9.3.1 Supply and active Miller clamp

The ISO5851-Q1 supports both bipolar and unipolar power supply with active Miller clamp.

For operation with bipolar supplies, the IGBT is turned off with a negative voltage on its gate with respect to its emitter. This prevents the IGBT from unintentionally turning on because of current induced from its collector to its gate due to Miller effect. In this condition it is not necessary to connect CLAMP output of the gate driver to the IGBT gate, but connecting CLAMP output of the gate driver to the IGBT gate is also not an issue. Typical values of V_CC2 and V_EE2 for bipolar operation are 15-V and -8-V with respect to GND2.

For operation with unipolar supply, typically, V_CC2 is connected to 15-V with respect to GND2, and V_EE2 is connected to GND2. In this use case, the IGBT can turn-on due to additional charge from IGBT Miller capacitance caused by a high voltage slew rate transition on the IGBT collector. To prevent IGBT to turn on, the CLAMP pin is connected to IGBT gate and Miller current is sinked through a low impedance CLAMP transistor.

Miller CLAMP is designed for Miller current up to 2-A. When the IGBT is turned-off and the gate voltage transitions below 2-V the CLAMP current output is activated.

9.3.2 Active Output Pull-down

The Active output pull-down feature ensures that the IGBT gate OUT is clamped to V_EE2 to ensure safe IGBT off-state when the output side is not connected to the power supply.

9.3.3 Undervoltage Lockout (UVLO) with Ready (RDY) Pin Indication Output

Undervoltage Lockout (UVLO) ensures correct switching of IGBT. The IGBT is turned-off, if the supply V_CC1 drops below V_IT-(UVLO1), irrespective of IN+, IN- and RST input till V_CC1 goes above V_IT+(UVLO1).

In similar manner, the IGBT is turned-off, if the supply V_CC2 drops below V_IT-(UVLO2), irrespective of IN+, IN- and RST input till V_CC2 goes above V_IT+(UVLO2).

Ready (RDY) pin indicates status of input and output side Under Voltage Lock-Out (UVLO) internal protection feature. If either side of device have insufficient supply (V_CC1 or V_CC2), the RDY pin output goes low; otherwise, RDY pin output is high. RDY pin also serves as an indication to the micro-controller that the device is ready for operation.

9.3.4 Fault (FLT) and Reset (RST)

During IGBT overload condition, to report desaturation error FLT goes low. If RST is held low for the specified duration, FLT is cleared at rising edge of RST. RST has an internal filter to reject noise and glitches. By asserting RST for at-least the specified minimum duration, device input logic can be enabled or disabled.

9.3.5 Short Circuit Clamp

Under short circuit events it is possible that currents are induced back into the gate-driver OUT and CLAMP pins due to parasitic Miller capacitance between the IGBT collector and gate terminals. Internal protection diodes on OUT and CLAMP help to sink these currents while clamping the voltages on these pins to values slightly higher than the output side supply.

Table 1. Function Table⁽¹⁾