Categories: Electronics

Useful security in non-automotive BMS designs

Battery-powered purposes, which have develop into indispensable during the last decade, require a sure degree of safety to make sure secure use. This security is supplied by the battery administration system (BMS). The BMS displays the battery and potential fault circumstances, prevents any hazardous scenario because of the battery or its environment, and ensures that there’s an correct estimation of the battery’s remaining capability or the extent of battery degradation.

The primary construction of a BMS for a low- or medium-voltage battery is often made up of three ICs, as described beneath:

Battery monitor and protector: Also called the analog front-end (AFE), the battery monitor and protector offers the primary degree of safety because it’s chargeable for measuring the battery’s voltages, currents, and temperatures.
Microcontroller unit (MCU): The MCU, which processes the info coming from the battery monitor and protector, generally incorporates a second degree of safety, together with monitoring thresholds.
Gas gauge (FG): The gas gauge is a separate IC that gives the state-of-charge (SOC), state-of-health (SOH) info and remaining runtime estimates, in addition to different user-related battery parameters.

Determine 1 The BMS structure shows the important thing three constructing blocks. Supply: Monolithic Energy Techniques

Determine 1 exhibits the primary construction of a whole BMS for low- or medium-voltage batteries. The gas gauge is usually a standalone IC, or it may be embedded within the MCU. The MCU is the central aspect of the BMS, taking info from each the AFE and gas gauge and interfacing with the remainder of the system.

Whereas three essential parts represent the BMS, utilizing these parts with none extra consideration shouldn’t be sufficient to make sure that the system meets the protection degree required by sure industries. This text will clarify the function that practical security performs in non-automotive battery administration techniques and the best way to obtain the required security degree.

Useful security introduction

Useful security is a department of total security centered on lowering the danger produced by hazardous occasions resulting from a practical failure of an electrical/digital (E/E) system. The aim is to make sure that the residual threat is inside an appropriate vary.

Lately, the growing use of E/E techniques in several fields reminiscent of automotive, equipment, medication, business, and aviation has been accompanied by a better emphasis on practical security. These modifications have led to the event of various practical security requirements.

ISO 13849, titled “Security of equipment – Security associated a part of management techniques”, is a practical security customary centered on the safety-related components of management techniques (SRP/CS) within the equipment area. This can be a area that features a broad spectrum of purposes, from generic industrial equipment to mopeds and e-bikes. ISO 13849 defines totally different security ranges as efficiency degree (PL), which vary from PLa (decrease security degree) to PLe (increased security degree).

This security customary defines an correct course of for threat analysis and discount. It proposes a simplified methodology to find out the achieved PL based mostly on three parameters: class, imply time to harmful failure (MTTF_D), and common diagnostic protection (DC_AVG), which is calculated by averaging all of the DC related to the totally different security measures utilized within the system.

The class is a classification of an SRP/CS that describes its resistance to faults and the following habits within the occasion of a fault situation. There are 5 classes (B, 1, 2, 3, and 4).

Structure has the most important impression on the class. The fundamental structure of an SRP/CS consists of three practical blocks: an enter, a logic block, and an output (Determine 2). Determine 2 corresponds with the structure proposed for class B and class 1, and it’s known as a “single-channel” structure. A single-channel structure is taken into account probably the most fundamental structure to implement the nominal performance of the SRP/CS, nevertheless it’s not supposed for any diagnostic performance.

Determine 2 The above structure is proposed for class B and class 1. Supply: Monolithic Energy Techniques

Class B and 1 depend on the reliability of their parts (MTTF_D) to make sure the integrity of the protection features. If a part implementing the protection operate has a failure, a secure state can not be assured, as no diagnostics are carried out (DC_AVG = 0).

For class 2, the proposed structure is named “single-channel examined.” The bottom of this structure is identical because the single-channel structure, however with an added check gear block that may diagnose whether or not the practical channel is working accurately. If a part implementing the protection operate has a failure, the protection operate shouldn’t be carried out; nonetheless, a secure state might be achieved if the failure is identified by the check gear.

For class 3 and class 4, the proposed structure is named “redundant channels,” which is carried out with two impartial practical channels that may diagnose points on the opposite channel. If a part implementing the protection operate has a failure, the protection operate can nonetheless be carried out by the opposite channel. Designers ought to choose the SRP/CS class based mostly on the focused security degree of every security operate.

Reaching practical security step-by-step

The ISO 13849 customary defines an iterative course of throughout which the SRP/CS design is evaluated to find out the achieved PL and examine whether or not that security degree is enough or should be improved in a brand new loop. The method consists of three totally different strategies for threat discount: threat discount by way of secure designs measures, threat discount by way of safeguarding, and threat discount by way of info to be used. ISO 13849 helps threat discount by way of safeguarding (Determine 3).

Determine 3 ISO 13849 helps threat discount by way of safeguarding. Supply: Monolithic Energy Techniques

The safeguarding course of begins by defining the protection features of the SRP/CS, by which the required efficiency degree (PLr) is outlined after the danger evaluation is carried out. The PLr is the goal PL of the SRP/CS for every security operate.

The subsequent step consists of designing the SRP/CS for the required security necessities. This entails contemplating the potential structure, the protection measures to implement, and finalizing the design of the SRP/CS to carry out the related security features.

As soon as the SRP/CS is designed, consider the achieved efficiency degree for every security operate. That is the core step of your complete safeguarding course of. To judge the achieved PL, outline the class after which calculate the MTTF_D and DC_AVG of the SRP/CS for every particular person security operate.

The MTTF_D is calculated per channel, and it has three ranges (Desk 1).

Desk 1 MTTF_D, calculated per channel, has three ranges. Supply: Monolithic Energy Techniques

Desk 2 exhibits the 4 ranges for outlining the DC of every diagnostic measure.

Desk 2 There are 4 ranges for outlining the DC of every diagnostic measure. Supply: Monolithic Energy Techniques

The achievable PL might be decided utilizing the related parameters (Desk 3).

Desk 3 Related parameters assist decide the achievable PL. Supply: Monolithic Energy Techniques

The achievable PL can solely be confirmed when the remaining necessities and analyses outlined by the usual are carried out within the design. These necessities should adjust to systematic failures administration, widespread trigger failure (CCF) evaluation, security ideas and software program improvement, if relevant.

As soon as this course of is full, the PL achieved by the SRP/CS for a concrete security operate needs to be verified towards the PLr. If PL < PLr, then the SRP/CS needs to be redesigned, and the PL analysis course of should start once more. If PL ≥ PLr, then the SRP/CS has achieved the required security degree, and validation should be executed to make sure the right habits via testing. If there’s an surprising habits, the SRP/CS needs to be redesigned. This course of needs to be reiterated for every security operate.

Useful security degree in keeping with every market

Battery-powered units are utilized in numerous markets, and every market calls for totally different practical security specs in keeping with how harmful a failure might be for people and/or the setting. Desk 4 exhibits the practical security degree required by a number of the essential markets. Word that these ranges are consistently altering and could also be totally different relying on every engineering workforce’s design.

Desk 4 That is how PL is decided based mostly on market. Supply: Monolithic Energy Techniques

Though these are the present efficiency degree market expectations, electromobility and sure power storage purposes could transfer into PLd because of the fixed points in battery-powered units around the globe. For instance, defective power storge purposes have resulted in fires in U.S. power storage system (ESS) services. In U.Ok., greater than 190 individuals have been injured, and eight individuals have been killed by fires sparked by defective e-bikes and e-scooters.

All these occasions may have been prevented by a extra strong and dependable system. The fixed want for growing security ranges means it’s important to have a scalable answer that may be carried out throughout totally different efficiency ranges.

A practical security design proposal

Take the case of an ISO 13849-based BMS idea that Monolithic Energy Techniques (MPS) has developed by combining an MCU with its MP279x household of battery displays and protectors. This technique is oriented to realize as much as PLc security degree for a sure set of security features (SFs), as proven in Desk 5. PLr willpower relies on the danger evaluation, by which small variations can happen, in addition to the applying by which the BMS is used.

Desk 5 See the outlined security features for the BMS idea. Supply: Monolithic Energy Techniques

The answer proposed by MPS to realize PLc can meet class 2 or class 3—relying on every security operate—as for sure security features. There may be solely a single enter block and for others, there are redundant enter blocks.

Determine 4 exhibits the best way to implement SF2 and SF4 to forestall the battery pack from over-charging and under-charging. Within the implementation of the SRP/CS, there are two logic blocks: the battery monitor and protector (logic 1) and the MCU (logic 2). These logic blocks are used to diagnose appropriate performance of various components within the design.

Determine 4 Right here is the best way to implement SF2 and SF4. Supply: Monolithic Energy Techniques

The implementation of single or duplicated enter is decided by the complexity and value in every case. To make sure that the protection features for a single enter are compliant with PLc, extra security measures might be taken to extend the diagnostic functionality; an instance is a cell voltage plausibility examine to confirm that the cell voltage measurements are appropriate.

Useful security was once related for automotive merchandise, however these days most trendy markets demand the producer to adjust to a practical security customary. The perfect-known security customary for non-automotive markets is ISO 13849, a system-level customary that ensures an software’s security and robustness.

Miguel Angel Sanchez is purposes engineer at Monolithic Energy Techniques.

Diego Quintana is practical security engineer at Monolithic Energy Techniques.

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