Current ripple ratio

Figure 12-2 How the Current Ripple Ratio Affects Other Parameters in a Buck Converter...

Figure 2-2 The AC, DC, Peak, and Peak-to-Peak Currents, and the Current Ripple Ratio V Defined...

In Figure 2-2 we first introduced the most basic, yet far-reaching design parameter of the power supply itself — its current ripple ratio r. This is a geometrical ratio that compares and connects the ac value of the inductor current to its associated dc value. So... 

Figure 2-6 How Varying the Current Ripple Ratio r Affects All the Components...

Therefore, in general, a current ripple ratio of around 0.4 is a good design target for any topology, any application, and any switching frequency. 

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 ... 

Maintaining the converter in CCM, down to the minimum load of our application, enforces a certain maximum value for r. For example, if the minimum load is Io min = 0.5 A, then to maintain the converter in CCM at 0.5 A, the set current ripple ratio (r at 3 A) needs to be lowered. Back calculating, we get the required condition for this... 

We therefore need to set the current ripple ratio to less than 0.333 at maximum load, to ensure CCM at Io min-... 

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

Provided we accept this temperature rise in our application (that will depend on our maximum operating ambient temperature), we can validate the chosen inductor. We have already confirmed it does not saturate in our application, and further, the current ripple ratio it provides is acceptable too. 

In other words, if we know the voltseconds at Vinmin, we will know the corresponding current ripple ratio tdmax for the chosen inductor. But first we have to calculate... 

So we have to calculate rso, that is, the current ripple ratio at D = 50% (or whatever voltage within the specified input range of our application range is closest to this point). 

The most basic question in design invariably is — what input voltage represents the worst-case point at which we need to start the design of the magnetics (from the viewpoint of core saturation) For the forward converter choke, this should be obvious — as for any buck converter, we need to set its current ripple ratio at around 0.4 at Vinmax- But coming to the transformer, we need some analysis before we can make a proper conclusion. 

This is fairly high, but as explained, it is actually helpful here, because Rac goes down. Now, the current in the secondary looks like a typical switch waveform, with its center equal to the load current (50 A), and a certain current ripple ratio set by the output choke. Its RMS value is... 

Answer To reduce stresses at various points inside a power supply, and also to generally reduce the overall size of its components, a current ripple ratio (V) of about 0.4 is considered to be a good compromise for any topology, at any switching frequency. 

For a buck, we know that at turn-on, the instantaneous switch (and inductor) current is Io x (1 — r/2), where r is the current ripple ratio, and Io is the load current of the dc-dc converter. At turn-off, the current is Io x (1 + r/2). Usually, we can ignore the current ripple ratio and take the current as Io for both the turn-on and the turn-off analysis. So the load current of the dc-dc converter, Io, becomes the same as the Io used so far in the switching loss analysis. Similarly, in a boost and buck-boost, the current Io in our switching loss analysis, is actually the average inductor current Io/(l — D). 

In addition, the half-switching-frequency pole is sufficiently damped — by introducing the right amount of minimum inductance vis-a-vis the applied slope compensation (or the appropriate amount of slope compensation, based on the desired or optimum value of inductance — as for example for a current ripple ratio of 0.4). 

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