Can a battery be switched to an external power source?

Can a battery be switched to an external power source?

A similar version of this article appeared in the May 13, 2002 issue of Electronic Design magazine. Portable equipment that can operate from a battery pack or an external power source (such as a wall-adapter or external supply) needs to be able to smoothly switch between the two power sources.

How does a microprocessor switch between battery and wall power?

The microprocessor-supervisory circuit (U1 in Figure 1) acts as a wall-source detector and debouncer. It monitors the wall supply and switches from battery power to the wall supply only when the wall supply is stable and has been at or above U1’s trip voltage for a period of time.

How does a switching power supply circuit work?

Harmonics can generate distortion that may affect other power supplies and devices connected to the grid. In a switching power supply design, the goal of the power factor correction circuit is to minimize these harmonics by filtering them out. To do so, there are two options: active and passive power factor correction.

How can I switch between two power sources?

Portable equipment that can operate from a battery pack or an external power source (such as a wall-adapter or external supply) needs to be able to smoothly switch between the two power sources. This application note describes a circuit ( Figure 1) that switches power sources with good efficiency and without switching noise.

How is USB power fed to the diode?

USB power is fed to load via Schottky diode D1. Power may be fed via Ja and the LDO regulator or via JB as desired. If feed via JB is used the LDO must survive with external voltage on its output when it has no input.

What is the voltage drop of a diode?

A diode-OR connection is a common solution but the forward voltage drop of the diode limits efficiency. For a small battery pack of one to three cells, the voltage drop for a standard diode (0.6V to 0.7V) is a large percentage of the battery’s terminal voltage.

What happens when a battery is connected to a USB port?

When either USB or dc power is connected, the Power On (PON) output immediately shuts off Q3 to disconnect the battery from the load. This allows the system to operate immediately when external power is applied, even if the battery is deeply discharged or damaged.

How to reduce battery load when using USB?

USB power feeds via D1 either via Ja and LDo or Jb as above. Changed R1, R4, R5 to nominal 1 megohm each to reduce battery load when USB in use. A small MOSFET for Q2 and/or some more thinking will reduce required standby current.

Why are multiple transistors used in a bipolar transistor?

Sometimes the DC current gain of the bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors are used. Here, one small input transistor is used to switch “ON” or “OFF” a much larger current handling output transistor.

What happens when you use a transistor as a switch?

Transistors that are fully “ON” are said to be in their Saturation region. Transistors that are fully “OFF” are said to be in their Cut-off region. When using the transistor as a switch, a small Base current controls a much larger Collector load current.

When to switch from battery to wall power?

It monitors the wall supply and switches from battery power to the wall supply only when the wall supply is stable and has been at or above U1’s trip voltage for a period of time. The battery will be back-driven (charged) during this delay period, which is typically 185mS.