ORing diodes are a necessary evil of high-reliability systems. They're used to isolate redundant power sources so a failure of one source won't bring down the entire system. Several inherent problems are associated with this isolation system.
One problem is the insertion loss from a component introduced directly into the power path. The full load current must pass through this device, resulting in the associated power loss of load current multiplied by the voltage drop across the device. This can be a substantial amount of power. In addition to decreasing the efficiency of the system, this power loss generates heat in the isolation diode, which also must be addressed.
Another issue is the reverse recovery time. When a short circuit occurs in one of the sources, this device must limit the flow of many amps of current in a short period of time. If not, the bus can be degraded to the point where the load fails due to undervoltage or transient conditions.
Schottky diodes are a simple method of providing isolation in such a system. They are easy to use, have a lower voltage drop than a conventional silicon rectifier and have a fast recovery time. That said, two problems exist with Schottky rectifiers. First, the forward drop is still a good fraction of a volt. So, for example, if we use an MBR1635 16-A, 35-V Schottky rectifier with a 10-A load current and a 100°C junction temperature, the power loss is:
Even for a TO-220 package, this is a lot of heat to dissipate.
Schottky diodes also have a leakage problem. Depending on the worst-case junction temperature, the leakage can be significant. This diode has a leakage current of 8 mA at a 125°C junction temperature and a 10-V reverse bias. That's 80 mW of dissipation when it is in its off state.
An alternate solution is to substitute a power MOSFET for the Schottky rectifier. Although this has a lower forward drop, it has added complexity because of the required drive circuitry and slower turnoff time. This solution also requires an external bias voltage to operate the gate of the FET, assuming an N-channel device is used.
Just to equal the power loss as the Schottky diode in the above example, a FET would be required with an RDS(on) of:
This on-resistance easily can be obtained with available low-voltage FETs. The difficulty comes in designing a circuit that can accurately and quickly detect a change in polarity, so that the FET can be turned off before the bus voltage is degraded to the point where the system crashes.
Even with some of the high-speed comparators available today, it's a challenge to couple a comparator with a driver and minimize the switching delays to achieve speeds in the 100-ns range. That level of performance is needed for reliable fault protection in the event of a shorted source.
FETs with on-resistances as low as 2 mΩ exist today, with voltage ratings of 20 V to 30 V. A 2-mΩ FET would have a total loss of:
This results in a power savings of 4.8 W, or a decrease in ORing losses of 96%. It also eliminates any need for heat sinking of the ORing diode. However, in selecting the FET, a tradeoff must be made between lowest on-resistance and a higher gate charge. Because we also want quick reverse recovery from a failed supply, a slightly higher on-resistance may be preferable because it will have a lower gate charge and faster turnoff time.
Better ORing Diode Solution
Integrating as much of the comparator, driver and FET circuitry as possible can optimize the interface impedances between these circuits. Several such devices are on the market, each using a slightly different approach.
The Better ORing diode from ON Semiconductor is a hybrid rectifier consists of a high-speed analog control circuit; a 24-V 4.7-mΩ typical, N-Channel power MOSFET; and a decoupling capacitor all-in-one package. The MOSFET is operated in the third quadrant, making the source of the MOSFET act as the anode and the drain act as the cathode (Fig. 1).
This device was designed to function as much as possible like an actual diode. The comparator senses the polarity of the voltage across the “diode” and turns the FET on when the body diode is forward-biased and off when it is reverse-biased. It was necessary to design the comparator with a small offset voltage to keep the device stable.
When the current through the diode initially switches to the forward conducting mode, the FET is not on and the body diode conducts. Fig. 2 shows the waveforms for the voltage and current through the FET/body diode.
Because no comparator is perfect, there will always be some offset. If the offset were ever negative, the device would oscillate at low currents. This is because the forward voltage across the body diode will always be in the hundreds of millivolts range, but the drop across the FET may only be a few millivolts. If the offset is between these two levels, the device will switch between its off state where the body diode is conducting and its on state where the FET is conducting. In the latter case, the voltage drop is the current multiplied by the on-resistance of the FET.
The offset voltage was designed to be a positive 2 mV (typical). This ensures stable operation but increases the propagation delay time of the circuit, because the current has to actually go past zero and into the reverse polarity before the comparator can respond. The typical current required to trip the comparator will be Voffset/RDS(on), or 0.43 A. Typical propagation delays have been measured at 100 ns, with the total time required for the current to recover at 260 ns.
Fig. 3 shows the current through the Better ORing diode in the top trace, on a scale of 1 A/div, and the gate drive signal on the lower trace. The delay times are measured from the time at which the current drops to zero. The reverse current can be seen to be about 0.5 A and lasts for a duration of 260 ns. The gate pin is accessible and can be used as a logic level signaling a fault or to parallel additional MOSFETs for higher-current capability.
The turnoff speed of this device was the design's top priority, beginning with the electrical design of the integrated circuit. The comparator, driver and MOSFET were designed to optimize speed above all else. This included allowing for more shoot-through current in the driver than normally allowed. Even with speed as the priority, the bias current is only 2 mA when the device isn't switching.
After completing the electrical design, the layout of the die was done to make the high-current, high-speed paths as short as possible — especially the output stage of the driver. The package for the die was then designed for optimum speed. This package included the dice for the driver and power FET, and an internal bypass capacitor. This optimization resulted in an unique footprint for the Better ORing diode shown in Fig. 4.
Fig. 5 shows a schematic of a short-circuit test board. One 12-V power supply was set 50 mV above a second 12-V supply, and then shorted. The diode of the higher source was monitored for its current (see the Fig. 6 waveforms).
Before the event, 0.50 A of load current are flowing in the diode. The current probe measured the current in the diode between the short and the load, so that it can be seen going to zero once the diode has turned off.
This same setup was used to test other FET-based devices that perform the ORing function. Tests revealed significant differences in the speed and magnitude of the reverse-current pulse that occurred when the conducting input was shorted. This transient can have a major effect on the system bus voltage, and could cause the system to crash if excessive ringing or voltage spikes occur.
Turnoff speed is the critical factor in minimizing the disturbance to the bus for a solid-state ORing diode. It can make the difference between a robust, reliable system and one that experiences random failures.
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