AN3338 datasheet(29/72 Pages) STMICROELECTRONICS

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AN3338

Electrical characteristics and functions

2.3.9

RC filter network selection

Two options of shunt (1- or 3-shunt) resistor circuit can be adopted in order to implement

different control technique and short-circuit protection, as shown in

Figure 17.

A RC filter network is required to prevent undesired short-circuit operation due to the noise

on the shunt resistor.

Both solutions allow to detect the total current in all three phases of the inverter. The filter is

based on the RSF and CSF network and its time constant is given by:

Equation 5

In addition to the RC time constant, the turn-off propagation delay of the gate driver, tisd

(specified in the datasheet) and the IGBT turn-off time (in the range of tens of ns), must be

considered in the total delay time (tTotal), which is the time necessary to completely switch

off the IGBT once the short-circuit event is detected. Therefore, the tTotal is calculated as

follows:

Equation 6

also considering that the IGBT short-circuit withstand time (tSC) is 5 µs, the tSF is

recommended to be set in the range of 1~2 µs.

In the case of a 3-shunt resistors circuit, a specific control technique can be implemented by

using the three shunt resistors (RSHUNT_U, RSHUNT_V and RSHUNT_W) able to monitor each

phase current.

Figure 17. Examples of SC protection circuit

SLLIMM

SHUNT

CIN

SLLIMM

SHUNT

CIN

SHUNT_U

SHUNT_W

SHUNT_V

1-shunt resistor circuit

3-shunt resistors circuit

⋅

off

isd

Total

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