# PCB design and schematic for a half bridge power amplifier

## Main Question or Discussion Point

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.

https://www.eevblog.com/forum/proje...mplifier/?action=dlattach;attach=679062;image

https://www.eevblog.com/forum/proje...mplifier/?action=dlattach;attach=679068;image

Pl suggest any modifications or pointers that will improve the circuit.
Thanks.
TIV

## Answers and Replies

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tech99
Gold Member
A 10mH load has a reactance of 1240 Ohms at 20kHz, so the 24 volt supply will be unable to drive 3A into it. By Ohms Law, required voltage V=XI = 1240 x 3 = 3720 Volts.

A 10mH load has a reactance of 1240 Ohms at 20kHz, so the 24 volt supply will be unable to drive 3A into it. By Ohms Law, required voltage V=XI = 1240 x 3 = 3720 Volts.
Hi tech 99,

Thank you for the reply. 20khz is the frequency of the pwm signal to the mosfets from the micro controller. Operating frequency of the drive is about 100 hz, giving about 7 ohms reactance.

Thanks,
TIV

tech99
Gold Member
Hi tech 99,

Thank you for the reply. 20khz is the frequency of the pwm signal to the mosfets from the micro controller. Operating frequency of the drive is about 100 hz, giving about 7 ohms reactance.

Thanks,
TIV
Thanks, I did not notice the 220uF cap.
I notice that the ACS712 gives an AC output centred on 1/2 Vcc and assume you are aware of this.

tech99
Gold Member
It also occurred to me that it might be worth checking that everything starts correctly at switch-on, especially as you have a micro controller involved.

It also occurred to me that it might be worth checking that everything starts correctly at switch-on, especially as you have a micro controller involved.
Thank you for the pointer. I will look at the start up scenario in detail and post back.
I will also condition the ACS current sensor output signal before giving the feed back to the mcu.
Thanks.

Just some small remarks.

- Do you have any calculation about the expected dissipation on the FETs? 20kHz PWM mode with 3A current is not really a stressing matter for any decent FET but still, cooling might be needed and now is the right time to prepare for that. With moving only a few component you can make enough space for heatsinks up to ~10W in case cooling will be needed later on.

- The high current connector seems to be half-off of the PCB, but due the bigger plug with thicker wires this is the one which expected to have higher mechanical load. If it is not any special type or special mount of connector you might want to move it onto the PCB: that will give both the connector, both the holes better mechanical stability.

- You don't have any mounting holes.

- You have some acute angle problems around C4.

- It is better to have a bit wider copper for power lines even if the expected current does not means much requirement

- if you have two ground planes then tie them together regularly with some VIAs.

- avoid using too small VIAs. Makes them bigger, or use the component leads to switch between layers if possible. With some more work you don't need any VIA on this PCB at all (apart from the ones used to tie the GND planes).

These recommendations are admittedly a bit overkill since at 20kHz almost everything will work just fine. But the sooner you start to apply the general rules, the better: later on it takes a lot of work to get rid of the bad habits.

Tom.G
Just for a lifetime consideration, unless their is an electrical reason not to, move C6 away from the heat of the FETs. (Right now it is in an oven between the FETs.)

Rive
It also occurred to me that it might be worth checking that everything starts correctly at switch-on, especially as you have a micro controller involved.
When the micro-controller switches on, there are 4 possible states for the two mosfets.

Both mosfets conducting : 3A flows. Actuators have been designed for this current.
PMOSFET OFF, N MOSFET ON: Actuator in free wheeling mode, current has to decrease gradually from 3 A.
MOSFET OFF, P MOSFET ON: Actuator in free wheeling mode, current has to decrease gradually from 3 A.
Both mosfets not conducting : No current flows.

All these 3 states occur in the operation of power amplifier and should not be a problem.
Thanks for the pointer.
TIV