Preventing the domino effect
September 20, 2012 by Don Robinson
A DC power supply is a critical piece of a control system. Power supply failure can lead to a process shutting down, resulting in considerable lost revenue. To prevent unplanned shutdowns, users often use redundant power supplies (i.e. power supplies with their outputs connected in parallel) so that if one supply fails, there are enough remaining power supplies to deliver the required load current.
Redundant power supplies can be also be used for increased current capacity – in this case the concern is less about unplanned shutdowns but rather the fact one supply cannot deliver the load current required. For increased current capacity, it’s better to use power supplies with “active load sharing.” This involves the power supplies communicating with each other and adjusting their output voltages so that each supply delivers the same current. If two supplies are used, each one will deliver half the required current. If three supplies are used, each one delivers a third of the total current required. Without active load sharing, each power supply must be adjusted so that their output voltages are identical, they much be maintained at the same ambient temperature and the wiring between each power supply and the common point should be of equal lengths – all of which are difficult conditions to maintain.
Regardless of the reason for using redundant power supplies, it is common practice to connect a diode in series with the output of each power supply so that a failed power supply cannot draw current from any operating power supplies. If there is a short circuit in the output of a power supply, that unit will shut down. But it could also short circuit any power supplies connected in parallel, causing those to shut down as well. A diode on the output of each supply prevents this from happening.
Using diodes to isolate power supplies, often referred to as “ORing diodes,” has the disadvantage of increasing power dissipation within the control cabinet. A typical diode drops the voltage 0.7V when conducting a current, or 0.7W per amp of load current. If two redundant power supplies are delivering a total of 10A, then the diodes will dissipate 7W in the cabinet. Using Schottky diodes could lower the power dissipation to approximately 4W. Either way, this power dissipation is unwanted and increases the ambient temperature within the cabinet. Sometimes bridge rectifiers are used because they are relatively inexpensive, are designed for high power dissipation and can be mounted directly on the wall of the cabinet so that the cabinet acts as a heat sink.
Reliability is also a concern (or should be) when using ORing diodes. If a diode fails in a short-circuited state then there is no protection and the voltage from that supply appears to increase by 0.7V. Unless active load sharing is used, that supply may try to deliver the total load current. If the power supplies are connected in parallel for increased current capacity, the supply might go in to an overcurrent condition and shut down. If one of the ORing diodes fails in an open circuit state, then there is no longer any redundancy and a shutdown of the remaining power supply could occur.
An option is to use an active diode circuit. These are based on using a metal-oxide-semiconductor field-effect transistor (MOSFET), with additional circuitry, in place of the diode. MOSFETs have the advantage of a very low on-state resistance, meaning the power dissipation is reduced significantly. For example, at 10A, a diode will dissipate 7W while the MOSFET-based active diode circuit can dissipate as little as 0.4W. Obviously this produces a lower ambient temperature within the cabinet and also offers greater reliability.
Although the circuitry used in an active diode circuit is more expensive than a diode alone, this is partially offset by the fact there is no need for a heat sink (or only a small heat sink is required). Active diode circuits can be very compact and easily packaged in DIN rail mounted housings.
Two ORing modules can have the same rating – i.e. two 20A inputs and a 40A output – but when one uses diodes it requires more than 110mm of DIN rail and includes a cooling fan to allow it to operate at high ambient temperatures. When the other uses an active diode module, it requires just 38mm of DIN rail. It can operate at the same elevated ambient temperature but requires no cooling fan.
There is also a new active diode module to be released in early 2013. It is rated 15A and requires just 8mm of DIN rail. This unit features a single input and output – it does not perform an ORing function on its own. One of these modules could be installed per power supply for maximum effect.
Don Robinson is president of Emphatec, based in Markham, Ont. You can reach him at email@example.com.
This article originally appeared in the September 2012 issue of Manufacturing AUTOMATION.