OR’ing Diodes are designed to OR two sources to a single load. Ultra-low on-resistance means devices require no heat sinking and no airflow at currents up to 600 amps. OR’ing Diodes are epoxy encapsulated in a custom ABS enclosure and are well suited for hostile environments. Both programmable and non-programmable versions are available. Allow us to help determine which version is best suited for your application.
Programmable vs Non-Programmable OR-ing
Perfect Switch offers two varieties of OR’ing devices, the non-programmable type which is a simple two diode, common cathode configuration, and the programmable type which is comprised of two of our bi-directional relays whose load terminals are joined together by the customer. Below is a description of the differences between the two:
“The non-programmable OR’ing consists of two of our single rectifiers with their cathodes tied together built on a single board and housed in a single package. It behaves exactly as you would expect when two diodes are configured in that manner, by passing current from the higher of the two sources to the common load. The primary advantage of the non-programmable OR’ing device is that it allows for nearly instantaneous switching of the load between the two sources. If the higher source suddenly experiences a decrease in voltage below that of the other, the device will immediately switch the load so that there is no deadtime, or period when neither source is providing power. The major difference between the non-programmable OR’ing device and two standard discrete diodes is apparent when the two sources are close in voltage to one another. With discrete diodes, both sources may provide power to the load depending on the voltage difference between them and the level of matching between the two diode’s characteristics. Because POWER-GATE rectifiers are MOSFET-based, there is no ability to share the current load in an equivalent manner; one side is on and one side is off. As a result of system dynamics, there is the possibility of rapid switching of the load between the two sources. Let us explain with an example:
Lets say, for arguments sake, the generator is sitting at 26.5 V and the backup battery is sitting at 26.4 V. These sources are connected to the inputs of the non-programmable OR’ing, and the generator will be providing power to the loads (since it is 0.1 V higher than the backup battery). Then lets say that a large load is turned on which causes the generator to experience a dip in voltage to 26.3 V. At this point, the non-programmable OR’ing will automatically switch the load to the backup battery. When this occurs, the unloading of the generator will cause it to jump back up to 26.5 V (and the backup battery voltage may sag in response); the generator voltage is now higher than the backup battery voltage and so the non-programmable OR’ing will switch the load back to the generator. The generator will again experience a dip, causing the OR’ing to switch back to the second source. This process will continue indefinitely until the load is sufficiently small as to allow one source to always be higher than the other (under normal operating conditions, if a source becomes disconnected, then the device will power the loads from the remaining source which is exactly the purpose of the device). Because of this oscillation behavior, we generally recommend that the two sources be separated by at least 0.5 V so as to avoid rapid switching between them. The dynamics of any application’s sources and loads may not exactly match this description, but this example provides an illustration of how the non-programmable OR’ing device will operate.
The programmable OR’ing avoids the oscillatory behavior of the non-programmable OR’ing by using two of our bi-directional relays in place of the rectifiers. The primary relay monitors the primary source (using the same example generator as above) and will power the load under normal conditions. If the generator voltage falls below a user-specified trip voltage (for example, 23.0 V), the primary relay will disconnect the generator from the loads and command the backup relay to connect the backup battery to the loads. After this, the primary relay will continue to monitor the generator voltage. If and when the generator voltage rises back above a user-specified reset voltage (for example 27.5 V) for a given period of time, the primary relay will command the backup relay to disconnect the backup battery from the loads and will reconnect the generator to the loads. This configuration allows the user to have directed flexibility in deciding what voltage (or range of voltage) the primary source should be allowed to power the loads, as opposed to the non-programmable OR’ing configuration which simply powers the loads from the higher of the two sources. However, unlike the non-programmable OR’ing device, the programmable version has an associated deadtime during the switching process. Utilizing our current generation 4.1 relays, the deadtime is 50us (50 micro-seconds). Some of our customers using the programmable OR’ing use a “hold-up” capacitor to provide power to the loads during this deadtime, but the need to do so is completely dependent upon the nature of the attached loads.
In summary, the non-programmable OR’ing will allow for switching with no loss of power to the loads, but will only pass the higher of the two sources to the loads. Rapid-switching (oscillation) may occur if the two sources are close enough in voltage, application dependent. The programmable OR’ing allows for greater flexibility in directing which source provides power to the loads, but has an inherent deadtime that must be accommodated for.