Reduce diode losses in redundant systems with integrated power MUXes
Introduction
Many power management applications use Schottky diodes for the parallel operation of multiple power sources. This type of power redundancy is often found in systems with solid-state drives (SSDs), hard disk drives (HDDs), programmable logic controllers (PLCs), peripheral component interconnect express (PCIe) cards, network and graphic cards, and some others used in automotive, industrial, personal electronics and telecommunications infrastructure applications. The diodes do a great job of isolating redundant power sources to keep the system operational in the event that any one power source fails, while also preventing current flow from one supply to the other.
The diode power-muxing configuration gives a seamless transition from one voltage rail to the other (Figure 1). However, system and circuit designers need to find methods to reduce circuit losses associated with these diodes. The diode also reduces the available supply voltage at the system input. This becomes critical for the lower side of the input operating voltage range. A 0.5 V drop across a diode represents four percent of the power consumption in a 12 V system.
Figure 1: Multiple power sources muxing with diodes
A second consideration is overcurrent protection to prevent bus droops during overload events and short circuits. Maintaining load voltage above the undervoltage level prevents system interrupts while reducing downtime and increasing customer satisfaction.
For a number of years there has been a trend moving away from MUXing with power diodes and moving towards MUXing with ideal diodes. An Ideal diode is a circuit that “makes a FET act like a diode.” Although somewhat more complex than a simple diode, the ideal diode can significantly improve system efficiency and consume less power supply margin. There are many controllers in the market today that can make a FET behave like a diode. There are also integrated devices that have a FET and a controller in a single package. Typically these are available for lower voltages and currents. The devices in the spotlight of this article contain an ideal diode as part of a total, integrated solution.
To determine power loss in a diode simply multiply current by VF, the forward voltage drop of the diode. VF is temperature- and current-dependent, and typically ranges from 0.3 to 0.7 V.
To calculate power loss in a FET, simply multiply RDS(ON) by the square of the load current. (I2 x RDS(ON)). Modern MOSFETs have very low on-state resistance, RDS(ON), which results in a low-voltage drop even under load. In turn, this results in much lower power losses than the equivalent system using diodes. This means greater system efficiency, more available power supply margin, and fewer thermal issues during design.
Effective and reliable active ORing is not as simple as it may appear and comes with a few tradeoffs. When the MOSFET is turned ON by its associated controller, the current can flow in either direction through its channel. Should the input power source fail due to a short circuit or voltage drop at the input, this will not prevent a reverse-current flow. A longer period of reverse current will discharge the output bus voltage, causing system-level damage. These conditions mandate that the active ORing control be capable of detecting the reverse current accurately, and turn OFF the MOSFET immediately.
An example of an intelligent ORing control that provides seamless transition between two power sources is shown in Figure 2. This solution gives a distinctive feature set of true-reverse current blocking, auto-forward conduction, and fast switchover.
Figure 2: Example schematic of an auto-ORing implementation
Auto-power multiplexing
In addition to the best possible diode implementation, the schematic in Figure 2 limits inrush current and protects each rail from potential overload, short circuit, and over/undervoltage faults. Now let’s take a look at the operation and experimental results of this implementation.
When the main supply, VIN1, drops more than 10 mV below VOUT, the internal FET (master device) is turned OFF in less than 1 μs. This blocks the reverse current flow from VOUT to VIN1.
As the forward voltage drop between VIN2 and VOUT grows larger than 100 mV, the auxiliary supply,VIN2, turns ON the internal FET (slave device) in less than 4 µs. This creates a seamless transition between two voltage rails (Figure 3). Such swift switchover keeps the load powered with no undervoltage transients, which is often the case with diodes. However, this happens at a much lower loss compared to diode ORing.
The IMON1 shows the current drawn from the VIN1 power supply, and IMON2 represents current drawn from the VIN2 supply. These waveforms provide a clear indication of the power drawn during changeover from one rail to the other.
Figure 3: Active ORing changeover from VMAIN (VIN1=12V) to VAUX (VIN2=3.3V)
Figure 3 depicts an active ORing changeover from VMAIN (VIN1=12V) to VAUX (VIN2=3.3V), and VOUT jumps to 3.3V. Note the load-current transfer from 12V (IMON1) to 3.3V (IMON2).
Figure 4 shows the changeover (ORing) from VAUX (VIN2 =3.3V) to VMAIN (VIN1=12V). In active ORing, the priority is always to go with the higher voltage rail (for example, 12V). Whenever this rail is active, the load current is transferred to the 12-V rail (IMON1). Figure 2 can be extended for multiple power supply active ORing configurations, as is shown in Figure 5.
Figure 4: Active ORing changeover from VAUX (VIN2=3.3V) to VMAIN (VIN1=12V)
Priority power multiplexing operation
The load-feeding priority by default with diode ORing, and even with active ORing, always has a higher voltage input. For example, consider the case shown in Figure 1 where the VIN1 rail is 3.3V and the VIN2 rail is 12V. These two rails are ORed with diodes. The 12V always feeds the load until it falls below 3.3V, therefore, the 12-V rails have priority over the 3.3-V rail.
What if the system requires that a 3.3V (VIN1 rail) power the load until this rail voltage is within 2.7V to 3.5V? If the VIN1 rail voltage is out of this range, then the VIN2 rail needs to power the load. However, this is not possible with a Schottky ORing diode, nor with an active ORing mechanism.
A priority power multiplexing implementation is shown in Figure 6 using two devices (master and slave). Now let’s look at a priority power-muxing operation and its experimental results.
Figure 5: Multiple power supply active ORing configuration
When mains power, VIN1 is present and the master device in the VIN1 path powers the VOUT bus. Irrespective of auxiliary power, VIN2 is greater than or less than VIN1.
Once the voltage on the VIN1 rail falls below the user-defined threshold (can be programmed by R6 and R7 in Figure 6), the master device on VIN1 issues a power-good signal (PG) to the slave device on VIN2 (to OVP pin), to switchover to auxiliary power VIN2 to feed power to the output. The transition happens seamlessly in less than 125 μs, with negligible output voltage droop on the output bus. Combining the output capacitance, COUT, with larger load current demands minimizes the output voltage drop (VDROP) during changeover time. The required COUT can be calculated using equation 1.
When VIN1 recovers, the device connected to VIN1 is turned ON at a defined slew rate and the device in VIN2 path turns OFF. This allows a seamless transition from auxiliary to main voltage supply with minimal droop and without shoot-through current. Figures 7 – 8 show the smooth changeover from 3.3V to 12V rail, and vice versa.
Figure 6: Example schematic of priority power muxing implementation
Figure 7: Priority multiplexing change over from VMAIN (VIN1=3.3V) to VAUX (VIN2=12V)
Figure 7 shows a changeover from VMAIN (VIN1=3.3V) to VAUX (VIN2=12V), and how VOUT jumps to 12V when 3.3V collapses. See the load-current transfers from 3.3V (IMON1) 12V rail (IMON2).
Figure 8 shows the priority changeover from VAUX (VIN2 =12V) to VMAIN (VIN1=3.3V), even though a 12V supply is present at VIN2. Still, when VMAIN (VIN1= 3.3V) becomes active, the load gets power from the 3.3V rail. Note the transfer of the load current from 12-V rail (IMON2) to 3.3-V rail (IMON1). This shows that the 3.3-V rail has priority over the 12-V rail.
Figure 8: Priority multiplexing changes over from VAUX (VIN2 =12V) to VMAIN (VIN1=3.3V)
Conclusion
Diode implementation with the TPS25942A active ORing feature effectively replaces a lossy Schottky diode. Also addressed are the inrush current limit and overload, and the short circuit and over/undervoltage fault protections. A priority power multiplexing feature allows the designer to decide the priority of one rail over another, regardless of the voltage level of the input rails.