I am designing a circuit to quickly release the energy of a 60 mf capacitor bank into a low resistance load. The capacitor bank is charged at 240 volts and the resistive load is approximately 10 ohms. (Yes, this is meant to vaporize the load.) The energy is delivered for approximately 500 ms before shutting off. This operation is completed about once every 5 minutes.
I have chosen an IGBT (IXGK320N60B3) with a Vces of 600V, Ic90 of 320A and Vce of 1.6V to operate as a high-side switch. I would like to use the IR2117 gate driver for a (hopefully) simple solution.
I am using a 12V supply connected to Vcc to power the IR2117 and a TTL logic level control signal on IN.
I am very concerned about design errors in this part of the application so I would like help avoiding a disaster. Please address the following issues regarding IR2117 connections:
The positive output from the capacitor bank is connected to the collector of the IGBT. Is this same output connected to Vb, the high side floating supply voltage?
In the “typical connection diagram”, there is a resistor between Vho, the high-side floating output voltage and the IGBT gate. How is this value calculated?
In the “typical connection diagram”, there is a capacitor between Vb, the high side floating supply voltage and Vs, the high-side floating supply offset voltage. What is this capacitor’s purpose and how are its values (capacitance, voltage) calculated? A recommended capacitor would be appreciated.
I understand that in a high-side switch application, turning on the IGBT requires a gate voltage between 10 - 15 V above the collector voltage. Please excuse my ignorance but I don’t understand how this voltage, which is above the voltage supplied to Vb (240V in my case) is provided by the Vho output driving the IGBT gate.
Thank you for your help.
Plan on having the circuit violently self-destruct several times before you end up with something that works. Spare parts, protective eyewear, proper procedures for working around DC of significant voltage. Somebody else nearby to call for assistance/provide CPR in the event you make a mistake.
No, very much no.
It’s a function of desired rise time, switch characteristics, layout, and several other factors. Sometimes it may not be explicitly used at all.
That’s called a bootstrap capacitor; using N-type devices as high-side switches requires a potential higher than that of the one being switched. In typical applications where switching occurs at a high frequency, it charges from Vcc during the off period, and provides a reserve of charge from which the switch can be maintained in an on state during the on/output high period.
The potential at the Vb node varies with respect to the potential at the HO output. Again, you don’t want to be connecting your capacitor bank to the Vb input.
AN-978 and AN-6076 are a couple of resources that help explain the use of this type of driver product. Reading and understanding those might be a good investment.
The plan to have the load open as a result of circuit function introduces some uncertainties; what its actual resistance at any point in time may be is not something that’s subject to some uncertainty.