Discontinuous conduction mode

In many applications it is preferable to perform PFC and isolation within a single converter stage. This is presently common in low-power requirements up to about 40 W using SEPIC and discontinuous conduction mode flyback topologies. 

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. 

As discussed previously, under various conditions, we may enter discontinuous conduction mode (DCM). From Figure 2-5 we can see that just as DCM starts to occur, the current ripple ratio is 2. However we can pose the question in the following manner — what if we have set the current ripple ratio to a certain value r (i.e. the current ripple ratio at the maximum load current, Io max)- And then we decrease the load current slowly — at what load does the converter enter DCM ... 

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). 

Now consider the equation for the output of a buck-boost in discontinuous conduction mode ( DCM )... 

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. 

The expression for the voltage conversion in (8.14) is to be replaced by a more complicated one in case the boost converter is operated in discontinuous conduction mode. 

Oilman, G.K., Hofmann, H.F., Bhatt, A.C., Lesieutre, G.A., 2003. Optimized piezoelectric energy harvesting circuit using step-down converter in discontinuous conduction mode. IEEE Transactions on Power Electronics 18, 696—703. 

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