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What's the difference between MOSFET high side and low side switching?

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narithota

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Hi

I need some information regarding the Power MOSFET switching.

1.what is the difference between high side & Low side switching
2.when we have to use High side switching
3.when we have to use Low side switching

Regards,
nari
 

Re: MOSFET switching.

narithota said:
Hi

I need some information regarding the Power MOSFET switching.

1.what is the difference between high side & Low side switching
2.when we have to use High side switching
3.when we have to use Low side switching

Regards,
nari
Click to expand...

Hi nari,

The highside and lowside MOSFET is chosed by your circuit topology.

The highside MOSFET which means it's body can be floating, you can short the body to source or other place according your requirement. Of course, the body is tied to source generally. The lowside MOSFET, its body is substrate. So its body is not floatable.
 

MOSFET switching.

Hi,
High side switching is required when your MOSFET is in high side configuration. This is when the drain is connected to +ve and load is taken through source to ground. As a result source has no direct connection to ground. Here when +ve is 200v, your VGS has to be 200+10 = 210V because source is not directly connected to ground, and when on has a potential of 200v.
Low side is where load is taken from drain and source is connected to ground. Here, VGS is always eg 10v as source is connected directly to ground.

In bridge circuits you need both high and low side switching.

Hope this helps.
Tahmid.
 
Re: MOSFET switching.

Hi

what is boost trap capacitor,why we need this circuit for High side MOSFET.
Can give some explanation or online materials regarding the boost trap.

Thank you.

Regards,
Nari
 

Re: MOSFET switching.

this datasheet may help explain high side and low side switching.

**broken link removed**
 

MOSFET switching.

Hi,
Say your V+ is +12V. And your bootstrap diode has a Vf of 0.6v. In that case, when the capacitor is being charged, it is charged to (12-0.6) = 11.4V. When being discharged, there is 11.4v across the cap, and there is 12v on the line. So, voltage at source = 12V on load, and therefore, voltage on gate = (12+11.4) = 23.4V
This is more than 10V greater than your source voltage and therefore, you can successfully turn the MOSFET on in high side configuration.

Take a look at this diagram [Taken from electro-tech-online, posted by Audioguru]:


Hope this helps.
Tahmid.
 
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Discrete driver for MOSFET .

Hi
Thanks for the valuable information.
i want to make H-Bridge circuit using 75NF75 MOSFET.For this i have to design the discrete driver. please guide me how to design the driver for H-Bridge.
how to calculate the Gate current of the 75NF75 MOSFET.

Regards

nari
 

Re: MOSFET switching.

Hello Tahmid how are you I have a great pronlem with a inverter I am building it uses a H bridge output but I cant get the output voltage to pass about 44 volts AC I have 196 v on the drain of the fets but I dont know how to get the voltage up could you help me here . I am using tl494 as pwm and sg3525 for the high voltage side, I am running the high voltage side frequency as high as 160khz. Please help me here thank you
 

Re: MOSFET switching.

joelyoung said:
Hello Tahmid how are you I have a great pronlem with a inverter I am building it uses a H bridge output but I cant get the output voltage to pass about 44 volts AC I have 196 v on the drain of the fets but I dont know how to get the voltage up could you help me here . I am using tl494 as pwm and sg3525 for the high voltage side, I am running the high voltage side frequency as high as 160khz. Please help me here thank you
Click to expand...

Which high-side driver are you using? Probably the problem is in the high-low side driver. Either the configuration of the circuit and drivers is wrong. Or the value of the bootstrap capacitor is too low. You should upload the circuit you're using.
 

Re: MOSFET switching.

Thank you for your reply. I have uploaded the circuit please tell me if you see it. Thank you
 

Re: MOSFET switching.

joelyoung said:
Thank you for your reply. I have uploaded the circuit please tell me if you see it. Thank you
Click to expand...
Please check to see if you can see the circuit.
 

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Re: MOSFET switching.

The only difference is I am using SG3425 for the pwm for the high voltage inverter insted of tl494
 

Yes, I can see the circuit. I'm having a look at it now.

---------- Post added at 19:56 ---------- Previous post was at 19:53 ----------

What transistor are you using for VT5 and VT8?

What are the resistances of resistor R16, R17, R18, R19, R20, R21, R22, R23, R24, R25, R26, R28?

What are the capacitances of C12, C13 and C15?

What are you using as a load? How much power does it draw?
 

Thank you for your reply I have uploaded the parts list, I am actually putting two circuits together to make one , This inverter is for 220 volts output but I am building it for 120v Ac so I am using NOW Irf630 for vt6 and vt9 but I intend to use IRFP260n finally. Today I got a bit further so I tell you what is hapening now. I changed values for R30 R29 and I got the volts to move up. Now when I adjust the high volts to 170v Dc I only get about 95v AC when I turn it down to about 115 to 120 v dc I get 120 v AC My understanding is 150 to 170DC volts should be giving you about 120 v AC maybe I am wrong with this knowledge . Please advice me on this. The other problem I am having is this voltage is not stable it fluctuates like 95 93 87 95 and the cycle continues I really need your expert help on this my first switching mode inverter
 

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This is the other circuit. I use the top section with the sg3425 to produce the 170v voltage Dc
 

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  • 300VA_Power_Inverter_Schematic.pdf
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Change value of R20, R22, R26 and R28 to around 10Ω.
Change value of R21 and R25 to 2.2kΩ.
Increase value of C12 to about at least 100µF and connect an inductor with inductance about 1mH between the bridge rectifier + output and C12.

With about 170VDC output, you should get 120VAC output (depending on the duty cycle).

Try with the changes mentioned above and with 160VDC and let me know the results.

Also, you may want to place pull-down resistors between each MOSFET gate and source.

Hope this helps.
Tahmid.
 

Thank you very much I really appreciate this help and I will let you know as soon as I do it tomorrow. this has been a problem for me several weeks now . Please explain to me in simple terms what is duty cycle Is this the frequency the 170v side inverter is running at. Thank you for your support and looking forward to ur reply.
 

Hello m y friend please remember to explain to me in simple terms the pratical meaning of Duty cycle. I am getting the 170v circuit organised i was using sg3525 but I was having some issues of blown ic so I am trying to find a circuit that uses tl 494 as pwm for the high voltage driver but not succesful yet if you have a link could you send it to me Thank you

- - - Updated - - -

Hello m y friend please remember to explain to me in simple terms the pratical meaning of Duty cycle. I am getting the 170v circuit organised i was using sg3525 but I was having some issues of blown ic so I am trying to find a circuit that uses tl 494 as pwm for the high voltage driver but not succesful yet if you have a link could you send it to me Thank you

- - - Updated - - -

when I use this circuit meaning the dc to dc converter section I am not getting no oscillation at pin 9 and 10 of ic 1 TL494
 

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Thank you very much Tahmin I really appreciate your support I am getting to understand about duty cycle. I am having some problem with the dc to dc 170v circuit seem some feed back voltage correction is not drawn so as I connect the h bridge the voltage falls . Could you please post a dc to 170v schematic using TL494 as pwm and having the feedback connected to the output for voltage correction. Thank you I am awaiting your reply.Thanks for your help so far
 

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