What are the effects of placing TVS tubes in front and behind? Which one is correct? Why?

kal9623287

What are the effects of placing TVS tubes in front and behind? Which one is correct? Why? [Copy link]

 

What are the effects of the following two TVS placement positions? Which design is correct? Why?

langtuodianzi

Figure 2 is correct. The fuse is in front of the TVS tube. If the high current fuse blows, the TVS tube will not be damaged.

kal9623287

langtuodianzi posted on 2022-5-10 10:43 Figure 2 is correct. The fuse line is in front of the TVS tube. If the high current fuse blows, it will not cause damage to the TVS tube.

Thanks, could you tell me the difference between the two placements?

kal9623287

A colleague said that in the following figure, the voltage mutation first drops to the inductor and capacitor in front, and then to the TVS. The impact is borne by the front. How does TVS protect?

How to explain

chunyang

TVS should be connected in front, but must be connected after the fuse. As for the above figure, if the input is connected to high voltage by mistake, what will be the consequences of connecting it as shown in the figure? As for the connection method in the figure below, if TVS is turned on, even if the fuse blows, the subsequent circuit can be protected, but L1 needs to withstand a large current before the fuse blows. Obviously, the specification requirements for L1 will increase greatly, bringing unnecessary cost expenditure.

Fred_1977

It is not easy to define which one is right or wrong, but a main guideline is that the TVS should be placed close to the interface.

kal9623287

chunyang posted on 2022-5-10 11:57 TVS should be connected in front, but must be connected after the fuse. As for the above picture, if the input is connected to the high voltage by mistake, what will be the consequences of connecting it as shown in the picture? As for the following...

This means that the TVS in the above figure should be connected after the fuse

The following picture can be used, but the cost is high

I have a question. In the above figure, if the TVS should be connected after the fuse, how should the TVS parameters be selected? The fuse is 3A now.

chunyang

kal9623287 posted on 2022-5-10 15:57 It means that the TVS in the above picture should be connected after the fuse. The picture below can be used. There is a question about the high cost. In the picture above, the TVS should be connected after the fuse...

The following diagram is not good, the fuse should be connected at the front end. The current specification of the TVS after the fuse is slightly larger than the fuse, and the voltage specification should take into account your input voltage.

Gen_X

I support the second option.

The main function of this circuit is to provide power supply pulse (interference) protection, and to provide simple protection for power supplies with higher subsequent transient voltage requirements.

It is mainly used to prevent surges, pulse coupling interference between power supplies, etc.

The RV in front provides absorption of smaller sharp pulses with large-scale error interference but smaller energy, and the TVS in the back provides more accurate instantaneous overvoltage protection than the RV.

There was a foreign product that broke down in batches in different factories in a short period of time. It was a similar design problem: the TVS was placed in the front. After a modification similar to Figure 2, the equipment has been running for 2 years without any damage. Later, the Asia-Pacific "factory" asked for our modification plan, and all new products were modified.

Gen_X

As for the position of the fuse and inductor, it would be more reasonable to interchange the inductor and the fuse.

However, it is generally best not to have a large capacitor in parallel with the TVS, otherwise it will affect the transient performance of the TVS.

kal9623287

Gen_X posted on 2022-5-11 16:13 I support the second solution. The main function of this circuit is to provide power pulse (interference) protection, and the subsequent transient voltage requires a higher power supply...

Makes sense,

If there is a TVS tube in front of the inductor, can it suppress part of the reverse current?

Does the location of the inductor have a big impact on where the TVS tube is added?

se7ens

Figure 1 is correct. TVS has the function of absorbing high voltage spikes. If it is placed at the end, it will have no effect. It is better not to put it at all.

Gen_X

kal9623287 posted on 2022-5-12 08:37 It makes sense. If there is a TVS tube in front of the inductor, can it suppress part of the reverse current? Where the TVS tube is added is closely related to the position of the inductor...

No, the main purpose is to reduce the direct parallel relationship between TVS and capacitor.

秦天qintian0303

Figure 2 is equivalent to adding a layer of protection, but here we usually choose between TVS and varistor. Generally, TVS can be directly used, as close to the input terminal as possible.

hjl2832

Each has its own advantages. Most people choose the solution in Figure 1. Why? Because the solution in Figure 1 relies on the TVS action to force the input power adapter to enter the short-circuit protection mode and stop working, thereby protecting the back-end components from damage due to overvoltage.

Moreover, this connection will not cause damage to components, eliminating the problem of after-sales maintenance. The second solution, although it will burn out the fuse and will not damage other components, but the equipment will not work and the fuse needs to be replaced. Ordinary users may not understand it and will contact the manufacturer for after-sales service.

kal9623287

hjl2832 posted on 2022-8-17 12:41 Each has its own advantages. Most people choose the solution in Figure 1. Why? Because the solution in Figure 1 relies on the TVS action to force the input power adapter to enter the short circuit protection mode...

Thank you for your reply

The second solution you mentioned will burn the fuse but will not damage other components.

Is it better to choose a fast or slow insurance?

hjl2832

kal9623287 posted on 2022-8-18 07:16 Thank you for your reply. Although the second solution you mentioned will burn the fuse, it will not damage other components. Is it better to choose a fast or slow type for this type of fuse?

This depends on the needs. First of all, you need to understand the difference between fast insurance and slow insurance.

Slow-speed fuses are usually used when there are large capacitors or inductors in the subsequent stage. At the moment of power-on, there will be a large inrush current. At this time, if fast-speed fuses are used, this inrush current (usually N times the normal working current) is likely to cause the fuse to operate and burn out, which is like a common switching power supply.

Therefore, in the actual design of many circuits, in order to alleviate this large current impact, the chip will have a soft start protection mode. At the same time, many power supply circuits recommend connecting a small resistor (less than 10 ohms) in series with the large capacitor to limit the instantaneous impact current.