Hey,

I'm trying an ambitious project, making a DC converter using a microcontroller.
I'm using a PNP transistor to drive both the high side and the low side MOSFET. When I'm trying the circuit, even if I keep the duty cycle as low as 20%, there is a path to the ground for the current (I'm able to observe this because my power supply goes to constant current mode from constant voltage mode)

I'm using the inbuilt PWM capability of the controller with complementary outputs and a dead time.
Please find attached my schematic and help me figure out the issue.

Thanks
Siddharth

 

The full bridge Mosfet drive is completely wrong and must be discarded.

Search the web for full bridge drivers, there are many ICs that perform that function, which you will see it is far more complex than your simplistic circuit.

 

I noticed it is actually a half bridge. Much easier.
Use the IR2111.

 

1) With no gate-to-source pull-down resistors, and no active pull-down until the micro boots up and configures its ports correctly, there is likely a period when significant shoot-through current occurs with both high and low side MOSFETs conducting at the same time. Hence your power supply behaviour.
2) The high-side MOSFETs can't turn on fully to pull the source high unless the gate goes several volts above the positive rail. Gate-drive ICs (e.g IR21xx) use a bootstrap arrangement to do this.

 

1) With no gate-to-source pull-down resistors, and no active pull-down until the micro boots up and configures its ports correctly, there is likely a period when significant shoot-through current occurs with both high and low side MOSFETs conducting at the same time. Hence your power supply behaviour.
2) The high-side MOSFETs can't turn on fully to pull the source high unless the gate goes several volts above the positive rail. Gate-drive ICs (e.g IR21xx) use a bootstrap arrangement to do this.

Thank you Alec. I understand the reason behind the power supply behaviour.

You recommend that I use IR2111. Would it be in place of the BJT's or in addition to them.
Additionally, the Vin for IR2111 requires a voltage between 10-20V. How do you recommend I handle that?

 

I noticed it is actually a half bridge. Much easier.
Use the IR2111.

Thank you for getting back to me.
Would IR2111 be an addition to this schematic or would it be used to replace the BJT's?

 

This guy has posted a few blogs on the IR21xx series.
https://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html

 

Thank you so much for all your inputs. Based on them, I have added an IR2110 in my circuit.
If possible can you please validate the schematic before I give my PCB for production.
please find attached the schematic.

Thanks

 

Your bootstrap diode is the wrong way round.
The bootstrap capacitance seems unusually high.
What is the switching frequency?
Are you providing dead-time in software (the IR2110 inherent dead-time may be insufficient for MOSFETs with very high gate capacitance)?

 

Your bootstrap diode is the wrong way round.
The bootstrap capacitance seems unusually high.
What is the switching frequency?
Are you providing dead-time in software (the IR2110 inherent dead-time may be insufficient for MOSFETs with very high gate capacitance)?

I'm switching at 30khz frequency. I'm providing a dead time from the Microcontroller that I'm using.
Also corrected the values for the capacitors and their direction.

 

The diode between pin 6 (VB) and Vcc is the wrong way round.
Why do you think you need as much as 47uF for the bootstrap?

 

The diode between pin 6 (VB) and Vcc is the wrong way round.
Why do you think you need as much as 47uF for the bootstrap?

Are you referring to the ECvb capacitor?
I just assumed that we want the capacitor to supply the charge continuously for a long time. And since I'm using 30khz, I thought we need a large capacitor.

Would you recommend a bootstrap capacitor of 4.7uF here?

 

Would you recommend a bootstrap capacitor of 4.7uF here?

No; and certainly not a polarised one. I would have expected ~100nF to be sufficient at 30kHz with typical MOSFET gate capacitances.