Table 2. Design Parameters

10.2.2.1 Recommended ISO5852S-EP Application Circuit

The ISO5852S-EP device has both, inverting and noninverting gate-control inputs, an active-low reset input, and an open-drain fault output suitable for wired-OR applications. The recommended application circuit in Figure 49 shows a typical gate-driver implementation with unipolar output supply. Figure 50 shows a typical gate-driver implementation with bipolar output supply using the ISO5852S-EP device.

A 0.1-μF bypass capacitor, recommended at the V_CC1 input supply pin, and 1-μF bypass capacitor, recommended at the V_CC2 output supply pin, provide the large transient currents required during a switching transition to ensure reliable operation. The 220-pF blanking capacitor disables DESAT detection during the off-to-on transition of the power device. The DESAT diode (D_DST) and the 1-kΩ series resistor on the DESAT pin are external protection components. The R_G gate resistor limits the gate-charge current and indirectly controls the rise and fall times of the IGBT collector voltage. The open-drain FLT output and RDY output have a passive 10-kΩ pullup resistor. In this application, the IGBT gate driver is disabled when a fault is detected and does not resume switching until the microcontroller applies a reset signal.

Figure 49. Unipolar Output Supply

Figure 50. Bipolar Output Supply

10.2.2.2 FLT and RDY Pin Circuitry

A is 50-kΩ pullup resistor exists internally on FLT and RDY pins. The FLT and RDY pins are an open-drain output. A 10-kΩ pullup resistor can be used to make it faster rise and to provide logic high when FLT and RDY is inactive, as shown in Figure 51.

Fast common-mode transients can inject noise and glitches on FLT and RDY pins because of parasitic coupling. The injection of noise and glitches is dependent on board layout. If required, additional capacitance (100 pF to 300 pF) can be included on the FLT and RDY pins.

Figure 51. FLT and RDY Pin Circuitry for High CMTI

10.2.2.3 Driving the Control Inputs

The amount of common-mode transient immunity (CMTI) can be curtailed by the capacitive coupling from the high-voltage output circuit to the low-voltage input side of the ISO5852S-EP device. For maximum CMTI performance, the digital control inputs, IN+ and IN–, must be actively driven by standard CMOS, push-pull drive circuits. This type of low-impedance signal source provides active drive signals that prevent unwanted switching of the ISO5852S-EP output under extreme common-mode transient conditions. Passive drive circuits, such as open-drain configurations using pullup resistors, must be avoided. A 20-ns glitch filter exists that can filter a glitch up to 20 ns on IN+ or IN–.

10.2.2.4 Local Shutdown and Reset

In applications with local shutdown and reset, the FLT output of each gate driver is polled separately, and the individual reset lines are independently asserted low to reset the motor controller after a fault condition.

Figure 52. Local Shutdown and Reset for Noninverting (left) and Inverting Input Configuration (right)

10.2.2.5 Global-Shutdown and Reset

When configured for inverting operation, the ISO5852S-EP device can be configured to shutdown automatically in the event of a fault condition by tying the FLT output to IN+. For high reliability drives, the open drain FLT outputs of multiple ISO5852S-EP devices can be wired together forming a single, common fault bus for interfacing directly to the microcontroller. When any of the six gate drivers of a three-phase inverter detects a fault, the active-low FLT output disables all six gate drivers simultaneously.

Figure 53. Global Shutdown With Inverting Input Configuration

10.2.2.6 Auto-Reset

In this case, the gate control signal at IN+ is also applied to the RST input to reset the fault latch every switching cycle. Incorrect RST makes output go low. A fault condition, however, the gate driver remains in the latched fault state until the gate control signal changes to the gate-low state and resets the fault latch.

If the gate control signal is a continuous PWM signal, the fault latch is always reset before IN+ goes high again. This configuration protects the IGBT on a cycle-by-cycle basis and automatically resets before the next on cycle.

Figure 54. Auto Reset for Noninverting and Inverting Input Configuration

10.2.2.7 DESAT Pin Protection

Switching inductive loads causes large, instantaneous forward-voltage transients across the freewheeling diodes of the IGBTs. These transients result in large negative-voltage spikes on the DESAT pin which draw substantial current out of the device. To limit this current below damaging levels, a 100-Ω to 1-kΩ resistor is connected in series with the DESAT diode.

Further protection is possible through an optional Schottky diode, whose low-forward voltage assures clamping of the DESAT input to GND2 potential at low-voltage levels.

Figure 55. DESAT Pin Protection With Series Resistor and Schottky Diode

10.2.2.8 DESAT Diode and DESAT Threshold

The function of the DESAT diode is to conduct forward current, allowing sensing of the saturated collector-to-emitter voltage of the IGBT, V_(DESAT), (when the IGBT is on), and to block high voltages (when the IGBT is off). During the short transition time when the IGBT is switching, a commonly high dV_CE/dt voltage ramp rate occurs across the IGBT. This ramp rate results in a charging current I_(CHARGE) = C_(D-DESAT) × d_VCE/dt, charging the blanking capacitor. C_(D-DESAT) is the diode capacitance at DESAT.

To minimize this current and avoid false DESAT triggering, fast switching diodes with low capacitance are recommended. As the diode capacitance builds a voltage divider with the blanking capacitor, large collector voltage transients appear at DESAT attenuated by the ratio of 1+ C_(BLANK) / C_(D-DESAT).

Because the sum of the DESAT diode forward-voltage and the IGBT collector-emitter voltage make up the voltage at the DESAT-pin, V_F + V_CE = V_(DESAT), the V_CE level, which triggers a fault condition, can be modified by adding multiple DESAT diodes in series: V_CE-FAULT(TH) = 9 V – n × VF (where n is the number of DESAT diodes).

When using two diodes instead of one, diodes with half the required maximum reverse-voltage rating can be selected.

10.2.2.9 Determining the Maximum Available, Dynamic Output Power, P_OD-max

The ISO5852S-EP maximum-allowed total power consumption of P_D = 251 mW consists of the total input power, P_ID, the total output power, P_OD, and the output power under load, P_OL:

With:

and:

then:

In comparison to P_OL, the actual dynamic output power under worst case condition, P_OL-WC, depends on a variety of parameters:

Equation 5.

where

• f_INP = signal frequency at the control input IN+
• Q_G = power device gate charge
• V_CC2 = positive output supply with respect to GND2
• V_EE2 = negative output supply with respect to GND2
• r_on-max = worst case output resistance in the on-state: 4 Ω
• r_off-max = worst case output resistance in the off-state: 2.5 Ω
• R_G = gate resistor

When R_G is determined, Equation 5 is to be used to verify whether P_OL-WC < P_OL. Figure 56 shows a simplified output stage model for calculating P_OL-WC.

Figure 56. Simplified Output Model for Calculating P_OL-WC

10.2.2.10 Example

This examples considers an IGBT drive with the following parameters:

• I_ON-PK = 2 A
• Q_G = 650 nC
• f_INP = 20 kHz
• V_CC2 = 15 V
• V_EE2 = –8 V

Applying the value of the gate resistor R_G = 10 Ω.

Then, calculating the worst-case output-power consumption as a function of R_G, using Equation 5 r_on-max = worst case output resistance in the on-state: 4 Ω, r_off-max = worst case output resistance in the off-state: 2.5 Ω, R_G = gate resistor yields

Equation 6.

Because P_OL-WC = 72.61 mW is less than the calculated maximum of P_OL = 88.25 mW, the resistor value of R_G = 10 Ω is suitable for this application.

10.2.2.11 Higher Output Current Using an External Current Buffer

To increase the IGBT gate drive current, a non-inverting current buffer (such as the npn/pnp buffer shown in Figure 57) can be used. Inverting types are not compatible with the desaturation fault protection circuitry and must be avoided. The MJD44H11/MJD45H11 pair is appropriate for currents up to 8 A, the D44VH10/ D45VH10 pair for up to 15 A maximum.

Figure 57. Current Buffer for Increased Drive Current