SLLSEQ1A September 2016 – December 2016 ISO5851-Q1
The ISO5851-Q1 is an isolated gate driver for IGBTs and MOSFETs. Input CMOS logic and output power stage are separated by a capacitive, silicon dioxide (SiO2), isolation barrier.
The IO circuitry on the input side interfaces with a micro controller and consists of gate drive control and RESET (RST) inputs, READY (RDY) and FAULT (FLT) alarm outputs. The power stage consists of power transistors to supply 2.5-A pullup and 5-A pulldown currents to drive the capacitive load of the external power transistors, as well as DESAT detection circuitry to monitor IGBT collector-emitter overvoltage under short circuit events. The capacitive isolation core consists of transmit circuitry to couple signals across the capacitive isolation barrier, and receive circuitry to convert the resulting low-swing signals into CMOS levels. The ISO5851-Q1 also contains under voltage lockout circuitry to prevent insufficient gate drive to the external IGBT, and active output pulldown feature which ensures that the gate-driver output is held low, if the output supply voltage is absent. TheISO5851-Q1 also has an active Miller clamp function which can be used to prevent parasitic turn-on of the external power transistor, due to Miller effect, for unipolar supply operation.
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 VCC2 and VEE2 for bipolar operation are 15 V and –8 V with respect to GND2.
For operation with unipolar supply, typically, VCC2 is connected to 15-V with respect to GND2, and VEE2 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.
The active output pulldown feature ensures that the IGBT gate OUT is clamped to VEE2 to ensure safe IGBT off-state when the output side is not connected to the power supply.
Undervoltage lockout (UVLO) ensures correct switching of IGBT. The IGBT is turned-off, if the supply VCC1 drops below VIT-(UVLO1), irrespective of IN+, IN– and RST input till VCC1 goes above VIT+(UVLO1).
In similar manner, the IGBT is turned-off, if the supply VCC2 drops below VIT-(UVLO2), irrespective of IN+, IN– and RST input till VCC2 goes above VIT+(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 (VCC1 or VCC2), 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.
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.
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 lists the functional modes for the ISO5851-Q1 device.
InISO5851-Q1 OUT to follow IN+ in normal functional mode, FLT must be in high state.