Power systems have become complicated over the years due to the requirement for multiple power supply rails. To meet these requirements, multiple switching supplies are often employed. One key decision to be made is whether to synchronize switching frequencies or to let each power supply switch independently. Not synchronizing results in less circuitry, while synchronizing may help to reduce filtering costs and lower electromagnetic interference (EMI).
To illustrate the impact of asynchronous operation, the circuit of Figure 1 was simulated with P-SPICE. The current sources represent the input currents to the supplies. Figure 1 shows the waveforms of the two power supply currents and the ripple voltage on the capacitor.
Figure 1: Beating effect on two asynchronous power supplies increases input ripple voltage.
Ripple voltage is at a minimum when the two converters are out of phase, and it is at maximum when they are in phase. In this case, there is almost a 2-to-1 difference in output ripple voltage. This is one of the strong arguments for synchronizing power supplies. You can reduce input voltage ripple by judiciously selecting the phasing of the various supplies. The second benefit is that the ripple current also can be reduced. In this case, when the supplies are out of phase with a 0.25 duty factor (DF) on each phase, the effective DF is 0.5, so the ripple current is:
Figure 2: This FFT of input voltage simulation showsno sum and difference components.
The current sources were operating at 180 kHz and 200 kHz, and there are only fundamentals and harmonics of those switching frequencies. To generate sum and difference products, a multiplication is required, which is usually done with a nonlinear device like a switching diode or transistor (remember sin(u)sin(v)=1/2(cos(u-v)-cos(u+v))?).
However, if you examine the average voltage from which the power supply draws power during each cycle in Figure 1, you will find that it has a variation related to the difference