# Grounding problem is coming between boost converter and mosfet driver

#### Arijeet

Joined Dec 27, 2019
85
I am designing a boost converter which is rated on 100 watts, 12 volts to 24 volts, switching frequency is 20 khz, in open-loop conditions. when I give the input power supply to the circuit as 12 volts, my driver (HCPL 4506 Mosfet Driver) will take VCC as 24 volts due to My microcontroller can generate a 3.3-volt output PWM 20khz switching frequency so when low-level amplitude generated by a microcontroller will be given to the low side of mosfet driver to get a large amplitude of voltage from the high side of the driver. the voltage should be large enough which can turn on the gate terminal of mosfet. the threshold voltage of my mosfet is 15 Volts for turning on the gate.

So suppose if I provide VCC as 12 Volts in the driver, so they will give low amplitude voltage as volts approx, it is not enough to turn on the gate of the mosfet. So when we are working in the circuit we checked we need to provide 24 volts to the driver circuit (VCC) then it can generate 15 volts approx, it is enough to turn on the gate of mosfet.

now when I have built this circuit with proper orientation in an open loop, the problem is coming I have 24 volts Dc supply (SMPS) for the input power supply where I am using a voltage regulator 7812 IC which will convert 24 to 12 volts for the boost converter(power circuit) and 24 volts directly given to the Driver VCC. when I powered on the circuit, the output voltage of the boost converter is coming 4 volts, I should come 24 volts. I checked this circuit, all the things are good. I think I am getting a grounding problem. Anyone can help me with this.

#### Ian0

Joined Aug 7, 2020
9,436
the threshold voltage of my mosfet is 15 Volts for turning on the gate.
Are you sure? It seems a lot. Most are <4V.

#### Arijeet

Joined Dec 27, 2019
85
Are you sure? It seems a lot. Most are <4V.
Yes, I am sure.
Please see the datasheet, it is clearly written.

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#### DickCappels

Joined Aug 21, 2008
10,120
From the datasheet you posted:

In your circuit, what do you expect the peak drain current to be?

#### Ian0

Joined Aug 7, 2020
9,436
Yes, I am sure.
Please see the datasheet, it is clearly written.
You might have mentioned it was a SiC FET, but we know now!
As @DickCappels said it only takes an average of 2.7V to start to turn it on.
I can see from the datasheet that there is some advantage to be gained in terms or Rds(on) from driving it with more than 12V.

The HCPL 4506 is not a MOSFET driver - it is only a gate-drive interface. It has a positive-going drive that is limited to 20kΩ which is less than 1mA, so it cannot drive a MOSFET gate directly. You need an additional low-side gate driver.
Most low-side gate drivers will run from up to 18V, which should be enough to get the SiC FET fully turned on. The HCPL4506 will take 30V so can be run from the same supply.

#### Arijeet

Joined Dec 27, 2019
85
Please see the schematic of the boost converter.

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#### Arijeet

Joined Dec 27, 2019
85
You might have mentioned it was a SiC FET, but we know now!
As @DickCappels said it only takes an average of 2.7V to start to turn it on.
I can see from the datasheet that there is some advantage to be gained in terms or Rds(on) from driving it with more than 12V.

The HCPL 4506 is not a MOSFET driver - it is only a gate-drive interface. It has a positive-going drive that is limited to 20kΩ which is less than 1mA, so it cannot drive a MOSFET gate directly. You need an additional low-side gate driver.
Most low-side gate drivers will run from up to 18V, which should be enough to get the SiC FET fully turned on. The HCPL4506 will take 30V so can be run from the same supply.
Sir , can you tell me about another mosfet driver which can work. please see the schematic.

#### Arijeet

Joined Dec 27, 2019
85
I
From the datasheet you posted:

View attachment 243903

In your circuit, what do you expect the peak drain current to be?
In the boost converter circuit, I will get a maximum drain current of 4 amps on the output side.

#### Ian0

Joined Aug 7, 2020
9,436
I

In the boost converter circuit, I will get a maximum drain current of 4 amps on the output side.
Not if you are supplying it via a 7812. A 7812 is limited to 1.5A.
Why do you need a 900V 32A MOSFET? Won't a 5A 30V MOSFET do?

What's the point of this circuit? It appears to have a 24V input and a 24V output? Can't you replace all of it by a piece of wire?

#### Arijeet

Joined Dec 27, 2019
85
Not if you are supplying it via a 7812. A 7812 is limited to 1.5A.
Why do you need a 900V 32A MOSFET? Won't a 5A 30V MOSFET do?

What's the point of this circuit? It appears to have a 24V input and a 24V output? Can't you replace all of it with a piece of wire?
sir, actually I will design a boost converter which is going to work on a solar inverter to the input voltage at the solar side will be 150 volts, and using a boost converter will give 400 volts and will charge the DC link capacitor. and further inverter designing will work.

Reason for the need for 900V 32A MOSFET? Actually, the solar inverter is based on 1KW rated so my boost converter will be a 1 KW so that's why I have chosen this mosfet.
The purpose of doing this circuit is to check the drivers, control circuit, power circuit, and I am going to convert this into a closed loop. once it starts working in an open loop, I am going to check my logic for closed-loop using PI controllers by DSP.
Not if you are supplying it via a 7812. A 7812 is limited to 1.5A. sir for this can you tell me another voltage regulation part which can do easily. actually, I know transformer - bridge rectifier circuit can do but the size of the transformer will increase.

#### Ian0

Joined Aug 7, 2020
9,436
Do you really need isolation on the gate drive? If so, you will also need isolation on the voltage and current measurements.
As the frequency is relatively low, and the gate charge of the MOSFET quite low, a driver such as MCP1401/2 would probably work. If you need the opto-isolator place the MCP1402 between the isolator and the MOSFET.
Remove C2 - slowing down the MOSFET gate drive is a really bad idea.