# Generating a floating voltage

Tags:
1. Jan 11, 2019

### marino

In this circuit i want generate the floatin voltage from the external supply connected by a load to drain of transistors (suppose this is constant). So for me is possible to command the gates without an auxiliary low voltage battery.
Taking advantage of the high sensitivity of the mosfet I connected the output of the logic gates (controlled by the external generator) directly only to the transistor gates, without the ground connection of the sources (otherwise I create a short circuit in the mosfet below) but I was wondering if there were some other circuit for generating a floating voltage.

2. Jan 11, 2019

### Baluncore

You need to define what you mean by a floating voltage.
What determines the floating voltage?

Is it a differential voltage that appears across a load? What is that load?
Is there a DC supply available? Does the floating voltage always remain between the DC supply rails?

What is represented on your diagram by the line IAC ?

3. Jan 11, 2019

### marino

Suppose the generator on the right that produce IAC is 30V (ac or dc).
I want a voltage of 5V dc to alimentate the logic gates and then supply the correct voltage between the drain and gates of the mosfet.
I cannot use simply a voltage regulator because then the drain of the transistors are connect to ground.
Possibly I can use a couple of resistors to create a voltage partitioninig.
But, there is another way?

4. Jan 11, 2019

### CWatters

How fast must they switch? Perhaps generate the power supply for the gates from the AC and use an opto isolator to control them.

5. Jan 11, 2019

### Svein

The circuit you have drawn makes no sense, neither does your explanation. Creating a "floating voltage" usually involves a transformer of some sort (although it is possible to do without).

So (as several others have already asked):
• What is the expected voltage difference between the source generator an the floating voltage? The answer to that defines the isolation requirements of the transformer (obviously a voltage difference of 15V needs a far simpler and cheaper transformer than a voltage difference of 15 000V).
• What value should the floating voltage have (5V, 12V, ±15V, 100VAC)?
• What current should the floating voltage supply to the load?
Answer that and we can steer you in the right direction.

6. Jan 11, 2019

### sophiecentaur

I have a feeling that what you want may not be a "floating" control voltage but just an 'offset' for your control voltage, which is easier. There are many ways to achieve either. You could even use a magnetic relay (a common solution in byegone days), a transformer with primary and secondary windings suitably isolated from each other or an Opto-Coupler. I think that yours is a common requirement so there must be a convenient solution to the problem. you may even get away with a resistor network or a suitable Zener Diode for shifting your control volts if the operating volt offset can be the same all the time.
As @Baluncore says, your diagram is too vague (and has errors as well) to get a good answer. I would suggest that, unless and until your knowledge of Electrical circuits is adequate (good enough to produce a feasible circuit).

7. Jan 11, 2019

### jim hardy

8. Jan 13, 2019

### alan123hk

Hi marino,

I am not sure if I can fully comprehend about the circuit diagram and the specific requirements involved. I just want to share some ideas for your reference.

There are certain methods to provide a stable voltage between the gate and source of the two transistors. The final solution depends on your specific situation and requirements.

The figure below shows two possible methods, one is very simple circuit and low cost, the other is complicated circuit and expensive, namely the Zener diode circuit and the other just AC / DC converter, of course if you treat the AC/DC converter as a module and are lucky enough to find a suitable one to buy off the shelf, then it may become simple and not so expensive.

There is a peculiar and interesting thing here. I personally believe that the gate and source voltages of the two transistors should not be considered as a floating voltages when the voltage difference between the gate and source is established, namely when the two transistors are turned on. This is because those voltages can actually be referenced to the system ground.

However, the gate and source voltages could possibly become floating level when the two transistor are completely turned off. That is, when the voltage difference between the gate and source is zero, but at this time there is no need to create any voltage from the outside, it can just become floating by itself.

Last edited: Jan 13, 2019
9. Jan 13, 2019

### Svein

There seems to be several possible interpretations of the OP.
1. The creation of a power supply totally disconnected from the main supply. This was my original interpretation.
2. Using a MOSFET pair with source and drain disconnected from the power supply. This is usually called a "transmission gate" and is basically an analog switch:
3. Using a "floating gate" which is the principle behind flash memory etc.:

10. Jan 13, 2019

### marino

Sorry, here the circuit:

The question: the simple method to eliminate VL?

#### Attached Files:

• ###### upload_2019-1-13_21-7-41.png
File size:
6.3 KB
Views:
32
11. Jan 13, 2019

### marino

... getting the voltage from VS!

12. Jan 13, 2019

### Baluncore

One answer is to use a TRIAC.

What does the control logic do?

13. Jan 14, 2019

### Svein

To start with: The circuit you have drawn is very wrong. I suspect that you meant to draw a complementary pair, but you have not done so.

Anyhow: You do not control AC directly from logic. Use a relay of some sort. For AC up to 0.5A, try a DIP relay.

For lager currents, search the web.

14. Jan 14, 2019

### Windadct

I think the language you want is you want an ISOLATED voltage source to drive the MOSFET Gates? ...

So the application is an AC switch - using series MOSFETs ... in my experience this is done for one of 2 reasons, modulate the AC from Vs, or to make a switch that can control the Vs to the load and turn off sub-cycle ( not wait for a zero cross) -- is this close?

You need a power source to drive the gates, if you do not want the battery - what power source do you want to use to drive the gates?

15. Jan 14, 2019

### marino

The control logic is simply an astable with an asymmetric retard for every state. This net give a signal of 0V or 5V to mosfet's.
The mosfets are a pair of IRF740 used as an inexpensive SSR.
The circuit works correctly, it works the same if I disconnect the connection from the logic net to the drain of mosfet.

16. Jan 14, 2019

### marino

How thinked by Windadct the source is ISOLATED, sorry for my misandestend!
My question is: how to get a source voltage for net logic from Vs? The simplest way!

17. Jan 14, 2019

### Windadct

Ahhh - google Basic Optop MOSFET driver - lots of options. You MAY want to make a basic regulated 12V or 24V DC from the Vs. - about 10 parts.

A single 5V logic can drive an opto for each MOSFET.

18. Jan 14, 2019

### marino

Intresting.
Photovoltaic MOSFET drivers: TLP3905
Power MOSFET Photovoltaic Relay: PVG612PbF

19. Jan 29, 2019

### Henryk

If your current isn't too high, you can replace the entire circuit by a photovoltaic relay. These are one-chip solutions that combine photovoltaic MOSFET driver with two source-to-source connected MOSFETS.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?