How to get through the problem related to MOSFET Switching?

[Venkadesh_M]

Here is its block diagram they are leveling up but still taking Ho from Vb can not understand.....

 

[SACHIN C]

FvM,

Still a bit below 800V switched voltage you are targetting to?



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[FvM]

Yeah i also have the think of pulse transformers but I believe it is impossible to integrate it inside a IC.. so here is my problem how it is possible to level up to such a high voltage, is there any way without pulse transformer??

You can't do everything with "a IC". I guess that these days some developers are hoping to get around developing circuits when they can use an application specific IC. But the truth is, you must know even more about electronic circuits to understand what exactly happens inside an IC, in case your design possibly fails.

High voltage, high power electronics will require most of the occupied space for isolation, heat sinks, large capacitors etc. The saving of using ICs instead of discrete devices in the control circuit doesn't count much in this case. There are still consideration about minimizing part count and number of solder joints to increase reliablility. But particularly in the HV interface, reliablility of ICs may be problematic to determine.

Here is its block diagram they are leveling up but still taking Ho from Vb can not understand.....

This is only the level shifting driver without the external bootstrap circuit. I think, you can understand from the internal schematic how the driver might work with an external floating power supply.

 

[Venkadesh_M]

I know it was better using driver IC but what i want is to understand how they are triggering a floating mosfet, I believe a circuit will only work if we know how every component works......

Y
This is only the level shifting driver without the external bootstrap circuit. I think, you can understand from the internal schematic how the driver might work with an external floating power supply.

yeah i know it was driver IC without power mosfets but still how they are turning the mosfet with Vb.....?

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I believe this file will be use full for every one interested in power electronics......

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The driver IC is explained here.....

 

[FvM]

There are at least:
- a power supply (e.g. 12-15V) grounded to the negative bus voltage
- a bootstrap diode
- a bootstrap capacitor
- a low-side MOSFET that periodiaclly pulls the source node of high-side MOSFET to the negative bus voltage, so the bootstrap capacitor can be charged

A detailed explanation can be found in IRF, Fairchildsemi or other manufacturers driver datasheets and application notes.

 

[Venkadesh_M]

FvM,

Still a bit below 800V switched voltage you are targetting to?



Thanks & waiting for support to enhance the knowledge,:smile:

Why you need two 600V mosfets in series for 210 V supply???

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There are at least:
- a power supply (e.g. 12-15V) grounded to the negative bus voltage
- a bootstrap diode
- a bootstrap capacitor
- a low-side MOSFET that periodiaclly pulls the source node of high-side MOSFET to the negative bus voltage, so the bootstrap capacitor can be charged

A detailed explanation can be found in IRF, Fairchildsemi or other manufacturers driver datasheets and application notes.

I think I got it.....

 

[SACHIN C]

Hi Venkadesh_M,
I need 230Vrms at the output of Half bridge Inverter,for this reason voltage across each capacitor in half bridge need to be 325VDC(total 650VDC) for peak,400VDC is the maximum voltage across single capacitor,likewise 800VDc(across series connection of bus link capacitor for half bridge),

Actually using two Mosfet of 600Vdc capacity i have tested for 365VDC(105Vrms) Output,where i have used floating supply for high side driver

Thanks & waiting for support to enhance the knowledge,:smile:

 

[Venkadesh_M]

Hi Venkadesh_M,
I need 230Vrms at the output of Half bridge Inverter,for this reason voltage across each capacitor in half bridge need to be 325VDC(total 650VDC) for peak,400VDC is the maximum voltage across single capacitor,likewise 800VDc(across series connection of bus link capacitor for half bridge),

Actually using two Mosfet of 600Vdc capacity i have tested for 365VDC(105Vrms) Output,where i have used floating supply for high side driver

Thanks & waiting for support to enhance the knowledge,:smile:

Actualy this half bridge inverter is designed for SCR(because a natural commutation occurs here) but why you want to do it with Mosfet..?

A mosfet can do a lot more than SCR and you can design a full bridge PWM inverter with mosfet, If inverter is your AIM..

If you want to make a half bridge inverter then you can choose a SCR which will be cheaper than MOSFET and will be available in very high voltages....

 

[SACHIN C]

Hi Venkadesh_M,
current rating is also 40A?
can we design Half Bridge Inverter using Mosfet?
can you please explain me how natural commutation takes place in this case?

Thanks & waiting for support to enhance the knowledge,:smile:

 

[Venkadesh_M]

Assume there are two SCRs T1 in upper side and T2 is in lower side...

If you trigger the T1 SCR it will make the current through Positive T1 inductor Load to C2.... now the C2 will charge to 2*Vs(because charging through a inductance)...
And if the capacitor fully charged the current will be zero and the T1 will automatically turned OFF(current less than holding current)..

the same happens for T2 and will make the reverse current in load and make nearly sine wave on the load....

This circuit is specially designed for SCR and the voltage across it will be 2*Vs that can be the maximum....

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So with the same four MOSFETs you can comfortably design a full bridge PWM inverter without any conflicts.............

 

[SACHIN C]

Venkadesh_M,
The Two SCR will behave like an Half Bridge circuit,



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[Venkadesh_M]

Yes It is also called as series resonant inverter....

 

[SACHIN C]

Dear All,
Do series connection of MOSFET divides voltage across each equally?
Ex :If 600VDC max capacity MOSFET connected in series for 800VDC MAx voltage across series combination,will it divide equally


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[FvM]

Do series connection of MOSFET divides voltage across each equally?

Not automatically, generally speaking. Some balancing means, e.g. passive zener diodes or active clamping circuits are probably necessary, also careful adjustment of driver signal timing to achieve simultaneous switching. MOSFETs are available with up to 1200V rated Vds, although there performance isn't mind-blowing, I would clearly prefer them over a fragile cascade circuit if still possible for the intended operating voltage.

 

[SACHIN C]

Hi FvM,
If we connect Zener Diode for regulation,basically it will also appear in series i.e back to back,but will the current flow will be related to zener wattage during OFF state of MOSFET ?
During ON state of MOSFET,Zener will not act or it will also get short?

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[FvM]

The current through a zener diode is commanded by the external current/voltage source, not the zener diode. Rated power is something you need to check for your design, otherwise the zener diode overheats and fails.

During ON state of MOSFET,Zener will not act or it will also get short?

Why should it "get short"?

 

[SACHIN C]

If we apply 600VDC across MOSFET & the current would be in ON state 35A through MOSFET,when MOSFET is OFF zener need to regulate the voltage across MOSFET to 450VDC,is 450VDC Zener available & if it is rated to say 1W then at 450V the current flowing through the zener will be 2.22mA?



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[SACHIN C]

Dear All,
How to read SOA graph of an MOSFET?



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[SACHIN C]

How to read SOA graph of an MOSFET?



Thanks & waiting for support to enhance the knowledge,:smile:

 

[SACHIN C]

Dear All,
As i load half bridge inverter upto 400W,it was generating pure sinewave with proper Gate Signal,but as i increased the load(Resistive) the MOSFET blow.Both MOSFET blown that means it was short,but i observed the gate waveform before blowing it was proper.
Is Snubber creates problem,because it will also stores the transient energy?
Snubber =22ohm(47ohm parallel with 47ohm) & 10nF/1KV Ceramic Disc type Capacitor
MOSFET Capacity =1200VDC Max/20A each

Thanks & waiting for support to enhance the knowledge,:smile: