Running a Mosfet-Full-Bridge with a single PWM-Signal ?!

[Hexer379]

Hello ... @all ...

i have a problem with my full-bridge and the PWM-signal ... i will use this for a SNT (30-45V -> 27V 15A)
my pwm-signal comes from a µC with 50kHz and a DC=50% at this Time.
this Signal go´s to the Input in the circuit that is attach ...
@this point ... it works , i see 1x PWM with 50kHz on Input and 2x 25kHz PWM on the 2 Output-Pins ... (8 & 11).
i planned to use this 2 signals to drive the H-Bridge .

is this OK for driving a SNT-full-bridge or is this a general fault ?!

Thanks , Thomas ...

PS: Sorry for my english ...

[titi]

Hi Hexer379,

Just to have a good design, K and R (reset pin) of HC73 must be connect to GND or VCC, between pin 8 of U1:C and C4 put a small resistor like R1 and R2 to create a dead time, same for pin 11 of U1:D and C3.
HC00 and other IC dont like drive directly a capacitive load even 1nF.

To drive a H Bridge it is a good practice to make a small dead time before switching one branch to the other.

It is also good idea to attach the DSN file to your message to simulate your design.

Best regards.

[thunderer]

Sorry for off-topic: If you decide to use a PIC (capable of H-bridge full driving - hardware included), let me know. I'd be more than happy to share my knowledge with you. Obvioulsy, it will be via this forum.

OR: see here http://www.sonsivri.to/forum/index.php?topic=50781.msg146690#msg146690

[Gallymimu]

echoing titi:

Having the capability to put some dead time between both the high and low side of a full bridge half is a really good idea.  Shoot through current because of difference in the high and low side switch times can cause serious problems if not blowing up the parts.

[Faros]

Dear Gallymimu, dead time is not just a good idea at 50KHz relatively high DCV and high current , it is a mandatory . also MOSFET drivers are factors that will affect MOSFET temp. and dead time gap., not to forget the MOSFET Bridge gate driving levels, protection.
driving such bridge through logic level series is not recommended (if ever possible). he will end up with a desk with some burned up MOSFETs surrounded by  lots of time and frustration 

[solutions]

He can use logic level MOSFETs, FAROS.

Not all FETs need 10-15V Vgs these days....

[flo0319]

A lot of MCUs have already implemented PWM module optimized for motors control (a special register for dead time), but in general are used 2 PWMs in complement modes for each half from bridge. With just a single PWM signal, I think, you need to put a hardware delay for 1 half from H-bridge (1 diagonal). Also, just a dead time between 2 PWM pulses is like  9n% (98%) PWM scale control, and a lowest frequency.

[Hexer379]

Hello ...

thank you all for the fast reply ... :-)
i attach the simulation of this ...
i planned this to switch from one half-bridge to another for each pwm-Pulse ...
this is why i use a FlipFlop with falling edge inputs ... !

This simulation is not up2date ... the deadtime-resistors are not include ... :-(
the next point is the mosfet-drivers ... i use for many devices IR2104 ... but for this , i need a driver that can switch up2 30kHz and can handle max.50V with one output ... i don´t need a high/low-driver ...
with or without bootstrap this is the question ... or is a discrete driver better for this ?!

Thank you very much ... Thomas ...

[MTong]

Once you build the circuit, you could use full bridge drivers with built in dead time so you don't worry about it with RC and logic. Some of them have programmable dead time.
IR2184 and IR21844 have respectively fixed and programmable dead time

[thunderer]

In any case, the better way to drive properly a MOSFET is to use a MOSFET driver. IR or any other driver is the wisest choice.

[Hexer379]

Hello ... @all ...



thanks for the help ... i will try to order a sample for IR21844 ...


Best Regards , Thomas ...

[MTong]

check if it is good for you, that is if the minimum dead time available with that driver is not too high for you. It depends on you switching frequency.
If your switching frequency is very high, the minimum dead time that you get with the driver could be too long.

[solutions]

^ That depends as much on the MOSFET switching speed as the driver

[MTong]

yes; I was pointing out that it would eat a good part of the available switching period if sw frequency is high. The consequenses are more losses, lower maximum duty cycle and some distortion

[thunderer]

It would be more useful if the OP will explain all his project to better understand the application and choose a proper frequency in consequence. I would say 20kHz is a high frequency for some applications (like a motor), but not so high for others.