic parameters must be low. But lowering dynamic parameters means a higher Rdson and will impact the conduction loss.
The bottom line is to select the MOSFET carefully to get the best conduction and switching loss. Making the upper MOSFET and lower MOSFET different is also a good idea.
13. The Size of the Inductor Dictates the Operation Mode of the Buck Converter
There are three operating modes a buck converter has. They are CCM, DCM and transition. CCM requires a big inductor. DCM needs only a smaller one. While transition should have an inductor somewhere in between the value of CCM and DCM.
To know more about the operation mode, read Sizing the Inductor of Buck Converter and Setting its Operation.