Understand the system load carefully before connecting the charger

introduction

With the rapid development of electronic technology, battery chargers, as a traditional industry, are undergoing unprecedented changes and penetrating into all areas of life. In order to adapt to the needs of the market and the development of electronic equipment technology, intelligent charger detection systems have emerged, and research has been developing in the direction of high frequency, integration, intelligence and greening. Intelligent charger detection systems have also quickly become the first choice for various electronic charging equipment detection with their simple maintenance, high detection efficiency, strong expansion capabilities and long service life.

It is becoming more common for systems to operate on a charger without a battery. This situation may occur in normal applications or during manufacturer product testing. In Figure 1, there is no battery power to power the system during any transient or power-up conditions. If the design is not appropriate, the charger will latch off in a short-circuit condition. To solve these design problems, we must fully understand the charger's output source specifications and input system load requirements.

Figure 1 Charger power supply and system load structure diagram

No battery operation issues

Lithium-ion (Li-Ion) battery chargers are often thought of as controlled current sources at a regulated voltage. Typically, these devices are paired with a battery pack that acts as an energy reservoir (bulk capacitor) to continue powering the system through power transients. If the system load exceeds the source current for a moment, the battery supplies the extra current. Without the battery, if the system load current exceeds the charger's source current, the system voltage drops rapidly. The trouble is, the charger is a three-stage current source: short-circuit, pre-charge, and fast-charge. Exceeding the available current causes the system voltage to drop and may also cause the charger to enter a pre-charge and then a short-circuit phase with less current. Conversely, if the load current is less than the charger current, the system voltage rises until it reaches a regulated 4.2V voltage. Finally, the charge current drops to equal the load current.

To achieve battery-free operation, the charger and system must be designed so that the charger can always provide the required current to the system. To solve this problem, the IV characteristics of the charger must match the IV characteristics of the system load.

Output characteristics of the charger

A charger usually refers to a device that converts alternating current into low-voltage direct current. Chargers are widely used in various fields, especially in the field of life, where they are widely used in common electrical appliances such as mobile phones and cameras. A charger is a static converter that uses power electronic semiconductor devices to convert alternating current with a fixed voltage and frequency into direct current. In power applications where batteries are used as working power or backup power, chargers have broad application prospects. There are many types of chargers, such as lead-acid battery chargers, valve-regulated sealed lead-acid battery testing and monitoring, cadmium nickel battery chargers, nickel-metal hydride battery chargers, lithium-ion battery chargers, portable electronic equipment lithium-ion battery chargers, lithium-ion battery protection circuit multi-function chargers, electric vehicle battery chargers, car chargers, etc.

We will discuss a Li-Ion battery charger because it has several charging stages, but the concepts we have discussed can be easily applied to other charger chemistries. Figure 2 shows the charger current as it relates to the charger output voltage VSYS. If there is no battery and the charger is not powered, the initial voltage is 0V. When power is applied to the charger, the charger output voltage starts to rise due to the internal pull-up resistor (~500 Ohms) between the input and output. The short-circuit mode is below 1V and is designed to minimize power dissipation during a short circuit on the OUT pin.

Once above the short circuit threshold (0.8~1.4V), the charger enters pre-charge mode. Pre-charge is used to recover deeply discharged cells or to determine if the battery pack is damaged and needs to be terminated. The pre-charge current is about 1/10 of the fast charge current, but some chargers can program this level individually. The pre-charge mode switches to fast charge constant current at ~3V, at which point the programmed fast charge constant current is achieved. The charger will never provide more current than this programmed current level. After the voltage reaches constant voltage mode at 4.2V, the output is regulated and can provide current up to the programmed current level. If the load current drops to the termination threshold, charging is terminated unless the termination function is disabled.

Figure 2 Li-ion charging overview – charging current and voltage output

Table 1 lists the current for each stage.

Charger Mode Typical Voltage Range Available Current Equivalent Power

Short circuit mode 0~1.0 V 500 ohms or ~8mA 8 mW

Pre-charge mode 1~3 V 100 mA 100~300 mW

Fast charging mode 3~4.2 V 1000 mA 3~4.2 W

Now that we know how much current is coming from the charger, a system load analysis is needed to confirm whether the design is suitable for running without a battery.

System load characteristics

Resistive loads draw current that is proportional to the applied voltage and can appear during the power-up phase. A resistance of less than 125 Ohms (ISINK = 1V/125 Ohms = 8mA) can prevent the charger from exiting short circuit mode when powered without a battery.

Generally speaking, a DC/DC buck converter is enabled when its input voltage is close to its regulated output voltage. Since the converter has a fixed output voltage, its load is usually constant, so its input current is inversely proportional to its voltage. The two curves in Figure 3 show the input current into a 1.8V and 3.3V DC/DC converter versus input voltage. You can see that as the voltage goes up, the current goes down, and vice versa. Generally speaking, capacitive loading at the input of the converter is not a problem and it slows down power-up unless a timing event ends to cause a reset or further loading. Capacitive loading of the converter output at power-up can result in peak power requirements and can be reduced if the converter has a soft-start feature.

Pulse loads are imposed on otherwise static loads and can occur at any time, so special care should be taken when operating without a battery to ensure that the peak load does not exceed the available charger source current.

Figure 3 Relationship between DC/DC converter input current and input voltage: A) Incorrect power-on sequence; B) Correct power-on sequence

Source current and system load current comparison

When an asynchronous motor is running at no-load, the current passing through the three-phase winding of the stator is called no-load current. Most of the no-load current is used to generate a rotating magnetic field, called no-load excitation current, which is the reactive component of the no-load current. There is also a small part of the no-load current used to generate various power losses (such as friction, ventilation and core loss, etc.) when the motor is running at no-load. This part is the active component of the no-load current, which can be ignored because it accounts for a very small proportion. Therefore, the no-load current can be considered as reactive current.

There are two types of comparisons that should be considered: static DC comparisons and real-time power-up and operation comparisons. The DC comparison compares the system load current to the available charger source current at a specific system voltage only. Figure 3 shows the total load current and available charger current as the system voltage varies. Initially at power-up, the resistive load current is close to the short-circuit current of the available charger. Therefore, the designer may want to ensure that the output voltage can charge into the pre-charge region. In pre-charge, when the 1.8V converter is turned on at 1.6V, the total current will slightly exceed the pre-charge current. One solution is to turn on the converter at VSYS = 1.8V, so that the load current will drop, as shown in Figure 3b. Similarly, the 3.3V converter can be turned on at 2.8V. Delaying the turn-on until VSYS reaches 3.1V will move the load into the fast charge region, preventing loading issues. Now that the static problem has been analyzed, it is best to do an operational test immediately.

A real-time run comparison helps understand the load transient times and confirm that the peak load will not exceed the available source current. A simple test can be performed by connecting the system load to a lab power supply. Insert a 100m Ohm resistor in the loop and set the power supply voltage to 4.2V. Connect the oscilloscope probe as shown in Figure 4 to capture the voltage and current. When using a single sequence trigger, set the oscilloscope to the voltage waveform and turn on the lab power supply. The test can be repeated using hot swap. A continuous run test can be achieved by using a current trigger (set just below the charger programming control current threshold) while running the system in various operating modes. This test should be performed over the entire system VSYS operating range. If the oscilloscope is triggered, check the current pulse and determine if the load is too high.

Figure 4: Setup to capture real-time operating current and voltage waveforms

System: operational, cycling on/off, or locked up (crashed)

The ideal mode of operation when no battery is present is for the available charger current to always be higher than the system load current, allowing for stable operation. In this mode, the system capacitors charge to the regulation voltage and the fast charge current tapers off to equal the system load current. The system remains in this stable state mode as long as the system current is below the programmed fast charge current. If the load current exceeds the available charge current, a cycling or latch-up state is entered, as the DC/DC converter requires higher current at low system voltages. If the system voltage drops, shutting down the converter, the system voltage will recover before the next overcurrent load. This cycling mode is commonly referred to as "hiccup" mode.

No Battery Operation or Testing Design Tips

Create a table similar to Table 1, or plot a charger current curve like Figure 3, to define the absolute maximum load boundary of the system. Operate the system in all operating modes over the system voltage range and define when the system can be on and when it is below the maximum load boundary. The best solution is to only turn on the system when the charger is in fast charge mode. Never allow the load to exceed the effective minimum fast charge power (for example: 3 watts for fast charge mode in Table 1). Since both the charger output power and the system load power are functions of VSYS, you can compare the power or current to reach the same conclusion.

Therefore, designers should maintain the system power requirement below the minimum charger power output or the peak system current requirement below the programmed charger output current to ensure continuous system operation.

Summarize

Driving electronics from adapters and batteries is simple, as the battery is always available as a backup source for any peak loads that may occur. The only problem is that the average charger current is greater than the average load current, so the battery is not discharged. If battery-free operation is required, special care must be taken that the load current does not exceed the charger source current. Otherwise, the system voltage may collapse and get stuck in a low-power current limit state. Typically, short circuit and pre-charge modes are where problems occur. Avoiding full load operation in these modes solves most problems.