IGBT｜Application

IGBT IPM: Protection Functions and Operation Sequences Short Circuit Current Protection (SCP) Function of IGBT IPMs

2024.03.13

Points of this article

・The short circuit current protection (SCP) function is implemented by connecting an external shunt resistor to an IPM and feeding back the voltage appearing across the resistor to the CIN pin.

・The CIN pin requires an RC filter to prevent malfunctions.

・Short circuit current protection operates only for the lower arm.

・When the short circuit current protection is actuated, operation must immediately be halted to avoid an anomalous state.

・The shunt resistor value and the RC filter time constant are set to close to recommended values that are based on equations provided, and are ultimately finalized by confirming operation of actual equipment.

table of contents

We begin with the first item, short circuit current protection of IGBT IPMs.

  • Short circuit current protection（SCP）
  • Control power supply undervoltage lockout（UVLO）
  • Thermal shutdown（TSD）
  • Analog temperature output（VOT）
  • Fault output （FO）
  • Control inputs （HINU, HINV, HINW, LINU, LINV, LINW）

Short Circuit Protection (SCP) Function of IGBT IPMs

Short circuit current protection (SCP) is made possible by connecting an external shunt resistor (resistor for current detection) to an IPM and feeding back the voltage that appears across the resistor to the CIN pin. When protection operation begins, IGBTs for all phases of the lower arm are turned off and an FO signal is output. In order to prevent malfunctions due to recovery currents and noise during switching, an RC filter (R2 and C9 in the dashed-line oval in the diagram below) should be installed at the CIN pin input. A time constant of 1.0 µs is recommended.

Operation Sequence of the Short Circuit Protection Function (Protection using an external shunt resistor and RC filter)

Below are shown the short circuit current protection (SCP) operation sequence and timing chart. In the explanation of operation, a1 to a9 are operations at the points shown in the timing chart.

  1. a1： In normal operation, when the IGBTs are turned on an output current Ic flows
  2. a2： Overcurrent detection (SCP) is triggered. The optimum shutoff time is set: the RC time constant should be set so that current is shut off within 2 µs (1.0 µs recommended).
  3. a3：Gates shut off for all lower arm phases (soft shutoff)
  4. a4：IGBTs shut off for all lower arm phases
  5. a5： FO output (45 µs Min）, case in which SCP is H for 45 µs or less. If SCP is H for 45 µs or more, over the SCP=H interval, FO signal is output （FO＝L）
  6. a6： LIN=L
  7. a7： While SCP=H, IGBTs are off even when LIN=H
  8. a8： FO output ends (returns to H). Even when SCP goes from H to L while LIN=H (dashed line), IGBTs are in the off state until the next LIN rising edge (each phase returns to normal state at LIN input for the phase)
  9. a9： Normal operation. IGBTs are on, output current Ic flows

※Points to be noted

  • ・Short circuit current protection acts only on the lower arm.
  • ・When short circuit current protection is actuated, if a fault signal is output, operation should immediately be halted to avoid an anomalous state.

Setting the Shunt Resistor Value for Short Circuit Protection

The following is the method used to set the value of the “shunt resistor” in the area surrounded by the dashed line in the above block diagram. The shunt resistor value RSHUNT is set using the following equation, based on the short circuit current protection trip voltage VSC and the protection current setting value ISCP.

The maximum value of ISCP must be set so as to be below the minimum value of the IGB saturation current, taking into consideration variation in the shunt resistor value and in VSC. The recommended value for IPM short circuit current protection is 1.7 times the rated current or less.

As an example, the method for setting the value for a BM64375S when ISCP is to be 34 A (1.7 times the rated current of 20 A) is shown below. Variation in VSC (for Tj=25°C, VCC=15 V) is shown in the following table.

Item Symbol Limit Unit Condition
Min Typ Max
SCP trip voltage VSC 0.455 0.480 0.505 V

The relationship between the shunt resistor value RSHUNT and the short circuit current setting value ISCP is as follows, taking variation into account.

From (3),

34A = 0.505V / RSHUNT(Min)
RSHUNT(Min) ≈ 14.85mΩ

If the variation in the shunt resistor value is ±5%, then

RSHUNT(Typ) ≈ 14.85mΩ / 0.95 ≈ 15.63mΩ
RSHUNT(Max) ≈ 15.63mΩ × 1.05 = 16.41mΩ

From the above, given Tj=25°C and VCC=15 V, the ISCP operating range is as shown in the following table.

Item Min Typ Max
RSHUNT setting range 14.85mΩ 15.63mΩ 16.41mΩ
ISCP operating range 27.7A 30.7A 34.0A

Due to resonance waveforms arising from the parasitic inductance and parasitic capacitance of external wiring, the protection function may operate at a current below the set value. It will ultimately be necessary to adjust the shunt resistor value after more detailed evaluations using actual equipment.

Setting the RC Time Constant for Short Circuit Protection

As explained in the section ” Short Circuit Current Protection (SCP) Function of IGBT IPM “, in order to prevent malfunctioning of the short circuit current protection circuit due to noise occurring in the shunt resistor, an RC filter (R2 and C9 in the dashed-line oval of the diagram above) is required at the CIN pin input. The RC time constant τ is set according to the duration of noise occurrence and the IGB load short circuit tolerance; a value of τ=1.0 µs is recommended.

After a voltage that exceeds the level of VSC has occurred in the shunt resistor, the time t1 until the voltage VSC is applied to the CIN pin via the RC filter circuit is calculated as follows.

Further, the IPM internal delay time t2 shown below elapses from the time the CIN pin reaches VSC level until the IGBT gate is shut off.

Item Min Typ Max
IPM internal delay time – – 0.65µs

Hence the entire time ttotal from the short circuit occurrence until the IGBT gate is shut off is as follows.

Detailed evaluations using actual equipment should be performed in order to decide on an RC time constant such that ttotal is within the IGBT load short circuit tolerance.

【Download Documents】 Basics of IGBTs

IGBTs are one of the typical power devices and are used in a wide range of applications including motor drives. This handbook provides a basic understanding of IGBTs, including their application scope and application picture based on their features, their structure and principle of operation, and how they compare with and use other power devices.

Short Circuit Current Protection (SCP) Function of IGBT IPMs
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