IGBT won't open in spark circuit

Question

I'm building a EDM machine for removing broken metal-working taps. I'm using the following circuit, but having issues with the IGBT transistor

R1 = 2Ohm resistor
C1 & C2 = 1000uf 100V capacitor
IGBT = IRG4PC40KDPbF IGBT
P = from the positive side of 5V 1A power supply

According to the datasheet (page 2), the Gate Threshold Voltage is 3 (min) to 6 (max) volts. When I apply 5V to P, I would expect the resistance across the collector & emitter to drop low, but it doesn't.

I have tried also connecting the negative from the 5V power supply to the IGBT emitter, but with no change.

Any thoughts?

Answer 1

The IGBT may have a Vth of 3-6V, but it'll need 10-15V to properly turn on to its minimum on-state resistance. The gate threshold voltage is determined under this condition:

Take a look at the Vge vs Ic graph in the datasheet

From the graph, at Tj = 25, you can see that the IGBT needs close to 6V for an amp of current to start flowing through it.

Presonally, I would use 15V to turn it on.

Answer 2

"Threshold" in MOSFETs and IGBTs is a trap for newbies. It's a deep technical term that has to do with the shape of the current vs. gate voltage curve, but isn't something that you can just measure. Probably the simplest way to put it is it's the gate voltage where the device just barely starts to conduct -- but even that is kinda-sorta wrong.

For switching applications, you don't care about the threshold voltage. You care about the rated gate-emitter voltage to turn the device fully on, the rated gate-emitter voltage to turn the device fully off, and getting the thing turned on and off fast enough so it isn't damaged (as mentioned above), and for big IGBTs, not turning it on so fast it goes into CMOS lockup.

As mentioned, that prescribed gate-emitter voltage is probably around 10 to 20V, depending on the IGBT you're using.

Answer 3

If your load resistance is below 0.75 ohm (with safety margin and at room temperatures,) a pulsed inrush current from capacitor C2 could surpass the datasheet specification for maximum pulsed current of 84A.

That IGBT adds 0.17 ohm for a voltage drop of 1.7V for 10A measured to load resistance and on about 4V for 84A that calculates to only 0.012 Ohm. Minimum acceptable load resistance would be around 0.75 ohm.

Below that resistance fully switching on possibly destroys this IGBT in this circuit, if it is conducting at lowest resistance, because of (inrush) over-current. Within 0.4-0.5 milliseconds that electrolytic capacitor C2 will discharge to half of its initial (60V) voltage on a load resistance at about 0.75 ohm and it will need these 1/2000 second for reducing its initial maximum current of around 80A to about (its half) 40A also. Load resistances below about 0.75 ohm will let the initial current start at values above 80-84A. Suitable guess for the endurance of the over-current (for to avoid) is http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html#c2 .

For breakdown voltage of an arc within air
(or other gases: \$H_2\$ (A=38.3, B=1041), \$He\$(A=22.5, B=255), \$Ar\$(A=102.0, B=1763), \$CO_2\$(A=150.0, B=3495) for A,B=[\$(kPa*cm)^-¹\$] ),
Paschen's Law online calculator (e.g. \$p=\$100000Pa, \$d=\$0.004mm, \$\gamma_s,_e=\$~0.5-2.0 (or values diagram) ) might provide further insight into possible minimum voltage values, that, from theory, hardly undercut ~30-35V for metals and air, hydrogen, argon (exceptions: \$He\$ and minor effective \$CO_2\$ atmosphere) (Sidenote: While knowing, that in reality even on car battery's low, nominal 12V sparks can be visible between positive wiring and vehicle shell).

For researching IGBTs, for example Infineon IRGP4069 and understanding parameters during switching on

1. Discrete IGBT Simulator, for example: 100V, 80A, 10Hz
2. datasheet IRGP4069, page 9
3. CircuitLab Simulation
   

^{simulate this circuit – Schematic created using CircuitLab}