Parallel and Series Connection of Power Semiconductor Devices

Posted on 23 May 2019

 

 

 

 

 

 

 

Peak current and blocking voltage of power semiconductor devices is limited. It is therefore often necessary to connect similar power semiconductor devices of in groups in order to enlarge the capacity of the equipment under development. The main connection types include:

  •  Parallel – used when it is necessary to increase peak current
  • Series – used when it is necessary to increase maximum blocking voltage
  • Mixed  - parallel + series

When connecting thyristors and diodes in parallel, it is necessary to strive for equal distribution of load current among the devices. It is important to provide suitable operating conditions for power semiconductor devices and equality of voltage-current characteristics while taking into consideration the scatter of technological parameters. In order to accomplish this, the following requirements must be met.

  • Installation of inductance and ohmic current dividers in parallel with each semiconductor device
  • Selection of semiconductor devices according to static losses in the operating point (according to VTM(FM) at operating current). It is worth noting that a certain technological scatter of semiconductor power device parameters always exists.
  •  When designing converters with semiconductor devices connected in parallel,  it is recommended to choose operating currents that are above the reversal points of the voltage-current characteristics of the power semiconductor devices. In this case, current equalization in parallel paths is carried out automatically since in the voltage-current characteristic area lying above the reverse point, negative feedback operates. In other words, when junction temperature increases, its resistance increases and direct current diminishes, resulting in junction temperature decrease.
  • In order to minimize the influence of separate thyristors turn-on times, and as a consequence  nonequilibrium current division in branches,  it is necessary to use strong gate-pulses with high di/dt, leading to a decrease in thyristor turn-on delay time and to the minimization of the effect on current division in parallel branches.
  • For circuits with high voltage power thyristors, thyristors with elements of  big diameters (more than 56 mm), and in situations where there are high induction coefficients in the power part that limit the rate of power current change, it is additionally necessary to take into consideration the time properties of the thyristor in the circuit. This is connected with the fact that power thyristors are first turned on in the limited area nearb the control electrode, and after that the longitudinal propagation of the engaged condition takes place during the restricted time.
  • Packaging of parallel branches should provide the equality of conductor lines resistance, including fuses.
  • Refrigerating conditions for all the devices entering the connection should be equal.

When connecting thyristors or diodes in series, it is necessary to strive for equal sharing of blocking voltage (forward and/or reverse) both in steady-state condition and in dynamic modes, that is, at thyritor startup and blocking condition recovery at the time of a thyristor or a diode turning off. Reasons for nonequal distribution of blocking voltages can be the following:

  • Differences of leakage in series connected devices resulting from natural parametric scatter and/or different operating temperatures due to, for example, different refrigerating conditions (for instance, on average, 8°C temperature change leads to a twofold change of leakage). Excess voltage emerges in devices with lower leakage current value.
  • Spread of turn-on time of separate thyristors connected in series in branches leads to redistribution of voltage among thyristors that are turned on prior and those that are turned on with delay. Excess voltage emerges in the thyristors that are turned on with delay.

The scatter of reverse recovered charge values in the series connected devices leads to the situation that at the moment of recovery, the devices accept reverse voltage at different times. Excess voltage emerges in the thyristors with lower reverse recovered charge value.

Ways of blocking voltage distribution equalization

  • To minimize of the influence of leakage current irregularity of series connected power semiconductor devices,  high value bypass resistors are turned on in parallel to each semiconductor device (a diode or a thyristor). Higher requirements of voltage equalization in this mode imply lower required values for bypass resistors.
  • Imbalances of blocking voltage sharing that arise from the spread of reverse recovered charge values of power semiconductor devices are minimized by using RC snubber circuits turned on in parallel to each semiconductor device. Higher snubber capacity value turned on in parallel to the device implies lower imbalances of blocking voltage distribution. However, the capacity increase is not always rational which is why selection of devices for series connection according to reverse recovery charge is required. As a rule, the spread of charges should equal 5-10%.
  • The spread of power semiconductor devices turn-on time is minimized by application of strong gate pulse current with high di/dt, leading to a decrease in thyristor turn-on delay time and minimization of the influence of this effect on voltage division. The presence of snubber circuits in parallel to each device has a positive influence since before turning on, the same direct voltage being applied to the thyristors is applied to the snubber capacitors. The after the thyristor turns on, the voltage of the snubber capacitors is applied to the thyristor and provides balanced voltage distribution.

Parallel connected or series connected power semiconductor devices encompass a wide range of power converters. When designing, maintaining, and repairing such systems, it is important to consider the above mentioned requirements and the peculiarities of device group turn-on. This will allow the power semiconductor device resource to be increased to the maximum extent possible, and assist in the development and design of reliable and long lasting equipment.

 

For more information, please read:

Criteria for Successful Selection of IGBT and MOSFET Modules

 Criteria for Successful Selection of Diodes and Thyristors

Parallel and Series Connection of Fuses

 

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