Solution for balancing input ripple current using programmable power supplies

In this article, we will explore how to use programmable power supplies and phase shifting to reduce the stress on the input capacitors while retaining the benefits of synchronization. (Synchronizing the switching frequency of the on-board buck converters is necessary to easily control EMI and block unwanted beat frequencies. However, synchronizing every buck converter on the board has an undesirable consequence. This puts considerable stress on the input capacitors.)

Before we look at the solution, let's analyze the problem in detail. When the input power supply of the system mainly delivers DC current, the various input capacitors of the buck converter will deliver discontinuous pulse current. The design of this pulse current basically takes into account the required ripple and RMS current of each converter. That's simple. The unexpected thing happens when there are multiple converters on the board. No converter will take the reverse current exactly from the capacitor designed as its input capacitor. Undoubtedly, most of it comes from the nearest low impedance source, but in reality, the switching stage will also draw the necessary current from the entire capacitor network on the board. In a fully synchronous system, the sum of all converter pulse currents will be drawn at the same time. Therefore, the RMS current obtained by a single capacitor will be formed. This makes sense because the power dissipation of the capacitor in the ESR is proportional to the square of the RMS current. This will have the unexpected effect of stressing all capacitors multiplying, which will reduce reliability. In addition, this will increase the peak value of the conducted emission surface.

Phase shifting is a simple solution. This approach is to delay the edge of the clock pulse for each converter so that it arrives at the appropriate time within the original clock period. If done correctly, this will minimize the amount of pulse current overlap in the input capacitors. Therefore, the RMS current in each capacitor will be reduced appropriately, and the peak value of the conducted emission surface will also be reduced.

In the past, phase-shifting synchronous clocks could be accomplished using analog circuits or FPGAs. Unfortunately, this adds extra components, cost, and development effort. Fortunately, there are several digital PWM controllers on the market today that integrate synchronization and phase shifting. In this article, Exar’s PowerXR technology demonstrates the benefits of phase shifting, as shown in Figure 1.