High voltage brushed motor control problem - back EMF

[Acetronics2]

Hi, George

For relays, I sometimes use a RC series ladder to kill the back EMF ... but never tried it with such voltage and power levels.

also seen use of 2 Power Avalanche controlled diodes ( connected in series by anodes or cathodes ) in the high power welding equipment ...

Alain

[George]

Thanks for the comments, I'm not sure a TVS would do the job - unless it was huge. It would be dissipating a lot of power near enough to continuously? Arent they only made for transient voltages?

What do you guys think of an SCR triggered by a Zener?

I'm unfamiliar with them although I have set a pic to trigger triacs before for light dimming. I guess a Triac is jst 2 SCRs? So rather than dissipating 350-400V like a TVS would be doing it would only dissipate around 2V?

My understanding of them is when they see a voltage, they latch. They then unlatch when the voltage across them returns through zero.

So the main IGBT will turn on, charge up the motor to set flux (full flux is 10ms) then turn off for a set period. The SCR gate would be set with a zener - probably a standard diode also to prevent main power going through it. Then EMF spike rises, latches SCR when it reaches Zener voltage, this bypasses the voltage back upto the top rail. Voltage then drops and SCR unlatches?

Not sure - confirmation would be nice.

Thanks

[aratti]

Exactly Henrik, There is going to be power supplied to the motor when it's driven backwards (overrun) so unfortunately the relay idea wont work. It's also going to be run it at high freq PWM so would guess the IGBT would see a few thousand plus volt spikes before the relay made contact on it's first switching.

George, I think Herink's suggestion is still the best one!

You can activate the relay via a dedicated pic port (hope you have a spare one) so you can control the timing. Activate the relay, wait few millisecs and then fire your IGBT via pwm.

Reverse the concept when you will stop the motor: set pwm duty cycle to zero, wait a while till the motor stops turning, then open the relay.

All very easy to implement using PBP.

Al.

[George]

I've tried lots of things so far, to a point I have it working. But it's not working as smoothly or as nice as I would like, so I dont think there is anyway out of driving the high side also. - but how???

I'm planning on using the same NPN IGBT and driver on the top side as I do on the low side, this should keep switching time consistant. (Also I have a tonne of 600V 70A IGBTs)

The question is, what's the best way to generate 12 or so volts above my 300V rail, also - what's the best way to trigger the drivers with such a huge potential across them.

It all has to be done quite quickly and presisely, for eg at the moment I'm using an opamp for current limiting, however the slew rate is far too lesuirly. It's taking 5us to respond, from what I've observed that's going to have to be cut right down a hundred fold to 50ns or so. So an optocoupler is just not an option (25uS typical fall time)

Any ideas on large potential high side driving?

[HenrikOlsson]

Have a look at the IRS21884, logic-level input, adjustable deadtime to avoid shoot-thru, shutdown input for cycle by cycle current limit or whatever as well as the bootstrap circuit for the high-side driver.

If you're not intending to use PWM (which is needed for the bootstrap circuit to operate properly) you'll have to provide an isolated 10-15V supply for the high-side, conneted between Vs and Vb. But IIRC you are using PWM, correct?

/Henrik.

[George]

thanks Henrik I couldn't find an IRS21884 but I found an IR2181 which appears to do a similar job. However I cant get it working! I've only used low side FET drivers before and had no trouble.

The max Vin current according to datasheet is 60uA, I have 12V coming in through a 1k resistor (I have a 5.1v zener there to prevent over voltage), the IR2181 is pulling the input down to 2 volts (have checked it's not the zener) - so that means about a 10mA draw, the output to the FET is also floating around a couple of volts.

The FETs, the IR2181 and my logic circuit all share common ground.

The FETs currently have a 24V supply and the logic circuit and IR2181 have a 12V supply.

I've tried a second IR2181 incase the first was faulty but no luck.

Any Ideas? The circuit I've used is the same as documented in datasheet. I'm using a .47uF polyester bootstrap capacitor (I initially thought it had to be high voltage but now realise it only has to be 20V or so). Also got an ultra high speed diode charging it..

Has anyone here used these before and had similar trouble, or first time success?

Thanks

Thanks

[George]

Sorry - I'm an idiot trying to be an electronics design engineer!!

I looked carefully at the datasheet and have the pinouts all wrong I followed the circuit on the top of the page assuming it was layed out in pinout form.

Will let you all know how it goes.

Thanks