by Scot Lester, Texas Instruments

Many systems require a power multiplexer to select between two different power sources. For example, a PCIbased board must be able to select between the main power supply rail or auxiliary supply rail. Another example is a battery- operated portable device that has to select between the battery or wall adapter.

This power switching feature could easily be implemented with a pair of diodes wired together to perform a logic “OR” function. However, this approach severely impacts the system’s efficiency and heat generation. In addition, the voltage available to the system would be one diode drop lower than the input voltage. On top of that, some systems require the use of the main supply if it’s available, regardless of the auxiliary supply’s voltage. The diode OR function can only select the highest input voltage to supply the load, which may not be the preferred main supply.

An approach to increase the diode OR’s efficiency is to use the body diodes of two P-channel MOS (PMOS) transistors as the diode OR function (Fig. 1). Once the body diode is conducting, its associated MOSFET can be turned on to provide a low impedance path to effectively short the diode and remove the associated diode voltage drop. This method reduces lost power due to the diode and improves the overall efficiency.

The two-PMOS-transistor circuit can suffer from cross-conduction currents. For example, in Figure 1, assume Q1 is ON and is providing a low impedance path from the main supply to the load, and Q2 is OFF and looks like a diode. If the voltage on the auxiliary supply increases above the main voltage, then the body diode of Q2 is able to be forward-biased. This will effectively short the auxiliary supply to the main supply, creating rather large cross-conduction currents and possibly damaging the MOSFETs or the input power sources.

This configuration can also produce large reverse currents when switching from a higher voltage to a lower voltage supply. For example, just before switching to a lower voltage auxiliary supply, the output capacitor (COUT) is charged to the level of the main supply. When Q1 turns off and Q2 turns on, there will be a large current flow from the output capacitor to the auxiliary supply. This is necessary in order to discharge the output capacitance down to the auxiliary supply’s voltage level. Not all power supplies can handle this large reverse current flow.

The circuit in Figure 2 uses an additional two PMOS transistors to eliminate cross conduction by forming back-to-back diodes with the body diodes of the MOSFETs. The circuit employs the TPS3803 voltage detector to monitor the voltage of the main supply. The detector keeps the main supply connected to the load until the main supply voltage drops below a preset threshold, set to 4.25V by R1, R2, and R3. Once the main voltage falls below 4.25V, the comparator will disconnect the main supply from the load and connect the auxiliary supply. The auxiliary supply will stay connected until the main voltage returns above the preset threshold. In the circuit shown, R3 provides 0.5V of hysteresis, so the main voltage must increase above 4.75V before it’s reconnected to the load.

When turned off, each transistor pair forms a back-to-back diode to keep current from flowing. Transistors Q1a and Q2a prevent current flowing from the supply to the load during off times. Q1b and Q2b keep current from flowing from the load to the input power source during off times.

The voltage detector and the inverter are powered through D1, which selects the higher voltage of the main or auxiliary supply. This allows the circuit to continue operating even if one of the input supplies is shorted to ground. Furthermore, the inverter will always have enough voltage to turn off the PMOS transistors, since the output voltage of the inverters will always be close to the highest voltage available in the system. Either or both of the main or auxiliary supplies can fall between 1.8 and 5.5V for proper operation.

Figures 3 and Figure 4 shows the output voltage and the supply currents during the switch over from one supply to the other with a 3.0A load current. In both cases, there are no cross-conduction currents. The circuit was designed to handle loads up to 3A, but can be scaled to any load current by selecting transistors with a higher current capability.

As the current flow in the circuit decreases, it increases the possibility of reverse current flow. If the load currents are small, the output capacitor may not discharge down to the auxiliary voltage level before transistor Q2 is turned on. This would produce a large reverse current into the auxiliary supply, which may be undesirable.

Three resistors and a transistor can be added to eliminate the possibility of reverse current flow into the auxiliary supply (Fig. 5). Transistor Q3 forces Q2b off until the system voltage is equal to the auxiliary voltage level. With the input and output voltages equal, no reverse current will flow.