BCM, many manufacturers have different names and may be divided into many modules, but anyway, for the convenience of discussion, we still call BCM uniformly.
Throughout the past 20 years, the development of BCM technology has made considerable progress. However, there is a problem that has not been resolved. Harness experts criticized it, but it hasn't improved much.
We all know that the boundary between BCM and the so-called SJB (smart Junction box) is becoming more and more blurred. It seems that one is that mobile phones have added entertainment functions, and the other is that iPod has added mobile phone calls. They have the same goal. , Whether there is a fuse to distinguish it.
Through this passage, everyone should already know that whether it is BCM or SJB, its functions are divided into two parts: power distribution and control.
Regarding control, this piece has indeed developed rapidly. More and more functions have been integrated (I said integration-not all in one module, but also in several modules (such as domain control)- But they are all put into the module to control, from the original switch/sensor—(relay)—actuator mode to the "switch/sensor—controller—actuator" mode), through software logic, you can To achieve more and more complex functions between electrical devices that could not be correlated before, which is currently very fashionable: use software to define cars.
This is true. Many of the things that do traditional wiring harnesses are "very panic". It seems that conventional wiring harnesses will be abandoned and thrown into the trash can of history.
In fact, such a pessimistic mood is unnecessary, even if control develops again-electrical devices still need electricity to drive after all. Moreover, after so many years, another part of BCM---power distribution has not been done well. This is also the part that this article will talk about.
Let me talk about the way BCM drives electrical appliances.
Generally speaking, it is divided into these types:
1. Relay drive. In this part, motor loads are more common. There may be loads with inrush current, locked-rotor current, or sizeable steady-state current. Power chips that are small and difficult to drive will be placed here and use relays to drive.
2. Power chip driver:
A. The chip has a protection function;
This category is the most common in chip drivers. However, if you are careful, you will find that the short-circuit protection of the chip is different from the fuse, and the parameter difference is not slight. Generally speaking, for the short-circuit protection function, Not bad, but for overload protection, compared to fuses, it is much weaker. ---This is actually a big problem.
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B. The chip has no protection function and has an external protection circuit;
Sometimes from the cost aspect, the driver chip is not protected. At this time, if the external circuit is found to be short-circuited, it will cause the controller or the wiring harness to burn out. Therefore, this severe failure cannot be allowed to occur. Thus, in many cases, the chip driver circuit The downstream will design a current detection loop, using a Hall sensor or a sampling resistor to monitor the current. Once an abnormal current is found, the chip output will be terminated in time. The loop protection function can also be realized. The response speed depends on the frequency of the detection current. The response speed of the chip has been. So in actual applications, this is also a flexible design.
C. The chip has no protection function and no external protection circuit.
In this case, the status quo is very rare, so I won't discuss it here.
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Seeing this, the careful friend may have already begun to think. For case 2, there is built-in protection. We can regard this protection as a fuse (although there is still a big difference with the fuse), regardless of whether it is very mature or not, at least it has; but for case 1, what about the relay drive situation? We all know that a relay contact is equivalent to a switch. Once closed in a loop, you can treat it like a wire.
So here comes the problem.
We all know that there are very few fuses on the top of the BCM. Many times, one hand counts them, but how many loads does it drive? There are a dozen or more fuses, let alone more.
In addition to the loads controlled by case 2 above, there are still many loads controlled by relays. Then you will find that BCM has a separate fuse for the loads controlled by these relays? The answer is almost always no.
For example, the washing motor is controlled by a relay inside the BCM, but it shares a 30A fuse with many other loads. At this time, we have seen many situations where the pin terminal of the relay output is usually designed to only be able to The airport that carries the working current of the washing current.
There are two problems here:
1. The internal circuit (PCB trace) of the controller may not consider the fusing current of the 30A fuse. When an external short circuit occurs, it may be burned;
2. The output terminal is too tiny to crimp a larger wire-the wire cannot match the upstream fuse. When an external short circuit occurs, or the motor is blocked, the terminal/connector and wire may be burned.
The first problem is that if the BCM is only burned, it does not continue to cause more significant problems. From the perspective of the entire vehicle, if it is not an emergency, the harm is not that great.
However, such designs abound in current BCM designs.
For such problems, the author gives several solutions for reference:
1. For the load driven by the relay, give a separate fuse-this is the simplest and lowest cost implementation.
2. The fuse position is limited and must be shared; then, in the BCM, the relay drive circuit must add current monitoring. Once a short course occurs, cut off the relay.
3. When doing DV on the harness, bring BCM to verify together.
4. The plan I most want to give is this: Ask the harness system experts to participate in the BCM power distribution design let. Professional people do professional things. Those who do BCM must admit that power distribution is not your specialty, and you should listen to the opinions of wiring harness system experts appropriately.