UM0377

Glossary

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Usually DCM is preferred to CCM due to the following characteristics:

–

better system stability and closed loop response

–

lower switching losses in the switch and free-wheeling diode

–

smaller size and lower price of the output inductor

The disadvantages of DCM over CCM are:

–

higher peak currents, which increase the stress on the power devices and

decrease system reliability;

–

lower output current capability (in DCM the max output current is half of the

VIPer12A-E or VIPer22A-E peak current.)

For a given output current, the operating mode of the converter moves from CCM to DCM if:

–

output inductance value is decreased

–

output power is decreased

–

input voltage is increased

In order to avoid system instability, the duty cycle must be kept lower than 50% when

operating in CCM.

Input ripple:

The input bulk capacitor Cin, along with the input diode rectifier, converts the AC mains

downstream converter. This voltage features a line frequency ripple superimposed over its

DC level, as shown in Figure 9:

Figure 9.

Input voltage ripple

In order for the converter to operate correctly, the Cin must be large enough to make the

ripple very low. An excessively low value of the minimum input voltage could greatly

increase the power dissipation because of the high RMS current flowing through the Power

MOSFET.

A typical value of the input voltage peak to peak amplitude is 25-30%.

●

Hold - up cycles:

The hold up requirement in power supply converters is the capability to regulate the output

voltage even when main voltage cycles are missing. This is usually specified in terms of