Mechanical relay replacement

[jozef007]

Hi,
I just recently run into problem to replace functionality of mechanical relay with semiconductor. Little change from relay is that is should be unidirectional for conductive DC positive only as attached schematic bellow. I already check many articles on internet, there was some suggestions, but at end not all my needs was fulfill.

schematic 1

Solutions I checked:

1. P-MOSFET , fast enough also other characteristic satisfactory, but thanks to protective (body) diode can also conduct in opposite direction. Didn't find any MOSFET without diode.

2. Solid state relays:
They can carry only switch ground (either used NPN transistor or N-MOSFET) or they have triacs which will conduct even when control signal is turned off. I saw also with dual MOSFET, but I find it to carry maximum 1A, I expected to have at least 2A.

3. Use PNP transistor with diode. I have low voltage battery in system, cannot drop more than 0.25V. For such currents (2A or more) drops are usually much higher.

So far best I came with is dual MOSFET schematic.

Even when open is bi-directional, I control directionality via G electrode, so I can close it (I didn't put details such as resistor to clamp G to VCC, etc. schematic is just for illustrative purposes). My concern here is if diode can handle the same current as MOSFET (I expect that voltage can), also not sure how much voltage drop will be and speed as well (expectation is somewhere around microseconds). Control signal will go from N-MOSFET which will be driven by 3.3V logic.

Will appreciate if you someone can help me out with my problem.

Thx

 

[Harald Kapp]

My concern here is if diode can handle the same current as MOSFET (I expect that voltage can), also not sure how much voltage drop will be and speed as well (expectation is somewhere around microseconds).

What voltage? What current do you need?
Why the need for unidirectionality? There may be other ideas to solve the underlying problem.

We need more detail to help you.

You can buy diodes with > 100 A for example. Voltage drop will be 0.7 V ... 1 V depending on the specific diode. Look for Schottky diodes for very low voltage drop.
Speed is the least issue. Power Schottky diodes act within a few 10 nanoseconds.

Depending on the power dissipation you may need a heatsink.

Alternatively a PNP transistor (which you have excluded) should work, Depending on collector current you can buy ones with |Vcesat| < 200 mV.

 

[jozef007]

Hi Harald, thank you for respond. Actually you gave me very good idea. I started looking for Schottky diodes and found with real low voltage drop (360mV@5A). So far is the best what I have. Purpose of switch is disconnect batteries in series in order to charge them. I read the is not good idea to charge in series due to voltage difference. Here is schematic:

Are you consider it as possible solution? Or maybe something else can serve better? Using MOSFET giving me low resistence open and also is fast enough. Trying to avoid heatsink, in current configuration (around 2A) MOSFET or diode won't need it. On Diode U*I
will be 0.4*2=0.8W , on MOSFET 20mOhm fully open which is R*I2 0.02*2*2=0.08W. Hopefully my calculation is correct.
Thx

 

[Harald Kapp]

I read the is not good idea to charge in series due to voltage difference.

Actually it is not the voltage difference but the difference in charge state.

Did you have a look at "battery balancer" circuits? These are designed to solve just this issue.

 

[jozef007]

You are right, not voltage but charge for the batteries. I'm using Li-Ti batteries, which is almost impossible to find charging IC for them, therefore my effort to charge them separately. I just my new calculation, I can live with such voltage drop so look like solution to go.
Thank you Harald for your help as usual was good.

 

[jozef007]

Hello again,
during my project I wasn't able to resolve the issue so I will be more straightforward regarding the issue.
I want to use following functionality:

Battery is normally connected to the load, but during charging load will be disconnect and charger will be connected. Connection to charger need to have the same abilities as relay connection - both ways, minimum resistance and when closed minimum leakage (maybe few nA). How I can replace this function with semiconductors? Thank for looking into it.

 

[Harald Kapp]

A small relay can do exactly this.
Howeve, as you want an electronic solution, here is my proposal:

The MOSFETs are both P-channel, but back to back so the body diodes do not interfere in either direction of current flow.
The type of transistor and MOSFETs in this schematic is arbitrary, I used what was at hand in the tool (LTSPICE). You'll have to select components matching your application, especially with regard to low Rdson of the MOSFTs as well as a low Vgsthr (threshold voltage) of the MOSFETS to be controllable by the low voltages involved here. Tip: use "logic level MOSFET" for your search.

 

[jozef007]

Thx Harald for thoughtful schematic.Will be diode on MOSFET handle such 1A and also little concern regarding voltage drop. Charger contain charging IC which is sensing voltage in order to modulate charging curve. Not sure if difference between charging voltage and sensing voltage(if going through diode) won't cause issue.

 

[Harald Kapp]

Use a low Rdson MOSFET (< 10 mΩ) and the voltage drop should not be a concern. If it is, then use a relay.

 

[jozef007]

Hi Harald, at end I used relay, since related voltage drop. Thank for your support and guidance. Please consider issue as resolved.