Latch-Up: Hints, Kinks and Caveats
The IGT is a rugged device, requiring no snubber network when operating within its published safe-operating-area (SOA) ratings. Within the SOA, the gate emitter voltage controls the collector current. In fact, the IGT can conduct three to four times the published maximum current if it’s in the ON state and the junction temperature is +150oC maximum.
However, if the current exceeds the rated maximum, the IGT could lose gate control and latch up during turn-off attempts. The culprit is the parasitic SCR formed by the pnpn structure shown in Figure 16. In the equivalent circuit, Q1 is a power MOSFET with a normal parasitic transistor (Q2) whose base- emitter junction is shunted by the low-value resistance R1.
EMITTER METAL | POLYSILICON GATE |
N+ | | N+ |
P | | P |
| PATH | |
N EPITAXIAL LAYER | CURRENT | |
| MINORITY |
| MAIN |
| CARRIER |
| INJECTION |
P+ SUBSTRATE | |
METAL COLLECTOR
COLLECTOR
FIGURE 16. THE IGT’S PARASITIC SCR IS RESPONSIBLE FOR THE DEVICE’S LATCH-UP CHARACTERISTICS.
For large current overloads, the current flowing through R1 can provoke SCR triggering. In the simplest terms, R1 repre- sents the equivalent of a distributed resistor network, whose magnitude is a function of Q2’s VCE. During normal IGT operation, a positive gate voltage (greater than the thresh- old) applied between Q1’s gate and source turns the FET on. The FET then turns on Q3 (a pnp transistor with very low gain), causing a small portion of the total collector current to flow through the R1 network.
To turn the IGT off, you must reduce the gate-to-emitter voltage to zero. This turns Q1 off, thus initiating the turn-off sequence within the device. Total fall time includes current- fall-time one (tF1) and current-fall-time two (tF2) components. The turn-off is a function of the gate-emitter resistance, Q3’s storage time and the value of VGE prior to turn-off. Device characteristics fix both the delay time and the fall time.
Forward-Bias Latch-Up
Within the IGT’s current and junction-temperature ratings, current does not flow through Q2 under forward-biased conditions. When the current far exceeds its rated value, the current flow through R1 increases and Q3’s VCE also increases because of MOSFET channel saturation. Once
Q3’s ICR1 drop exceeds Q2’s VBE(ON), Q2 turns on and more current flow bypasses the FET.
The positive feedback thus established causes the device to latch in the forward-biased mode. The value of IC at which the IGT latches on while in forward conduction is typically three to four times the device’s maximum rated collector current. When the collector current drops below the value that provokes Q2 turn-on, normal operation resumes if chip temperature is still within ratings.
If the gate-to-emitter resistance is too low, the Q2-Q3parasitic SCR can cause the IGT to latch up during turn-off. During this period, RGE determines the drain-source dV/dt of power MOSFET Q1. A low R1 causes a rapid rise in voltage - this increases Q2’s VCE, increasing both R1’s value and Q2’s gain.
Because of storage time, Q3’s collector current continues to flow at a level that’s higher than normal for the FET bias. During rapid turn-off, a portion of this current could flow in Q2’s base-emitter junction, causing Q2 to conduct. This process results in device latch-up; current distribution will probably be less uniform than in the case of forward-bias latch-up.
Because the gains of Q2 and Q3 increase with temperature and VCE, latching current - high at +25oC - decreases as a function of increasing junction temperature for a given gate- to-emitter resistance.
How do you test an IGT’s turn-off latching characteristic? Consider the circuit in Figure 17. Q1’s base-current pulse width is set approximately 2∝sec greater than the IGT’s gate- voltage pulse width. This way, the device under test (DUT) can be switched through Q1 when reverse-bias latch-up occurs. This circuit allows you to test an IGT’s latching current nondestructively.
The results? Clamped-inductive-load testing with and without snubbers reveals that snubbering increases current handling dramatically: With RGE = 1kΩ, a 0.02∝F snubber capacitor increases current capability from 6A to 10A; with RGE = 5kΩ, a 0.09∝F snubber practically doubles capacity (25A vs 13A).
Conclusions? You can double the IGT’s latching current by increasing RGE from 1kΩ to 5kΩ, and double it again with a polarized snubber using CS < 0.1∝F. The IGT is therefore useful in situations where the device must conduct currents of five to six times normal levels for short periods.
Finally, you can also use the latching behavior to your advan- tage under fault conditions. In other words, if the device latches up during turn-off under normal operation, you could arrange it so that a suitable snubber is switched electroni- cally across the IGT.
