Continuous conduction mode

Figure 3.4 Ideal voltage and current waveforms for the buck converter operating in the continuous conduction mode.

Note that the ratio r is defined for CCM (continuous conduction mode) operation only. Its valid range is from 0 to 2. When r is 0, AI must be 0, and the inductor equation then implies a very large (infinite) inductance. Clearly, r = 0 is not a practical value If r equals 2, the converter is operating at the boundary of continuous and discontinuous conduction modes (boundary conduction mode or BCM ). See Figure 2-5. In this so-called boundary (or critical ) conduction mode, Iac = Idc by definition. Note that readers can refer back to Chapter 1, in which CCM, DCM, and BCM were all initially introduced and explained. 

It can be shown that to avoid subharmonic instability, we need to ensure that the amount of slope compensation (expressed in A/s) is equal to half the, slope of the falling inductor current ramp, or more. Note that in principle, subharmonic instability can occur only if D is (close to or) greater than 50%. So slope compensation can be applied either over the full duty cycle range, or just for D > 0.5 as shown in Figure 2-10. Note that subharmonic instability can also occur only if we are operating in continuous conduction mode (CCM). 

The Current Ripple Ratio r in Forced Continuous Conduction Mode ( FCCM )... 

The CCM-type mode that replaces the DCM mode in synchronous regulators is distinguished from the usual (normal) CCM mode, by calling it the forced continuous conduction mode (FCCM). The main switch is usually identified as the top (or high-side ) mosfet, whereas... 

One of the most notable features of the synchronous buck topology is that on decreasing the load, it does not enter discontinuous conduction mode as a diode-based (conventional) regulator would. That is because, unlike a bjt, the current can reverse its direction in a mosfet (i.e. it can flow from drain to source or from source to drain). So the inductor current at any given moment can become negative (flowing away from the load) — and therefore continuous conduction mode is maintained — even if the load current drops to zero (nothing connected across the output terminals of the converter) (see Chapter 1). 

Let us discuss the three major topologies separately here. Note that we are assuming voltage mode control and continuous conduction mode. Further, the ESR zero is also not included here (introduced later). 

Ensure that the crossover frequency is well below any troublesome poles or zeros — like the RHP zero in continuous conduction mode (boost and buck-boost — with voltage mode or current mode control), and the subharmonic instability pole in continuous conduction mode (buck, boost, and buck-boost — with current mode control). The latter pole is discussed later. 

Now consider the dependency of a buck converter in continuous conduction mode ( CCM ) ... 

We can also use the fact that the output voltage of a discontinuous mode converter at a given duty cycle depends on its inductance. So we can tune the slave buck-boost to have the required output level (at its expected maximum load current) by a careful choice of inductance. Within a valid range, this technique provides completely adjustable auxiliary output voltages, something we cannot normally expect from composite topologies based only on continuous conduction modes. 

This chapter considers a simple boost converter often used in power electronic systems. Figure 8.1 depicts its circuit schematic. In this circuit, the MOSFET transistor and the diode may be considered non-ideal switches. The transistor is a controlled power switch. Boost converters are designed that they operate either in so-called continuous conduction mode or in discontinuous conduction mode. In continuous conduction mode the inductor current never falls to zero. Accordingly, the converter assumes two states per switching cycle. When the transistor is on, the diode is off and vice versa. The diode commutates autonomously and oppositely to the transistor. Hence, there are two system modes in a healthy boost converter. 

In a healthy system operating in continuous conduction mode, the switch and the diode open and close oppositely (mi + m2 = 1). Let / sw = Rd = Ron- Then the ARRs simplify and the dynamic behaviour of a correctly operating boost converter is given by the state equations... 

Equation (8.13) holds under the assumption that the boost converter operates in continuous conduction mode and reduces for Rl =0 into the well known formula for the voltage conversion... 

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