Hi,
Currently I'm considering to design a Dynamic Brake Unit (DBU) for V.F.D.
DBU power range is around 15KW and VFD is 400V class. Recommended resistor value is 40 Ohm
2kW.
I know the basic of DBU and will manage the voltage selection and PWM things via Micro-Controller.
My major concern in this post is related to a safety issue relative to IGBT.
When DC Bus voltage of VFD rises above a certain limit, IGBT is triggered via Micro-Controller PWM signal which will further dissipate this excess power to a low value, high power resistor. (Attachment picture).

OK until is point.
Now problem comes in to my mind that if, for some reason, IGBT is burn and short, dc bus voltage will directly go through resistor which will damage the resister and VFD by overloading. Since DC bus connection is taken directly from capacitor bank, there is no built-in current sensors in this area. There may be a built-in fuse in this line but it will still leave the VFD for repairing. I didn't select any IGBT model at this time,
Any ideas and guideline to overcome this situation and also some useful advice for design is appreciated.
Thanks and regards.

 

Hello,

Wich IGBT are you using?
Is the gate voltage high enough to activate the IGBT?
When the gate voltage is to low, the IGBT is in linear mode and has to dissipate a lot of power, wich may burn the IGBT.
Wich microcontroller are you using?
What voltage comes from the microcontroller?

Bertus

 

Hello,

Wich IGBT are you using?
Is the gate voltage high enough to activate the IGBT?
When the gate voltage is to low, the IGBT is in linear mode and has to dissipate a lot of power, wich may burn the IGBT.
Wich microcontroller are you using?
What voltage comes from the microcontroller?

Bertus

Thanks for reply.
I did not prepare a practical schematic, therefore I've not decided IGBT yet, as I've mentioned in the post. Using any suitable PIC Micro-Controller. Micro-Controller PWM output voltage are 5V but can be buffered up accordingly using op-amp. IGBT will be working in full saturation mode so with minimum resistance and heating.
I want to know how to handle the situation in case IGBT burn and short.

 

An IGBT is the perfect device to drive the braking resistor. Of course you need to size the IGBT correctly to handle the off state voltage and the current drawn by the resistor when the IGBT is switched on. Choose a device with some safety margin (probably at least 25%) on both current and voltage. A fast fuse in series with the resistor would also be a good idea.

The really important thing is to drive the gate of the IGBT correctly. It is vital that the device is either OFF or ON - never partially ON. It also needs to be switched quickly - as the gate has a high capacitance and will need a high peak current to switch fast.

I would recommend using a gate driver IC such as the MC33153 (see: https://www.onsemi.com/pdf/datasheet/mc33153-d.pdf) there are many other types.
This will give you a number of useful advantages:

• Can be driven from a 3.3V or 5V micro-controller logic signal.
• Has a supply voltage lockout so the gate drive will only operate if the gate supply voltage is above about 12V.
• Can drive a peak current of 1A into the gate to quickly charge the gate capacitance.

It also has an optional "fault" output that could be fed back into the micro-controller to quickly switch off the load resistor if a a fault occurs. Usually, this will be "desaturation" - too much load current causing the voltage drop across the IGBT to rise above its normal on-state voltage drop. Your micro-controller code would cause a fault lock-out in this condition preventing further operation until the fault is reset.