A discrete power multiplexer with no cross current conduction

BANGALORE: Many systems require a power multiplexer to select between two different power sources. For example, a PCI-based board needs to be able to select between the main power supply rail or auxiliary supply rail. Another example is a battery operated portable device that must select between the battery or wall adaptor.

This power switching feature easily could be implemented with a pair of diodes wired together to perform a logic ‘OR’ function. However, this approach has a severe impact on the system’s efficiency and heat generation. Additionally, the voltage available to the system would be one diode drop lower than the input voltage. Also, some systems require the use of the main supply if it is available, regardless of the voltage of the auxiliary supply. 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 efficiency of the diode ‘OR’ is to use the body diodes of two P-channel MOS (PMOS) transistors as the diode ‘OR’ function, as shown in Figure 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.

Figure 1 – Two PMOS transistor diode OR function

The two PMOS transistors circuit can suffer from cross conduction currents. For example, in Figure 1, assume Q1 is ‘ON’ 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 will be forward biased. This effectively shorts the auxiliary supply to the main supply, creating 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 uses 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 4.25 Volts 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 is reconnected to the load.

Figure 2 – Voltage detector controlled power multiplexer

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 of the main or auxiliary supply. This allows the circuit to continue operating even if one of the input supplies is shorted to ground. Additionally, 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 be between 1.8 and 5.5V for proper operation.   

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

 Figure 3 – Switching from 5.0V main to 3.3V auxiliary with 3 Amp load

Figure 4 – Switching from 3.3V auxiliary to 5.0V main with 3 Amp load

As the current flow in the circuit decreases, the possibility of reverse current flow increases. 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, as shown in Figure 5, to eliminate the possibility of reverse current flow into the auxiliary supply. The 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.

Figure 5 – Additional transistor and resistors to prevent reverse current flow

The author is an applications engineer, Portable Power, Texas Instruments

 As published by Power Electronics Technology, © May, 2007