PCB design and schematic for a half bridge power amplifier.

[thambiTeaInnumVarala]

Hi,

I am making a half bridge power amplifier that operates at 24 V.
The bridge has to supply around 3A current to an inductive load (10mH)
The bridge will be controlled using 20kHz PWM from a microcontroller.
I have put a ACS712 current sensor in the load path to give feedaback to the microcontroller.
There is a IRF9Z24 P FET at the top and a IRFZ44 N FET at the bottom.
Both are driven by a single MC34125 Gate driver IC, which has a 12 V power supply.
The gate driver is connected to the microcontroller through a HCPL optocoupler.

I have maintained 80mil track width for the 3A paths, from a online track width calculator.
I have put necessary decoupling capacitors for the optocoupler and the gate driver ic.
There are 2 copper filled ground planes on top layer and bottom layer that I have not shown in the pcb schematic for visibility.

I have made a schematic and circuit diagram that i have linked here.
Pl suggest any modifications or pointers that will improve the circuit.
Thanks.

TIV

 

[FvM]

I wonder what MC34125 is, I don't know a gate driver IC of this type.

As for the layout, the traces are surely sufficient for 3A, but using copper pours can considerably reduce the circuit inductance and switching overvoltage.

 

[thambiTeaInnumVarala]

Hi FvM,

I typed MC34125 instead of MC34152 driver by mistake.
Regarding copper pours, I have put ground planes of solid copper on both top and bottom layer.
You have mentioned this reduces circuit inductance. I understand that it is helpful (Reduces oscillations in gate drive path?)
Am I right?

Thanks,

TIV

 

[KlausST]

Hi,

My pcb layout recommendation.
* use one side with GND copper pour, with at least as possible other traces.

Please show the complete schematic.

Klaus

 

[FvM]

The driver circuit is wrong, you can't drive a 24V supplied PMOS transistor with a 12 V ground referred driver. Q2 will be permanently on.

 

[thambiTeaInnumVarala]

Thanks for looking at the schematic.
I will keep only one bottom ground plane and remove the top ground one.
I will make a clean schematic and post it here.
Regarding Q2 being permanently on, i will check up on P channel mosfet and reply back.

 

[thambiTeaInnumVarala]

The driver circuit is wrong, you can't drive a 24V supplied PMOS transistor with a 12 V ground referred driver. Q2 will be permanently on.

Hi,
The Pchannel mosfet is connected as follows:
Gate to the driver.
Drain to the Load
Source to +24V.
When the MC34152 pulls its output pin to ground, the connected gate pin will be at -24V with respect to source and mosfet is on.
When the MC34152 drives its output pin to +12V, the connected gate pin will be at -12V with respect to source and mosfet is still on.
I understood the mistake. Will work on correcting it.
Thanks for the pointer.

 

[FvM]

In addition, Vgs of -24V is exceeding the maximum ratings and potentially dangerous for a MOSFET.