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2-3 Traditional Methods of Over-Current Shutdown

For shutting down an IGBT when an Over-Current condition is detected, a soft turn-off is preferred

because it reduces a high voltage spike across the collector and emitter of the IGBT device at fault.

Thus it provides a wider margin for the RBSOA limit during a short-circuit condition, allowing the

snubber circuit to be significantly minimized or even eliminated. When implemented in the discrete

circuit based on the floating ground reference architecture, a soft turn-off circuit can get very com-

plex. Each IGBT gate drive circuit requires an additional fast opto isolator and soft turn-off circuit with

the dedicated totem pole buffer transistors. Given circuit costs and complexities, a snubber circuit is

preferred to the discrete soft shutdown circuit. For low Hp devices, the snubber circuit can be imple-

mented with a high frequency type capacitor across the DC bus, near the IGBTs.

2-4 Problems with the Traditional Method

A. Circuit Control Problems

In a soft shutdown scheme, simultaneous shutdown of all 6 IGBTs is required to prevent a potential

false turn-on while the IGBTs are in the soft shutdown mode. If a high side IGBT is turned off softly

while the other IGBTs are in the middle of a switching transition within a PWM period, that IGBT can

unintentionally be turned back on. It is extremely difficult to synchronize simultaneous shutdown

using traditional architecture.

B. Part Count, Size, and Cost Problems

If the system requires full IGBT protection, including Ground Fault protection, and if it is based on the

floating ground reference architecture, then the gate drive and protection circuit require the following

major components (even without the soft shutdown feature):

•

6 fast opto isolators

•

2 fast Hall Effect sensors or fast linear opto isolators

•

2 comparators

•

4 floating 15V power supplies

Hall Effect sensors and opto isolators are relatively large and bulky, and require a lot more space

than a monolithic IC. In addition, if the system requires a soft shutdown function, 6 additional opto

isolators and 6 buffer circuits (with a provision for soft turn off capability) are required. Thus, whereas

low Hp AC drive systems are today moving toward smaller sizes, the above solutions provide neither

the simplicity nor the integration of the gate drive and protection circuitry that is needed.

The total cost, including assembly of the inverter system, is large due to the large number and

bulkiness of components. Furthermore, components such as the Hall Effect sensors are still subject

to manual assembly.

To circumvent these issues, a small, integrated IC device that is capable of performing all the above

functions is a necessity.