Figure 1: Power management system functional block diagram.

Evaluating candidates by the performance the functional blocks is a good place to start, and in particular the analog functional blocks. One core functional block common to battery power management systems is the battery charger. The correct choice of DC system power supplies, having a 'battery' of specialized features, greatly simplifies testing of the battery charger block in power management systems.

Typical Power Management System Functional Block Diagram
A representative power management system functional block diagram is shown in figure 1.? It reflects the high level of complexity typical of today's mobile devices. Functional blocks within the system can usually be categorized as either digital or analog in nature.

Figure 2: Example battery charger system.

Battery Charger Functional Block Details
Regardless of the differences in power management system designs, there is a high level of functional commonality in the battery charger block. Most chargers will identify the type of battery pack installed and apply the appropriate charge regiment.?? An example battery charger block and ancillary components are illustrated in figure 2.

Basic parameters for initiating, maintaining, and terminating battery charging include battery voltage, condition, and temperature, and adapter voltage and current. Control circuitry within the charger block monitors these inputs and in turn, regulates the charging while providing status back to the host device.

?Battery Charging Process
When conditions for charging the battery are met, a typical charge process sequences through three phases:

?Ø?Test Battery Condition
Battery condition is first checked.? One approach is to apply a short pulse of current to the battery while measuring the resulting voltage response.? Many details about the battery can be ascertained from its voltage response.

Figure 3a: Battery condition test.

Figure 3b: Li-Ion charging.

Figure 3c: Ni-Cad/Ni-MH charging.

?Ø?Apply Charge Regiment
Charging is dependent on battery type.? Ni-Cad and Ni-MH batteries are charged by applying a constant current.? Li-Ion batteries are charged by first applying a constant current, followed by applying a constant "float" voltage.

?Ø?Terminate Charging
Charge termination is likewise dependent on battery type. Charge termination for Ni-Cad and Ni-MH batteries is triggered by battery voltage peaking and temperature rise. In comparison, the dropping off of charge current being drawn by a Li-Ion battery triggers its charge termination.

The details of the charging process are illustrated in figure 3.

Testing The Battery Charger Functional Block
In test, power supplies replace and emulate the battery and charger. Being programmable they provide a controlled means of testing the battery charger block, along with additional equipment mostly used for making specific measurements.?

The key considerations for emulating and replacing the battery are extensive:

Discharging and Charging
During use, the battery sources current, powering the host device. During charging, the battery becomes a load, replenished by the charge current. Emulating charging and discharging requires a power supply having two-quadrant output operation, as illustrated in figure 4.

Battery Condition Voltage Response
Batteries typically have tens to hundreds of milliohms of series output resistance. It is the dominant factor governing the battery output voltage response in the battery condition test. A programmable series resistance and a means for capturing the resultant dynamic voltage response are needed for performing the battery condition test.

Battery State-of-Charge Voltage Dependence
A battery's voltage varies with its charge level and is used for establishing end of charge and discharge termination points. Precision power supply output voltage programming and measurement is needed for emulating and validating battery state-of-charge voltage dependence and charge termination.

Figure 4: Two-quadrant operation for battery emulation.

Battery Temperature
An internal negative temperature coefficient (NTC) thermistor often monitors temperature in a battery pack. When connected to the charger block, a voltage proportional to the battery temperature is produced. In test, a programmable voltage signal can be usually be used to simulate the battery temperature.

Li-Ion Battery Float Voltage and Charge Termination
In the second stage of charging a Li-Ion battery voltage 'floats' up to match the charger's output voltage. This, along with the battery's series resistance causes the charge current to taper off to a point at which the battery is deemed charged. The combination of precision output voltage programming and programmable series resistance provide a means for emulating Li-Ion battery charge termination.

Off-state Battery Drain Current
When the host device is turned off, there should not be more than a few micro-amps drawn from the battery. Micro-amp level current measurement is required for validating off-state drain current.

The key considerations for emulating and replacing the adapter are more modest:

Figure 5: Configuration for charger functional block testing.

Adapter Output Voltage
A given charger relies on the adapter to provide a specific output voltage level, within a few percent, for proper operation. The power supply having precision voltage programming is useful for checking charger operation at minimum and maximum adapter voltage limits.

Adapter Output Current Regulation
Many times the charger block relies on the adapter for regulating a constant charge current. The power supply requires a programmable constant current limit capability for emulating adapter current regulation.
?

Charger Current Read Back
Battery charge current read-back is often an integral feature of the charger block. Accurate external current measurement at the adapter port is useful for validating the charger's integral current read-back.

Adapter Powered Down
When the adapter is powered down it should not present any load on the adapter port of the charger. Disconnect relays are effective for isolating the adapter port during test.

Based on these needs, a test system configuration using a variety of general-purpose test equipment is shown in figure 5.??

Figure 6: Simplified test system implementation.

Reducing Test System Complexity
Using specialized test equipment that incorporates many of the test needs as standard features can greatly reduce system complexity and cost. One example is a DC source specially designed for replacing and emulating a battery during test, such as an Agilent 66319B. Its specialized capabilities are summarized in table 1.?

The more modest requirements of the adapter can be addressed by a good system DC power supply having precision programming and read-back, and additionally offers output disconnect relays, such as an Agilent 6612C DC Source. The resulting simplified test system configuration is shown in figure 6.

Summary
Today's power management systems provide unprecedented performance and features that extend the operating time of battery-powered devices. This creates a greater variety of choices, making the job of selecting and evaluating candidates for new designs challenging. A good place to start is with the battery charger functional block, being a core block of a battery power management system. Because of the greater sophistication and intelligence to better manage the battery, the charger functional block requires a major investment of time and equipment to test.
?
A better alternative is to use more specialized equipment to reduce test system complexity and cost. For battery charger functional test, specialized DC power supplies that provide battery of sourcing and measurement capabilities, like the Agilent 66319B and 6612C for example, greatly simplify the test set up, freeing up your resources to better spend on developing your next generation product.

Edward Brorein is with Agilent Technologies.