Several external things can cause the supply to have no drive pulses on the gates, even though it is good.
1) The control chip must get some power to start operating. Often this small current comes through high value resistors from the rectified high voltage DC. If those resistors are bad, or if the capacitor which filters the local power supply is leaky/shorted, the supply will not start up, because it does not get enough voltage.
2) some other signal may be controlled by a protection circuit, and is not reaching the control chip. Sometimes the internal feedback signal (often called "comp" on the data sheet) is used, by shorting the "comp" pin to ground in case of a fault. Whatever part does that may be bad.
3) The control chip may not get a "start" signal. Most chips only require voltage to start, but some also require an "enable" signal. Sometimes the "enable" is used as part of the protection circuit. If there is an "enable" required, it may not be getting a good signal. However 8 pin control chips do not usually have this.
4) bad IGBT may have damaged the gate drive. There is usually visible evidence of this, so I assume it is not the problem, but I have to mention it "for completeness".
These are just a few possible reasons for the control chip not working because of some external problem.
Originally Posted by BbbyBld
If you are making measurements on the primary side, I hope you are using an isolation transformer.
Yes indeed ..... +1 on that.....
But some oscilloscopes have isolated inputs...... very very very nice to use for this.