trigger IGBT

[bunda222]

Hi,

I'd like to know how a IGBT works with a pulse generated by a pulse amplitude modulator (PAM). For me the amplitude of the pulse sent to the gate of igbt serves only to trigger it. I don't know how varying the amplitude can make any difference in the output waveform.

 

[Skogsgurra]

There are PAM and PWM.

The PAM technique is obsolete and was used when the thyristors/GTOs weren't fast enough to modulate the motor voltage. Instead, the supplying DC voltage was modulated by some device - sometimes a simple SCR rectifier and sometimes with a crude transistor device while "blocks" of that voltage was switched onto the motor windings. Rather terrible performance with bad cogging/vibrations and extra heat and all that.

The IGBT isn't "triggered" like a thyristor, the IGBT is switched on or off with the gate voltage.

I think yo should tell us what kind of equipment you are referring to. Is it a drive or a battery charger or an electrolythic plant or something else?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[bunda222]

I was meant to say switch. What I am trying to understand is how the amplitude of the gate voltage can exert influence in the output waveform of 6 pulse converter for example. For me the gate is either on or off no matter what is the amplitude.

 

[Skogsgurra]

Yes you are right. In a VFD, the IGBT is either off or on. It is the ratio between on and off, the Duty Cycle, that is controlled so that the mean voltage (say that the duty cycle is 30%, then the mean voltage is .3*600=180 V if the DC voltage is 600 V. If the duty cycle is 60%, the mean voltage will be 360 V). By modulating the duty cycle (which is then called pulse width modulation, PWM) you can make the output mean voltage vary in a sine-shaped way.

The connected motor winding works as a low pass filter so that the current through the windings is sine shaped although the voltage is composed of square pulses. You may need to think twice about that. But that is how it is.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

It doesn't do as you're asking. The driving circuit in question will convert the analog voltage level into a corresponding PWM duty cycle.