Battery Tester --- Power Conversion

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With the rise of the lithium battery industry, the market for battery testing equipment has also become huge, and it is mainly used in the chemical composition of 3C batteries and power batteries. 3C batteries have a small number of strings, and the actual use does not require high consistency for each string of batteries. However, power batteries have hundreds of strings and are used in relatively extreme environments. In order to ensure a longer service life, the consistency requirements are much higher than those of 3C batteries. Therefore, the current accuracy required in the composition of batteries is higher. At present, according to market requirements, maintaining the requirement of 0.02% is a design challenge faced by battery testing equipment manufacturers. In order to gain a higher market share, the pursuit of accuracy, efficiency, power density and other performance has never stopped.

It is important to know that in battery equipment, there are three main parts, namely, bidirectional AC-DC power conversion, data processing unit, and battery testing unit. This article mainly analyzes the power conversion part of the battery testing unit, which is closely related to the key points of battery capacity division technology.

Power conversion

The power part of the battery test unit mainly includes analog controller, half-bridge driver, MOSFET, input and output protector. The most commonly used power conversion topology is the half-bridge rectifier Buck/Boost conversion circuit, mainly because the circuit structure is simple and the control is not complicated. At the same time, it can meet the current market applications with output currents as low as 0.5A or less and as high as hundreds of amperes. The difference is that in high-current applications, in order to ensure good heat dissipation performance and reduce the size, multi-phase Buck/Boost parallel connection is often used. Whether it is single-phase, two-phase or even multi-phase, the control scheme remains unchanged.

TL594 discrete solution

Currently, the more mature solution is the analog discrete solution of TL594. Texas Instruments has previously launched the TI Design of the TL594 solution, as shown in Figure 1. This solution is mature, stable, and has cost-effective advantages.

Figure 1 TI Design block diagram

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Half-bridge driver

When the output current of the test unit is around 10A or less, and the driving capability requirement is not large, generally around 1A can meet the requirement. The more commonly used ones are LM5109B and LM5106, as shown in Table 1.

When the output current is tens or hundreds of amperes, considering the driving current required for the fast switching of MOSFET, UCC27282 currently drives 2.5A source/3.5A sink current, and HS can withstand a large negative voltage and has high reliability. Because it has an interlock function, it will not be interfered by the di/dt of the converter half-bridge node and cause half-bridge direct conduction.

Table 1: Drivers

	LM5106	LM5109B	UCC27282
Bus voltage (Max) (V)	100	90	120
Peak current SINK/SOURCE (A)	1.8/1.2	1	3.5/2.5
HS pin minimum negative voltage (V )	-1	-1	-14

LM5060 and LM5069 are used to realize the protection of power interface

When the battery is charging, if the system does not have current backflow protection, the tested battery will discharge, resulting in unreliable system startup. At the same time, when the system is overvoltage, overcurrent, and undervoltage protected, if only the main circuit MOSFET is turned off, the battery still has a current backflow path, so it is particularly important to add input and output interface protectors. LM5060 and LM5069 are suitable for undervoltage, overvoltage, surge current and other protections of the input and output interfaces. The parameters are shown in Table 2.

Table 2: Load switch parameter table

	LM5060	LM5069
Minimum input voltage (V)	5.5	9
Maximum input voltage (V )	65	80
MOFET	external
Function	Adjustable current limit
	Inrush current limitation
	Circuit breaker function for overcurrent events

LM5170-Q1 Integrated Solution

When the current is large, multi-phase interleaved parallel connection is required, but a TL594 can only control one phase of power conversion current, and an external synchronization signal needs to be given to the TL594. Multi-phase parallel connection increases the complexity of the TL594 discrete component solution. The LM5170-Q1 solution contains two control loops, which can work independently or in parallel, and integrates overvoltage, overcurrent, undervoltage and other protections, supports diode simulation mode, and also has a protection circuit to prevent current backflow. Figure 2 shows a typical application solution of two parallel connections, TI Design: TIDA-01041, with a maximum output current of 100A, and a current accuracy of 0.02% and a voltage accuracy of 0.1%.

Figure 2: TIDA-01041

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The advantage of the LM5170-Q1 solution is that it not only leaves room for free expansion in the design of the signal chain, but also has a high degree of integration, providing a large space for the heat dissipation of the power conversion MOSFET. ti.com provides several reference designs of different current sizes, TID-A01041 and TIDA-01042.