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Bus to Bus Short Circuit Protection in an N+1 Power System

IP.com Disclosure Number: IPCOM000014448D
Original Publication Date: 1999-Dec-01
Included in the Prior Art Database: 2003-Jun-19
Document File: 2 page(s) / 42K

IBM

Related People

Randhir Malik: AUTHOR

Abstract

Problem In all N+1 or redundant power systems the over-voltage (OV) is sensed before the OR-ing diode. This is done to prevent shutdown of the whole power system in case of an over-voltage on any one of the outputs of any power supply in parallel. For example, if +5V Bus goes OV due to a fault in power supply 1, the other power supplies in parallel will not shutdown because their individual OV sense point is on the anode side of the OR-ing diode. The good power supplies never see the OV fault because OR-ing diode is reverse biased for an OV fault condition on the Bus. Now if one of the Bus voltage e.g.+12V shorts +5V bus or +3.3V bus, the power system will not shutdown because the OR-ing diodes on +5V and +3.3V are reverse biased by the OR-ing diodes and therefore the OV condition is not detected by the individual power supplies. It could mean damage to expensive hardware such as memory, processors and other chip sets in a computer or communication system! Problem solved

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Bus to Bus Short Circuit Protection in an N+1 Power System

Problem

In all N+1 or redundant power systems the over-voltage (OV) is sensed before the OR-ing diode. This is done to prevent shutdown of the whole power system in case of an over-voltage on any one of the outputs of any power supply in parallel. For example, if +5V Bus goes OV due to a fault in power supply 1, the other power supplies in parallel will not shutdown because their individual OV sense point is on the anode side of the OR-ing diode. The good power supplies never see the OV fault because OR-ing diode is reverse biased for an OV fault condition on the Bus.

Now if one of the Bus voltage e.g.+12V shorts +5V bus or +3.3V bus, the power system will not shutdown because the OR-ing diodes on +5V and +3.3V are reverse biased by the OR-ing diodes and therefore the OV condition is not detected by the individual power supplies. It could mean damage to expensive hardware such as memory, processors and other chip sets in a computer or communication system!

Problem solved

The proposed circuit uses a very simple principle to protect against bus to bus shorts. Each output is clamped by a zener diode whole clamp voltage is designed to be within the absolute maximum limits of the hardware the particular output provides power. Also, a resistor is added across the OR-ing diode to feedback a sample of Bus voltage to the anode side of the OR-ing diode. In case e.g. 12 volt Bus shorts to +5V or +3.3V bus the feedback sample to the anode side of the OR-ing diode is sufficient to shutdown all the power supplies in parallel and thus protect the expensive system from damage.

Circuit Description

Figure 1 shows a redundant parallel power system to provide two voltage outputs VA and VB.

In case of an internal fault to the power supply e.g. PS1, VA or VB or both may go over voltage (OV). In this case E-VA voltage on the anode side of the OR-ing diode exceeds the OV point. A latch in Power supply PS1 is activated and it takes itself output of operation without shutting down the power supplies in parallel. The output on Bus VA will go high but still remain within the safe limits of the output voltage...