half bridge gate drive power requirment

[Salvador12]

Hi folks , i'm having a weird problem , now my smps worked and all was ok until i needed a small repair and accidentally by mistake blew my switching IGBT'S , now I replaced them with new ones of a different kind but before turning them on I checked whether all is ok with my gate drive IC's , I replaced the IC (IR2110) and checked the supply voltage which is 18 volts and is stabilized with the help of 7818 voltage regulator transistor.
Now as soon as I added the gates to the IGBT's I switch on the smps and it doesn't work , I measure the gate voltage supply and it has fallen as low as 5 volts which is below threshold so the IC doesn't drive the IGBT gate in order to save it from half open failure.

I can't understand wha has gone wrong , my gate drive is very simple two IC's a small traffo and a diode bridge and voltage regulator, I checked the gate drive supply with a resistor load and I calculated the voltage drops under different resistances and my gate power supply is capable of about 180-200 mA of current , but that was with a 39 ohm resistor and the voltage dropped to some 8 volts , so I assume at steady 18 vols the amperage would be lower something like 100mA no more , is that enough for gate drive ?
But my previous IGBTS worked just fine , so I can't understand.

the IGBT's seem also similar ,
my previous IGBT was STGW80V60DF and they worked fine until I accidentally short circuited them.
the current IGBT's are FGH60N60SFTU.

Could it be that the IGBT'S are faulty and leak gate current or what else ? or could they be different as they need more gate current?
hard to understand , so yeah my question is basically how much of a current at what voltage is needed for a safe and good half bridge gate drive ?

 

[KlausST]

Hi,

Show your circuit with all exact part names.

And completely missing: any IGBT..driver...and circuit parameters like frequency, dead time, and so on.

Klaus

 

[Salvador12]

it's a 50khz half bridge , deadtime is 50/50


there are a few traces missing in this schematic as it has been redone a few times to add precise values for resistors etc but I think you will understand.

 

[KlausST]

Hi,

18 volts?
50/50 dead time?

Please clarify.

Klaus

 

[ElecDesigner]

I think if I were fault finding on this I would be temped to omit the mains supply to begin with and power the 18V directly from a bench PSU. Check if things get hot etc. check the current draw is reasonable and check the gate waveforms look sensible. I think that will help you a lot in narrowing down where the issue is.

 

[Salvador12]

Well you are correct about trying out a separate power supply for the gate drive although I have checked mine and there is nothing wrong with it , after all its simple to begin with , a small transformer , a bridge rectifier , a capacitor for smoothing , then a 7818 voltage regulator and that's it , the traffo is ok, i've checked the voltage regulator its also ok, under no load there is a steady 18 volts , putting a 39ohm resistor as the load the voltage drops to something about 7/8 volts , by calculus that would mean about 200mA going through the resistor and it does get hot to touch but I assume IGBT gate drive requires much less current than that.
Sadly I don't have any bench power supply and also not a transformer that would yield me he correct voltage for this application but as I said I really don't think the gate drive psu it at fault as it hasn't changed since the last time everything worked with the previous IGBT'S and they worked for quite a while and had no problems whatsoever.

either these IGBT have a higher gate charge requirment which they don't atleast according to the datasheets the gate charge is similar , or they are defective ,
notice I did a test , I added the gate drive with the main power switched off so the IGBT's ground leg as well as all others were floating and the gate drive voltage stayed at 18 v , as soon as I add the main psu rectifier and the ground to the main IGBT's the gate drive fails to drive them and the voltage sags down. So something takes up extra charge whenever the capacitor action is initiated in the IGBT I assume.

yes it's 50/50 deadtime and 18 volts of drive voltage supplied to the sg3525 osscillator and the ir2110 driver board.

 

[KlausST]

Hi,

Your schematic shows 7812 and 12V, I can't find 18V.

50/50 is no dead time, maybe 50/50 is duty cycle, but we need dead time.

Klaus

 

[Salvador12]

yes I apologize I got confused , 50/50 is a duty cycle , well I actually am not sure I think it's a bit less because my scope isn't that precise.
yes i know the schematic says 12v regulator but I have an 18 volt instead because with the 12 volt regulator due to voltage drop the gate drive wasn't with enough reserve to assure fully on and reliable switching of the gates.

But why would you say something about dead time here Klaus ? it's not like the devices overshoot , previously they worked fine , then by accident i shorted the transformer secondary which caused a too high of a load on primary and the IGBT's failed , even though the fuses blew fast , the mains rectifying caps had enough energy in them to destroy the devices.
after this the weird problem began when I put in these new different IGBT's that I had on hand.
as I've said I checked the voltage regulator even swapped it with a new one just for curiosity since i had a few spare , the small gate drive mains traffo traffo is fine has never been even warm. There is literally nothing that is wrong with the gate drive psu and that makes me wonder.
taht's why I was asking about how much current a typical half bridge like this requires for it's gate drive since I know the current output of mine and would simply like to know. I know its rather difficult to calculate but maybe an approximation ?
I simply have the feeling that either these IGBT's are more demanding than the previous ones or somehow they leak gate charge.

 

[KlausST]

Hi,

My idea is that your new IGBTs are slower than the original IGBTs....then the dead time will be important to avoid cross conducting.

Klaus

 

[sac1991]

Here are the things I suggest:
1. Use a parallel resistance across gate-emitter to avoid accidental turn on of IGBT.
2. STGW80V60DF require a gate charge of 448nC and FGH60N60SFTU require a gate charge of 198nC. So 2nd IGBT is faster.
3. For calculating Gate current read the data sheet either use total gate charge require for the IGBT to turn on. So frequency is also factor that determine the gate current requirement.
4. Turning off of IGBT/MOSFT require the capacitance C_CE to charge to the blocking voltage using load current. So try testing the circuit with load.

 

[Salvador12]

well that's the problem I don't know how high of a load should I put on the gate drive psu to mimic the original load of the IGBT's and the driving IC.

yes I too saw that these current IGBT have a lower gate charge , so i'm kind of puzzled as to why this happens. When I switch on the full mains rectifier to the IGBT at first when I power up there seems be be one or few cycles going on as the secondary gets about 1/2 volts of a small spike out of induction but then suddenly everything stops , as I am watching from a voltmeter attached to the gate drive psu at that very switch on moment what I see is at first the voltage rises to 18v as it should be but then it falls just as quickly to about 7/8 volts and the IC shuts down the gate drive to save the devices from being half open.

Klaus I don't think the new IGBT's will be slower , their datasheets are rather similar they are both among the newest power devices and can handle frequencies way above 50khz whihc is my frequency and is not that high to begin with, I really doubt that could be the problem here