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Posted on 06 March 2019

Switching Loss Reduction Networks - Snubber Circuits

 

 

 

 

 

 

 

Power electronic switches with conventional thyristors or GTOs require snubber circuits in order to guarantee operation within the safe operating area, i.e., these networks are indispensable if the components are to live up to their basic functions during switching. In contrast to this, the SOA (Safe Operating Area) characteristics of modern IGBTs and MOSFETs allow operation without the use of networks, meaning that additional networks may only serve to reduce switching losses or perform symmetry tasks in the case of cascading.

Figure 1 shows a conventional buck converter with IGBT and basic networks for turn-on and turn-off relief (reduction of switching losses).

Buck converter with IGBT and basic switching loss reduction networks

Figure 1. Buck converter with IGBT and basic switching loss reduction networks

Reduction of turn-on losses (RLD network)

To begin with, the IGBT is in off-state (VCE ≈ VDC), and the load current is flowing in the freewheeling circuit.

Commutation from the freewheeling diode to the IGBT is started by active IGBT turn-on. As soon as the snubber inductance L has reached a certain value, it will take up the commutation voltage almost completely (this corresponds to the input DC voltage of the converter) when the collector current rises, meaning that the collector-emitter voltage is quickly reduced to a very low level. At the same time, the network inductance will bring about a reduction in the current commutation speed.

Together, these two factors lead to a substantial decrease in IGBT turn-on losses. The characteristics of collector current and collector-emitter voltage correspond to soft switching.

In addition to the reduction of IGBT turn-on losses, the turn-off losses of the freewheeling diode will also be decreased during commutation, since the reduced current commutation speed will lead to low-level reverse recovery peak currents.

The combination of R-D will create a freewheeling circuit for the network inductance, which will limit IGBT and FWD overvoltages during turn-off.

Recommendations for dimensioning:

  1. The network inductance should not to be dimensioned any bigger than needed for loss reduction
  2. Reduce the inherent (internal) capacitance of the snubber inductor to a minimum
  3. R and L form the time constants (t = L/R) necessary for internal energy discharge in the inductor. This results in a minimum IGBT static off-time (duty cycle limitation) to achieve efficient reduction of turn-on power losses (no residual current in L). On the one hand, increasing R will reduce the minimum IGBT static off-time. On the other hand, however, it will lead to a higher voltage and, consequently, higher power dissipation in the power semiconductors that are turning off.

Reduction of turn-off losses (RCD snubbers)

To begin with, the IGBT is in on-state and conducts the load current. Commutation from the IGBT to the freewheeling diode is started by active IGBT turn-off. The load current quickly commutates from the IGBT to the parallel D-C branch, causing the collector current and the collector-emitter dv/dt to decrease at the same time.

This leads to a reduction in the IGBT turn-off losses. The characteristics for collector current and collector-emitter voltage correspond to soft switching. The loss reduction that can be achieved with a certain capacitance strongly depends on the given transistor technology. At the end of voltage commutation, the free-wheeling diode will turn on with low losses and take up the snubber capacitance current. As of the next time the IGBT turns on, the energy stored in the snubber network capacitor will be mainly converted to heat by resistor R.

Recommendations for dimensioning:

  1. The snubber inductance should not to be dimensioned any bigger than needed for loss reduction
  2. Use fast network diode with lower turn-on overvoltage (forward recovery)
  3. Use pulse-proof capacitors (film capacitors or similar) with low internal inductance
  4. Reduce loop inductance in the snubber network to a minimum
  5. R and C form the time constants (t = R*C) necessary for internal energy discharge in the capacitor. This results in a minimum IGBT static off-time (duty cycle limitation) to achieve efficient reduction of turn-off power losses (no residual voltage in C). On the one hand, decreasing R will reduce the minimum IGBT static on-time; on the other hand, however, it will lead to a higher current and, consequently, greater losses when the transistor is turned on.

Note that bigger inductive and capacitive snubber elements will always lead to longer commutation times!

In applications that use simple snubber networks, as described above, the total energy stored is converted to heat mainly in the network resistor but also partially in the transistor (dissipative snubber). Thus, the overall efficiency of the circuit will not be improved - irrespective of the reduced losses in the switches. All this measure does is move the losses from the semiconductor to the snubber resistor, thus allowing higher switching frequencies.

Furthermore, numerous low-loss snubber networks (non or low-dissipative snubbers), where the energy is stored in resonant circuits or fed back to the DC link, are well-known from relevant literature. However, circuit designs such as these are often very complicated to dimension, and the layout and circuitry requirements are very challenging, too [1].

References:

[1] Teigelkötter, J.: "Schaltverhalten und Schutzbeschaltungen von Hochleistungshalbleitern", diss., Ruhr-Universität Bochum, 1996, VDI-Verlag, 1996, ISBN 3-18-320621-8

 

For more information, please read:

AC Side Snubber Circuits

DC Side Snubber Circuits

Snubber Circuits Based on Silicon Avalanche Diodes

Protection of Thyristors with Snubbers

 

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One Response

  1. avatar priyanka says:

    how to reduce switching losses in the mosfet and give to switching losss equation

    how to reduce voltage sttressis in the semiconducroe device

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