This tutorial installment is: Power Supply Conduction Modes. This topic answers the following questions:
- What are power supply conduction modes?
- What are the effects of conduction modes on power supply performance?
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Power Supply Conduction Mode Explanation
The earlier tutorial installment titled Power Supply Capacitors and Inductors discussed how inductors are energy storage devices. The energy stored is proportional to the square of the current through the inductor. During times of light loading and when using passive rectifying diodes rather than synchronous rectification, the current through the inductor can fall to zero. This condition is known as discontinuous conduction mode (DCM) operation. When the inductor current never falls to zero, or when the power supply employs synchronous rectification, the condition is said to be in continuous conduction mode (CCM).
With synchronous rectification, active switches are used for rectifiers and current flow can actually reverse direction allowing the current to continue to flow. Therefore, unless the synchronous switch is commanded to turn off, the conduction mode will always be continuous.
Effects in Discontinuous Conduction Mode vs. Continuous Conduction Mode
Advantages of DCM over CCM:
- No right half plane zero (RHPZ) in boost, buck-boost, flyback topologies: The CCM boost, buck-boost, and flyback topologies have a RHPZ in their control to output transfer function. This means that if, for example, the load current increases causing the output voltage to initially dip, the response of these topologies will initially correct in the wrong direction before correcting in the right direction. The right half plane zero is nearly impossible to compensate for in the compensation loop. As a result, the control loop in these CCM coverters is typically made to cross over at a frequency much lower than the RHPZ frequency resulting in lower transient response bandwidths. The DCM version of the boost, buck-boost, and flyback converters do not have a right half plane zero and can have higher loop crossover frequency allowing higher transient response bandwidths.
- Single pole transfer functions: In the buck, boost, buck-boost, and all topologies derived from these, the input to output and control to output transfer functions contain single pole responses while operating in DCM. Converters with only single pole transfer functions are easier to compensate than converters having a double pole response.
Disadvantages of DCM over CCM:
- Conversion ratio is dependent upon load: While operating in CCM, the DC conversion ratio (the output voltage divided by the input voltage), is independent of the load, to a first order basis. This characteristic makes DC analysis of converters operating in CCM easier. However, while operating in DCM, the DC conversion ratio is dependent upon load, complicating the DC analysis.
- Ringing: When the inductor current reaches zero while in non-synchronous operation, the end of the inductor connected to the switch (also called the freewheel end), must immediately transition to the voltage at the other end of the inductor. However, there will always be inductive and capacitive parasitic elements which will cause severe ringing if damping is not implemented. It is possible for this ringing to exceed switch voltage ratings. It is also possible for this ringing to be a source of radiated and conducted emissions. Lastly, this ringing can produce undesirable noise at the output of the power supply.
- Higher peak and RMS currents for same power output in boost, buck-boost, and derived topologies including flyback: Boost, buck-boost, and derived topologies including the flyback are commonly operated only in DCM to avoid the adverse effects of the RHPZ described earlier. However, to achieve the same power in DCM as in CCM, the peak and RMS currents are substantially higher resulting in greater losses in the conduction paths and greater ringing because the energy stored in inductances is proportional to the square of the current. Energy stored that is not delivered to the output causes ringing and losses.
- Physically larger transformers and inductors required for same power output: In DCM, the inductance must be much smaller in value to allow the current to fall to zero before the start of the next cycle. Smaller inductance results in higher RMS and peak inductor currents. Because the RMS and peak currents are greater in DCM than in CCM, the transformers must be sized larger to accommodate greater flux swings and copper and core losses.
Next Topic
The next tutorial installment is Power Supply Isolation. This topic answers the following questions:
- What is power supply isolation?
- What are different types of isolation?
- What are the effects of non-ideal isolation?
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