H-Bridge using n MOSFET for rectification

Thread Starter

MAINAHNOUD

Joined Jul 24, 2019
67
In positive half cycle, current path through two MOSFETs and also pass through tga anti parallel diode of the other two MOSFET. In the negative half cycle the process is revesed

So, my question why current pass through anti parallel diode, I need waveforms of inductor current and diode current to under stand the concept
 

Attachments

wayneh

Joined Sep 9, 2010
17,153
In positive half cycle, current path through two MOSFETs and also pass through tga anti parallel diode of the other two MOSFET. In the negative half cycle the process is revesed

So, my question why current pass through anti parallel diode, I need waveforms of inductor current and diode current to under stand the concept
I believe you're looking at the body diodes of the MOSFETs. Conduction in the direction of the body diode is not typically used. When the gate-source voltage is high enough, the MOSFET conducts in either direction with very little voltage drop. If the gate voltage goes low, the MOSFET no longer conducts except in the direction of the body diode, and with a corresponding diode drop in voltage.

You haven't shown it but your high-side MOSFETs will need a gate drive that goes above the battery voltage.

I believe the inductor is present to protect against a transient shoot-through spike in current.
 

crutschow

Joined Mar 14, 2008
27,455
Below is the LTspice simulation of a simple MOSFET bridge rectifier with a plus and minus ramp input to show what's happening:
The flat spot at the bottom of the voltage/current to the load is when the voltage is below the MOSFET's Vgs threshold voltage, and the substrate diodes are carrying the current.
Understand that the MOSFETs are conducting here in the reverse direction when they are ON (same direction as their substrate diodes) since MOSFETs conduct equally well in both directions when ON , and are OFF in the normal direction.

Note that this bridge only works with a resistive type load.
You cannot add a capacitor to the output to generate a steady DC , since the stored charge will conduct back through the substrate diodes when the input voltage is below the capacitor voltage.

1611255618792.png
 
Last edited:

Thread Starter

MAINAHNOUD

Joined Jul 24, 2019
67
Below is the LTspice simulation of a simple MOSFET bridge rectifier with a plus and minus ramp input to show what's happening:
The flat spot at the bottom of the voltage/current to the load is when the voltage is below the MOSFET's Vgs threshold voltage, and the substrate diodes are carrying the current.
Understand that the MOSFETs are conducting here in the reverse direction when they are ON (same direction as their substrate diodes) since MOSFETs conduct equally well in both directions when ON , and are OFF in the normal direction.

Note that this bridge only works with a resistive type load.
You cannot add a capacitor to the output to generate a steady DC , since the stored charge will conduct back through the substrate diodes when the input voltage is below the capacitor voltage.

View attachment 228399
 

Thread Starter

MAINAHNOUD

Joined Jul 24, 2019
67
[/QUOTE]
Sorry I can't understand well.

I will discuss again what I need to know.

When I calculated the power loss of MOSFET, we have two part.
Firstly Conduction loss due to Ron of MOSFET. Secondly switching loss.

In condition loss as attached, there are loss during the anti parallel diode.

So my question why current in each half cycle pass through two diodes? @
 

Attachments

Thread Starter

MAINAHNOUD

Joined Jul 24, 2019
67
I believe you're looking at the body diodes of the MOSFETs. Conduction in the direction of the body diode is not typically used. When the gate-source voltage is high enough, the MOSFET conducts in either direction with very little voltage drop. If the gate voltage goes low, the MOSFET no longer conducts except in the direction of the body diode, and with a corresponding diode drop in voltage.

You haven't shown it but your high-side MOSFETs will need a gate drive that goes above the battery voltage.

I believe the inductor is present to protect against a transient shoot-through spike in current.
Did you mean that, for two MOSFET which is being on in first half cycle have high gate to source voltage. As the same time the other two MOSFET which in off state has low gate to source voltage, so the current pass through its diode of MOSFETs?

Yes, I have gate driver for high side MOSFETs.
 

crutschow

Joined Mar 14, 2008
27,455
Yes, in this example. It said that in each half cycle current pass through two MOSFETs and two diodes.

My question is, why current pass in the two diode besides through two MOSFETs?
You circuit is not complete enough for me to know what happens in your circuit.
It doesn't in mine.
Did you mean that, for two MOSFET which is being on in first half cycle have high gate to source voltage. As the same time the other two MOSFET which in off state has low gate to source voltage, so the current pass through its diode of MOSFETs?
No.
The current only goes through any of the diodes when the input voltage is below the threshold voltage of the MOSFETs.

The NMOSFETs are ON for a positive Vgs and the PMOSFETs are ON for a negative Vgs.
For either polarity of the input there is thus always one NMOSFET ON, along with the diagonal PMOSFET ON.

Edit: I see the difference in circuits is that mine uses complimentary MOSFETs. That's why yours conducts current through the diodes.
 
Last edited:

Thread Starter

MAINAHNOUD

Joined Jul 24, 2019
67
You circuit is not complete enough for me to know what happens in your circuit.
It doesn't in mine.
No.
The current only goes through any of the diodes when the input voltage is below the threshold voltage of the MOSFETs.

The NMOSFETs are ON for a positive Vgs and the PMOSFETs are ON for a negative Vgs.
For either polarity of the input there is thus always one NMOSFET ON, along with the diagonal PMOSFET ON.

Edit: I see the difference in circuits is that mine uses complimentary MOSFETs. That's why yours conducts current through the diodes.
All four MOSFET are n MOSFETs. Also, during S1&S3 are on, S2&s4 are off. So, did you mean when S2&s4 are off the current pass through their diodes of them?

Can you tell me what is complemntry mosfeet?
 

wayneh

Joined Sep 9, 2010
17,153
So, did you mean when S2&s4 are off the current pass through their diodes of them?
I said earlier that in most designs, MOSFETs act as switches and current does not flow in the body diode.

A notable exception is in "ideal" rectification. A comparator can be configured to trip when it sees a positive voltage (and current) across the MOSFET body diode. The comparator then turns on the MOSFET to conduct fully and eliminate the voltage drop due to the diode. When current reverses, the comparator turns off the MOSFET and no current flows in that MOSFET or its diode.
 
Top