Is it necessary to use D1 bidirectional TVS in this circuit?

kal9623287

Is it necessary to use D1 bidirectional TVS in this circuit? [Copy link]

 

I saw a senior give a circuit, and after seeing it, I was a little curious and wanted to ask everyone in the forum,

Is it necessary to use SMBJ15CA bidirectional TVS for D1 in the actual circuit in the figure?

And one end is grounded.
Besides, the price of bidirectional is higher than that of unidirectional .

呜呼哀哉

Yes, if you connect the input VCC in reverse and try using a unidirectional one, you will understand why a bidirectional one is used.

maychang

In the figure, which one is the power input terminal, VIN or VCC? It seems that VIN is the input terminal and VCC is the output terminal, which provides power to other devices.

秦天qintian0303

TVS is not expensive whether it is bidirectional or unidirectional.

chunyang

Whether or not to use bidirectional TVS for D1 requires analysis of the circuit function.

The circuit in the original post is a reverse connection protection circuit. Vcc is the input and VIn is the output (these two labels are not good, semantically speaking, they are reversed). When the input is reversed, Q1 will not conduct, and its parasitic diode and D2 will not conduct either, thus preventing the problem caused by reverse input.

Since it is a reverse polarity protection circuit, D1 must use a bidirectional TVS, otherwise what will happen? You may as well tell me about it yourself.

吾妻思萌

You never know how your users will access your circuit. Therefore, fuses are the most effective first line of defense.

qwqwqw2088

Unidirectional TVS is generally used on DC power supplies, and bidirectional TVS is generally used for differential signal lines.

不亦心

One-way is fine

se7ens

If VIN is an input, what is D2 used for?

Besides, if TVS is not used in bidirectional mode, what are the consequences of reverse connection?

langtuodianzi

The price of unidirectional and bidirectional TVS diodes of the same voltage is the same.

不亦心

se7ens posted on 2022-7-19 09:49 If VIN is input, what is D2 used for? Besides, if TVS is not used in both directions, what are the consequences of reverse connection

Blowing the fuse is not a problem, I have done my duty.

kal9623287

chunyang posted on 2022-7-18 16:18 Whether D1 should use a bidirectional TVS depends on the circuit function. The circuit in the original post is a reverse connection protection circuit. Vcc is the input and VIn is the output (these two labels are not good...

Is it ok to put TVS at VIN?

kal9623287

se7ens posted on 2022-7-19 09:49 If VIN is input, what is D2 used for? Besides, if TVS is not used in both directions, what are the consequences of reverse connection

Can I put the TVS at the VIN?

kal9623287

Bu Yixin posted on 2022-7-22 00:14 Blowing the fuse is not a problem, I have done my duty

Excuse me, how do you understand that burning the fuse is not a problem?

I have a question, is it okay to put the TVS at the VIN?

chunyang

kal9623287 posted on 2022-7-25 07:31 Is it okay to put the TVS at the VIN?

The fuse must be placed on the input side of the system, otherwise the circuit before the fuse will not be protected. For example, as shown in the original poster's picture, the TVS is a unidirectional TVS. What will happen if the input power is reversed and the fuse is placed at VIN?

chunyang

kal9623287 posted on 2022-7-25 07:33 Excuse me, you said that burning the fuse is not a problem, how do you understand that there is a problem, then is it okay to put the TVS at the VIN?

This means that the fuse will burn out, thus avoiding damage to the front-stage power supply. What will happen if you put the fuse at the back? You should understand.

However, although the fuse will burn out when the input is reversely connected with a unidirectional TVS, it will still cause a surge impact on the previous stage power supply. If the fuse is used with the wrong specifications, it is hard to say. Therefore, a bidirectional TVS should be used here, at least the fuse will not burn out when the power supply is reversely connected, and there will be no surge impact on the previous stage.

不亦心

kal9623287 posted on 2022-7-25 07:33 Excuse me, you said that burning the fuse is not a problem, how do you understand that there is a problem, then is it okay to put the TVS at the VIN?

What I mean is that unidirectional TVS can also provide protection, but it comes at the expense of the fuse. It is of course better to use a bidirectional one.

In fact, as far as the reverse connection protection function is concerned, TVS and diodes are redundant and are not needed.

kal9623287

Bu Yixin posted on 2022-7-26 00:06 What I mean is that unidirectional TVS can also play a protective role, but at the expense of the fuse. It is of course better to use a bidirectional one. In fact, it can only protect against...

This is how it works, thanks.