IGBT Overcurrent Protection Circuit

[MOSFET KILLER]

Hello All,
I have designed a circuit that will compare the collector voltage of an IGBT to a fixed variable referance voltage. When the IGBT gate voltage is low, the collector will be at the power supply voltage(in my case 320V), when the IGBT gate voltage is high, the collector voltage will be depednent on the current flowing through the IGBT. This is where the comparator comes in, if the collector voltage goes above a set referance voltage while the gate voltage is high, the comparator will give a pulse that will trigger a latch and shut down the IGBT gate. The purpose of this circuit is to protect an IGBT in the event of an overcurrent.

Question:
1. How will I limit the collector voltage going to the comparator while the gate voltage is low? The voltage will be 320V and that would fry my small LM393.

2.

 

[schmitt trigger]

The circuit you are looking for is an antisaturation detection circuit.

There are several options, but this is one of them:
1) Operate the LM393 at its maximum operating voltage (30v). Since the LM393's output is an open collector, you can have a pullup to +5v to interface with logic circuitry.

2) Use a resistor divider to limit the collector voltage to a value which would be within the comparator's input limits. A 220k (1watt) and a 22k resistor would limit the voltage to about 29 volts.

3) For safety reasons divide the 220k resistor in 2 or even better 3 individual resistors. The reason beign the resistor's maximum withstanding voltage value. So you could use three 75k (1/2 watt) in series and have plenty of safety margin.

 

[MOSFET KILLER]

The resistor divider idea on the collector would work for lowering the collector voltage to safe limits. But what about when the gate voltage is high, the collector will be at about 2 1/2 volts if the circuit is operating properly and with those resistor values the voltage would only be 0.2 volts, would the LM393 even detect that small of a difference?

 

[Hero999]

1) Operate the LM393 at its maximum operating voltage (30v).

Not meaning to be picky but the maximum voltage is 36V.

 

[schmitt trigger]

Of course it could.
As long that the voltage difference being compared is larger than the offset voltage (5 mV max, 1 mV typ) the output will change states

 

[schmitt trigger]

You are absolutely correct, the quoted 30 volts was from the top of my head.

 

[MOSFET KILLER]

Look at this picture. this guy used two diodes and a current limiting resistor.**broken link removed**

 

[schmitt trigger]

As I previously mentioned, there are several options to implement such a circuit...another would be to use a zener clamp.

 

[MOSFET KILLER]

But when the zener diode reaches its breakdown voltage there will be a direct short across the collector and emitter of the IGBT. How exactly do the diodes in the schematic clamp the voltage?

 

[schmitt trigger]

No, it is not a direct short... you have got a 470k resistor (R507) to prevent that.
The two diodes (D404, D405) clamp the voltage to the positive and negative supply rails, plus one Vf.

 

[tcmtech]

You could just use a premade IC that does all of that in one 8 pin package and save yourself a lot of time and effort.

 

[MOSFET KILLER]

I think I know what to do now, I will use a diode clamp and a current limiting resistor. The comparator will have a variable reference voltage set by a potentiometer. The output of the comparator will go to an AND gate with the gate voltage. If the gate and collector are high at the same time a capacitor voltage will be allowed to rise. If the capicitor voltage passes a fixed reference voltage the desaturation detector will close down the gate. Does anyone know of a way to charge a capacitor with a constant slope?

 

[MOSFET KILLER]

@ tcmtech
I am financially challenged at the moment. I just bought a pair of GE50NC60VD IGBTs and the pair plus shipping cost me $50. I will just build my desaturation detector with the parts I have.

 

[Ubergeek63]

@ tcmtech
I am financially challenged at the moment. I just bought a pair of GE50NC60VD IGBTs and the pair plus shipping cost me $50. I will just build my desaturation detector with the parts I have.

not sure about that one .... Digi-Key - 497-7486-5-ND (Manufacturer - STGW35NB60S) is 2 USD

 

[tcmtech]

Not sure about that cost limitation myself either. The IR2127 IC's are only around $2.50 each at Digikey as well.
I think you got ripped off if you spent $50 on the IGBT's unless you bought about 20+ of them!

 

[MOSFET KILLER]

Did not get ripped off Digi-Key - 497-7011-5-ND (Manufacturer - STGE50NC60VD)

 

[Ubergeek63]

Did not get ripped off Digi-Key - 497-7011-5-ND (Manufacturer - STGE50NC60VD)

those things are expensive! why did you choose that package? I am certain there is one that would work at $10 each or less

 

[MOSFET KILLER]

I chose the SOT-227 package so I can connect the IGBT directly to bus bars that hold the coupling capacitors for the halfbridge converter. Also I can screw my gate driver board and desaturation detector directly to the IGBT and keep leads short.

 

[Ubergeek63]

I chose the SOT-227 package so I can connect the IGBT directly to bus bars that hold the coupling capacitors for the halfbridge converter. Also I can screw my gate driver board and desaturation detector directly to the IGBT and keep leads short.

at what voltage, current and frequency?